APOD: Spiral Galaxy NGC 2841 Close Up (2011 Feb 19)

[dougettinger]

http://apod.nasa.gov/apod/ap110219.html

This flocculent galaxy, NGC 2841, appears to have definite plumes throughout the last 2/3's of its disk radial distance. It appears that the plumes are hiding the majority of stars including the galaxy's spiral structure. Could these plumes be the result of almost simultaneous(in cosmic time) of deaths of massive stars ? These plumes would be the envelopes of nebulae and supernova remnants. Perhaps a recent collision by another galaxy has caused a combination resulting in a proto-spiral galaxy and this galaxy is just now beginning the evolution of multi-generations of stars and the birthing of its spiral arms.

Doug Ettinger, Pittsburgh, 02/19/2011

[Ann]

This flocculent galaxy, NGC 2841, appears to have definite plumes throughout the last 2/3's of its disk radial distance. It appears that the plumes are hiding the majority of stars including the galaxy's spiral structure. Could these plumes be the result of almost simultaneous(in cosmic time) of deaths of massive stars ? These plumes would be the envelopes of nebulae and supernova remnants. Perhaps a recent collision by another galaxy has caused a combination resulting in a proto-spiral galaxy and this galaxy is just now beginning the evolution of multi-generations of stars and the birthing of its spiral arms.

Doug Ettinger, Pittsburgh, 02/19/2011

James D Wray talked a lot about galactic plumes in his book, "The Color Atlas of Galaxies", from 1988. To him, plumes were straight segments in a galaxy that didn't follow the curved shape of the spiral arms. I haven't heard other people talk about plumes until you mentioned them.

James D. Wray photographed galaxies in UBG. He was good at detecting the ultraviolet light of unobscured newborn hot stars, but he couldn't see the infrared light emerging from dusty cocoons of baby stars. So he couldn't see the earlier stages of star formation. That said, I don't remember that the plumes of his galaxies looked more ultraviolet - or usually even as ultraviolet - as the spiral arms of them. So I don't know if plumes are generally a more fertile breeding ground for stars than other parts of a galaxy.

But a galaxy can most definitely have many supernovae going off simultaneously (in cosmic time). A large number of supernovae at the same time can drive most of the gas out of a galaxy and prevent or strongly reduce further star formation. Nearby M82 is an interesting example:

Click to view full size image

In this image, the bluish structure running from upper left to lower right is the galaxy's disk. This disk appears to be made up of middle-aged stars, but, apart from some light structures near the center on the left and some rather wispy dust, the disk is remarkably smooth. There are no young stellar cluster, emission nebulae or dense dark dusty structures that might condense into stars in this disk. In view of the fact that we apparently see M82 edge on, it is remarkable that we don't see a dust lane running along the disk, either. In this respect M82 is like a lenticular galaxy, disk galaxies with smooth disks and no dust or star formation. Here is a link to an edge-on lenticular galaxy without a dust lane, NGC 3115: http://2.bp.blogspot.com/_w1kycNNBkOE/T ... 5-LRGB.jpg
Compare the smooth featureless disk of NGC 3115 with the dense starforming dust lane of edge-on spiral galaxy NGC 891:http://apod.nasa.gov/apod/image/0207/ngc891_cfht.jpg

The disk of M82 appears to have completely lost its ability to form new stars. This is unusual for a disk galaxy that has more activity in it than what comes from a central black hole. Of course, even spiral galaxies can run out of star forming material. James D Wray's book contains a picture of an all-yellow spiral galaxy with no hot ultraviolet stars and probably no middle-aged stars, either, NGC 5929:



This was the best image I could find. Sorry. As you can see, NGC 5929 is interacting with another galaxy, and they have almost certainly driven the gas out of one another through their interaction. NGC 5929 is also a Seyfert galaxy with an active black hole in its center, and this active black hole may have helped inhibit star formation.

But let's return to M82. M82 is no Seyfert galaxy, but look at those incredible red outflows from its center. They glow red because they are composed of ionized hydrogen, and they are believed to have been caused by a great number of supernovae going off more or less simultaneously in the galaxy's center, thus driving copious amounts of gas out of the galaxy. Interestingly, although the Hubble telescope has detected many massive young clusters in the central region of M82, my impression is that the galaxy's glory days of star formation are over. It will not be possible for the galaxy to keep forming many new stars when it has lost so much gas to space. And interestingly, the gas just might keep glowing red for a while even though M82 has not produced any new supernovae for a while, and even though it may not have all that many hot "un-exploded" stars left any more. Compare M82 and its mighty gaseous outflows with the gaseous Hanny's Voorwerp and the galaxy that produced it:

Click to view full size image

The green-glowing gas has been driven out of the galaxy directly behind it. That galaxy had an active quasar in its center, which expelled and ionized a lot of gas. Fascinatingly, the quasar has now turned off, but the expelled gas keeps glowing green with oxygen emission.

But my point is that M82 has been irreversibly changed, and this change has almost certainly occurred because of its interaction with its larger neighbour M81:

Click to view full size image

This image really brings out how outrageously red the central outflow from M82 really is, compared with anything else in its galactic neighborhood. But the image also brings out how non-blue the disk of M82 is compared with the blue spiral arms of M81.

So, in conclusion, M82 has had a huge number of supernovae going off at the same time, but all these supernovae have killed M82's ability to form new stars in its disk.

Ann

[dougettinger]

Hello Ann, thanks for your thoughtful response. Discussing such topics as Hoag objects, flocculent galaxies, and ring galaxies really expands my knowledge. Are you perhaps an astrophysicist specializing in galaxies. Irregardless, I have an interesting question. Is there any theory regarding the age of galaxies like there is for stars ? If our universe is 13.7 billion years old and our solar system is 4.6 billion, then how many progressive generations of stars in our own galaxy lived and died before their remnants produced our solar system ?

Doug Ettinger, Pittsburgh, PA 02/20/2011

[Ann]

Hello Ann, thanks for your thoughtful response. Discussing such topics as Hoag objects, flocculent galaxies, and ring galaxies really expands my knowledge. Are you perhaps an astrophysicist specializing in galaxies. Irregardless, I have an interesting question. Is there any theory regarding the age of galaxies like there is for stars ? If our universe is 13.7 billion years old and our solar system is 4.6 billion, then how many progressive generations of stars in our own galaxy lived and died before their remnants produced our solar system ?

Doug Ettinger, Pittsburgh, PA 02/20/2011

Hi, Doug!

I am strictly an amateur. My knowledge comes, first of all, from my awe and fascination for the majesty of space, and second from my love for blue things. I love blue stars, as well as spiral and irregular galaxies that contain moderate to large populations of blue stars. Therefore I need to know basic facts about how blue stars are made, and as well as basic facts about galaxies.

As for the theories about the age of galaxies, I am the wrong person to ask, I'm afraid. What I think I know is that astronomers believe that stars simply couldn't form until a certain time after the Big Bang. The universe simply had to cool sufficently for ordinary matter to form in the first place, and for matter to clump together to form stars. A problem when it comes to very early star formation is that no dust existed before the first stars had exploded as supernovae and ejected dust into their surroundings. Astronomers say that current star formation requires dust, since dust helps the condensation and cooling of gas clouds that are necessary for these clouds to collapse into stars. Before dust existed, however, stars must have formed through other means. A possibility is that collision between primordial gas clouds may have triggered the earliest star formation.

Astronomers keep pushing back the date when the first galaxies formed. Just recently the Hubble telescope spied the oldest galaxy that has ever been detected. See http://www.space.com/10722-oldest-galax ... scope.html.

Astronomers believe that galaxies are mostly formed "bottom-up", that is, small galaxies collide and merge, thus creating a larger galaxy. There was just recently an article here at Starship Asterisk about how the Andromeda galaxy has supposedly grown by colliding and merging with smaller galaxies. We can be sure that galaxies change over time, both because they collide and because their stellar content change over time. Massive stars live fast and die young. If an average O star lives for 10 or possibly 20 million years - let's compromise and say that they live for 13-14 million years - and the universe has existed for 13-14 billion years, that means that not only has the universe had time to churn out a thousand generations of O stars, but a thousand generations of O stars have had time to die, too. (Of course, if the creation of O stars is a process that goes on continuously, then many more than a thousand generation of O stars have been born, but I think you know what I mean.)

The situation is radically different for low mass M stars. The universe has had time to churn out just as many generations of M stars as of O stars, but not a single low mass M star has had time to die since it formed. The lifetime of a low mass M star is many times greater than the current age of the universe. So the total number of M stars has been growing ever since the first stars formed in the universe, but the number of O stars has decreased. We know that this is the case, because astronomers have proved that star formation was much more intense in the universe a couple of billion years ago than it is now.

Our own Sun supposedly formed 4.5 billion years ago, although I've heard the figure 5 billion years mentioned, too. The oldest globular clusters in the Milky Way are about 12 billion years old, or so I believe. But of course, it can be argued that a galaxy which truly resembled the Milky Way hadn't formed at the time when the earliest Milky Way globulars were formed. Let's say, in any case, that the Milky Way had been growing and evolving for 7 billion years by the time that our Sun was formed, and something like 500 generations of O stars had had time to go supernovae and eject heavy elements into the interstellar medium. But you mustn't forget that ordinary red giants, those that never explode as supernovae, nevertheless form many heavy elements during the end of their lives, and then they blow these heavy elements into the interstellar medium during their planetary nebula phase.

Ann

[dougettinger]

Hello Ann, you made a quotable statement in stating your own estimation of how many generations of O-type stars occurred in our galaxy. I like the way that your thoughts wander beyond the box and how you Create your very own ideas. You can both answer questions by "nuefing" them and by being creative.

I will try to expand on your quotable line. Since the beginning of the Milky Way Galaxy (MWG) approximately 500 successive generations of O-type and/or B-type stars have been produced. One or two generations of F-type and G-type stars like our Sun and only one generation of M-type starts have been produced in that same time period. For each successive generation of O and B type stars less material is available in a galaxy, because long-lived stars of lesser mass are produced after each generation of O-type stars. As the age of the galaxy increases, entropy in turn increases thereby reducing O and B type stars after each birth and death cycle.

The ratios of O-type stars to G-type stars and G-type stars to M-type stars should reveal the overall age of each galaxy. However, this logical thinking is thwarted. The linear progression from larger to smaller stars as time goes forward is interrupted by galaxial collisions and near encounters. Also, there arises the difficulty of analyzing the evolution of different types of galaxies such as elliptical and spiral types.

Ann, please edit this expansion to agree with your own ideas, if you wish. I am immensely interested in your opinions. You need not cite a reference. Be forewarned; we may get nuefed which is entirely OK by me.

Do you believe that all observable galaxies were created simultaneously within a very short period of cosmic time ?

Thinking outside the box, what is your opinion of the plumes mentioned in the description of NGC 2841 ? Your idea would naturally cover any flocculent type galaxy.

Very enthusiastic about O-type stars,
Doug Ettinger, Pittsburgh, PA 02/20/2011

[Ann]

Why, thank you very much, Doug!

As for how many generations of O stars our galaxy has seen, we must remember that star formation does not seem to be a linear affair. A typical galaxy doesn't form one new star, or ten new stars, every year for billions and billions of years. One of my books on astronomy that I like a lot, The Galaxies of the Local Group by Sidney van den Bergh, says that star formation in the Large Magellanic Cloud has been anything but "same, same" and predictable over the lifetime of this galaxy. According to van den Bergh, the LMC formed quite a few massive clusters about eleven billion years ago, then it went quiet for about two billion years, then it formed a small number of new clusters, then there was a five-billion-year hiatus, and then about three billion years ago it began making clusters again. About two billion years ago it simply popped its lid and started churning out new clusters at a furious rate, and although its star formation rate has gone down a bit since then, its starmaking ability has nevertheless remained prodigious. So we can be sure that the LMC has an unusually large number of O stars in relation to its total stellar population, simply because it went into "starburst mode" no more than two billion years ago and is still very actively forming stars.

Why does the LMC have such a strange star formation history? I can only speculate, so that is what I'm going to do. It could be that the proto-LMC came into existence when a relatively large gas cloud was a bit disturbed and started collapsing and forming stars. But it was probably not too disturbed, and it didn't make use of its full star forming potential by far. Perhaps it was a bit like dwarf galaxy NGC 6822:

Click to view full size image

There is a bit of star formation here - note the pink clouds near the top of the image - but not nearly as much as there could be. NGC 6922 sits inside a large reservoir of molecular gas, but for some reason, most of the gas has not collapsed into stars. I think the Large Magellanic Cloud may have started out of such a cloud, so that it originally only formed a relatively small number of stars. Then something happened. What?

I can't resist showing you an image of NGC 1313, because when it comes to this galaxy astronomers say, "Something has happened, but what?"

Click to view full size image

NGC 1313 is forming stars so violently that its entire shape is twisted and contorted. I believe that this galaxy too sits in a large reservoir of gas, but I'm sure that it has no neighbours. It has most likely not been disturbed from outside. Something happened in its internal workings to make it go "starforming-ballistic".

I believe, however, that the LMC was indeed disturbed from outside. I think I read somewhere just recently that the LMC may originally have been a satellite of the Andromeda galaxy, but if you ask me, it can't have been orbiting huge M31 for a very long time. The enormous gravity of the Andromeda galaxy would have siphoned the gas out of proto-LMC and thwarted its star forming ability, unless proto-LMC was in a very wide orbit around M31.

No, but what really happened was almost certainly a close encounter between the Large Magellanic Cloud and the Small Magellanic Cloud. Of course these two galaxies are interacting. The interaction would have compressed the gas in the LMC and set off a starburst. But since the Small Magellanic Cloud was almost certainly very gas-rich at the time when these two galaxies had their first real encounter, the LMC, with its higher mass and stronger gravity, could steal gas from the SMC and so keep up its own starburst.

This is my point: star formation is not a linear affair, and it must a be a rare galaxy indeed that produces the "same, same and equal" number of O stars for each time slot over billions of years.

Also remember that our own galaxy has a large number of star forming sites that produce only low-mass stars. NGC 7129 is one of those:

Click to view full size image

Yes, there is star formation here, but no O stars are being produced. But certainly low-mass M stars will hatch out of this gas cloud.

My point is that we can't just assume that our galaxy has churned out a steady stream of O stars during its lifetime. Does that mean that we were incredibly lucky that the nebula that our Sun condensed out of almost five billion years ago was sufficiently metal-rich to produce the Earth and us humans? Yes and no. Yes, because the Sun could have been a lot more metal-poor, so that it might have been unable to produce the Earth and us. Did you know that the Pleiades, which are just so much younger than the Sun - a hundred million years compared with more than four billion years for the Sun - are nevertheless more metal-poor than the Sun? How can that be? My answer is that not all gas clouds in the Milky Way are the same. Some are strongly enriched by heavy elements due to a recent supernova, and others may be mostly made up of metal-poor gas that has been siphoned off an unlucky dwarf galaxy that passed us by. As for the solar system, it is entirely possible that it condensed out of a gas cloud that had been enriched with heavy elements by a recent supernova.

On the other hand, supernovae are not the only source of heavy elements in the galaxy. I know I have read somewhere that most of the iron in the galaxy has been made by non-exploding red giants during the final stage of their lives, when they puff and pant, expand and contract and set off all kinds of weird chemistry in their outer envelopes just before they turn into planetary nebulae. On the other hand, most of the oxygen in the galaxy was supposedly made by supernovae!

This has turned into a very long post, so I'll finish it now, before I say something about galactic plumes. But I'm afraid I don't have very much to say about those.

Ann

[Ann]

<< reference to spam post removed >>

Dough, you asked this:

Do you believe that all observable galaxies were created simultaneously within a very short period of cosmic time ?

No, I don't believe that all observable galaxies were created simultaneously within a very short period of cosmic time, but I do believe that the vast majority of at least moderately large galaxies - and that includes very many galaxies that are smaller than the Milky Way - formed their first stars a very long time ago, probably 10-13 billion years ago.

Take a look at the picture of NGC 1313 again:

Click to view full size image

Robert Gendler's image of NGC 1313 shows off its young blue star clusters and pink emission nebulae, which are lit up by even younger stars, very well. But please note the beige-colored, somewhat distorted "envelope" of diffuse light that surrounds the "blue and pink antics". What is this light, and why is it beige in color?

My answer is that this is the old, pre-starburst stellar population of NGC 1313. The beige color of it shows that it is mostly made up of late-type (K and M-type) low-mass stars and evenly distributed, only moderately-bright red giants. Such stars can easily be twelve billion years old or more. I believe, therefore, that NGC 1313 may well have been around for perhaps twelve billion years, but it is only very recently that it went into starburst mode.

Many other smallish starburst galaxies do in fact have an underlying old population of stars similar to the one we can see in Robert Gendler's picture of NGC 1313. An example is LMC-type starforming galaxy NGC 4449:

http://cosmo.nyu.edu/hogg/rc3/NGC_4449_UGC_7592_irg.jpg

This image shows red emission nebulae as green, and you can indeed see greenish blobs of emission nebulosity scattered over the face of the galaxy, particularly near the top of the image. But note in particular the large beige diffuse envelope of old stars surrounding this young-looking starburst galaxy. NGC 4449 has a sizable old population of low-mass stars, and like NGC 1313, it may well have been around for twelve billion years or so.

Another example is small starforming galaxy NGC 4214. Here the old smooth beige population is more centrally condensed:

http://cosmo.nyu.edu/hogg/rc3/NGC_4214_ ... 36_irg.jpg

Personally I find spiral galaxy NGC 4395 a lot more perplexing. Yes, there is a small old beige population in the center of the galaxy, but it is really quite puny. And there is a hint of green in some of the blue clusters, but that hint is quite faint:

http://cosmo.nyu.edu/hogg/rc3/NGC_4395_ ... 48_irg.jpg

To me, NGC 4395 almost looks like a single-generation galaxy! It is as if all the stars in this galaxy were just a hundred million years old or so, like the Pleiades. Really, that is not the case since NGC 4395 does contain a noticable old population in its center, and it may contain a tiny bit of ongoing star formation, too. But to me, this "almost-the-same-young-age-all-over" galactic look is weird.

But weirder still is edge-on galaxy NGC 4656:

http://cosmo.nyu.edu/hogg/rc3/NGC_4656_ ... 26_irg.jpg

This is not a small galaxy. It may be as luminous as the Milky Way. Personally I can't detect even a bit of yellow or beige in it. Can you? (Or maybe there is a tiny suggestion of yellow close to the center of it?) I can't see a hint of green in it, either. Could this be a true young single-generation largish galaxy, one that burst into existence just a few hundred million years ago and then shut off its star formation just as suddenly? Is that possible?

For comparison I'm going to show you large spiral galaxy M100:

http://cosmo.nyu.edu/hogg/rc3/NGC_4321_ ... 96_irg.jpg

Note the bright yellow center surrounded by a bright circumnuclear blue starburst. Note the yellow population outside this inner blue ring, and note the yellow population between the spiral arms. Note the beige "envelope" surrounding all of the visible galaxy. Note the blue arms with the green knots in them, which correspond to red emission nebulae. M100 is most definitely a multi-generation galaxy, and it has most definitely been around for billions of years.

So to summarize, I do believe that the vast majority of at least moderately large galaxies of today contain stars that are probably at least ten billion years old.

Ann

[Ann]

John, you asked:

Thinking outside the box, what is your opinion of the plumes mentioned in the description of NGC 2841 ? Your idea would naturally cover any flocculent type galaxy.

I haven't given galactic plumes much though. The way I see it, they are just the result of instabilities in the galactic disk. In view of how huge most galactic disks are, it is more remarkable that there are extremely regular spiral disks than that there are flocculent disks and disks with plumes, at least in my opinion.

Take a look again at the image of majestic spiral M100. There is a bit of plume structure between the arms, particularly at the upper part of the galaxy in this image:

http://cosmo.nyu.edu/hogg/rc3/NGC_4321_ ... 96_irg.jpg

Well, in my opinion, there is not a lot of plume structure in M100, although there is some of it. But what does it take to get rid of plume structure altogether? I don't know if that is possible, but I do feel sure that the most regular spiral galaxies have only a small amount of star formation.

Perhaps the most regular long-armed spiral galaxy that I know of is NGC 2857:

Click to view full size image

It looks quite amazing, doesn't it? But there is not a huge amount of star formation in it. The enormous forces released in huge starbursts and the subsequent supernovae can disrupt the shape of a galaxy pretty thoroughly.

There is in fact some plume structure in NGC 5827 after all. You can see some structures that connect the arms on the lower right.

What does it take to have a beautiful two-armed spiral in the first place? James D Wray, the man behind The Color Atlas of Galaxies, thought that a central bar structure helped. And there are certainly barred galaxies that are strikingly two-armed:

Click to view full size image

NGC 1300.

Click to view full size image

NGC 1365.

These are typically two-armed spirals, yet the arms break up in some plume structure, particularly in NGC 1365.

The presence of a bar clearly makes it easier for spiral arms to form at the bar ends, one spiral arm for each bar end, so that you get a two-armed spiral. Interestingly, many seemingly non-barred spirals do in fact have small bar structures in their centers, which facilitate the forming of two-armed spirals:

Click to view full size image

This infrared image of grand spiral M101 shows a small bar-like structure in the center. It is possible to trace several arm- or dust features as originating from the ends of this tiny bar. Of course, I wouldn't say that M101 is two-armed!

James D Wray said that a strong two-armed structure in a galaxy could probably be traced to the way the strong dust lanes of those two spiral arms originated in or near the nucleus of the galaxy. I think he may be right. It could be that, or the presence of a bar which makes it easier for arms to "attach" to that bar. It is interesting to think of the bar as a feature that rotates as a more or less rigid feature, stirring up the interstellar medium and setting up "wakes" and waves that may turn into long spiral arms that "turn in the wind" as the bar they are "attached to" keeps turning.

So why are some galaxies flocculent? I have absolutely no idea, but chances are that they are non-barred and don't have any interesting dust structures that penetrate deep into the nucleus. Perhaps they don't have a rotating black hole, or perhaps there is no dust close to the black hole. In the case of NGC 2841, there appears to be a large dust-free yellow bulge that "cuts off" the connection between the central black hole and the dust structures outside the bulge. The galaxy rotates, and the dust will form structures that reflect the rotation, but there will be no strong dust lanes or other forces that can channel much of that rotational energy in two mighty spiral arms.

Here is a picture of flocculent galaxy NGC 2976. Note the tiny nucleus, the large beige "elliptical galaxy"-like envelope, and the chaotic central dust structure with little spiral shape and a bit of star formation.

Click to view full size image

Ann

[dougettinger]

Doug, you asked this:

Do you believe that all observable galaxies were created simultaneously within a very short period of cosmic time ?

No, I don't believe that all observable galaxies were created simultaneously within a very short period of cosmic time, but I do believe that the vast majority of at least moderately large galaxies - and that includes very many galaxies that are smaller than the Milky Way - formed their first stars a very long time ago, probably 10-13 billion years ago.

Take a look at the picture of NGC 1313 again:

Click to view full size image

Robert Gendler's image of NGC 1313 shows off its young blue star clusters and pink emission nebulae, which are lit up by even younger stars, very well. But please note the beige-colored, somewhat distorted "envelope" of diffuse light that surrounds the "blue and pink antics". What is this light, and why is it beige in color?

My answer is that this is the old, pre-starburst stellar population of NGC 1313. The beige color of it shows that it is mostly made up of late-type (K and M-type) low-mass stars and evenly distributed, only moderately-bright red giants. Such stars can easily be twelve billion years old or more. I believe, therefore, that NGC 1313 may well have been around for perhaps twelve billion years, but it is only very recently that it went into starburst mode.

Many other smallish starburst galaxies do in fact have an underlying old population of stars similar to the one we can see in Robert Gendler's picture of NGC 1313. But note in particular the large beige diffuse envelope of old stars surrounding this young-looking starburst galaxy. NGC 4449 has a sizable old population of low-mass stars, and like NGC 1313, it may well have been around for twelve billion years or so.

So to summarize, I do that the vast majority of at least moderately large galaxies of today contain stars that are probably at least ten billion years old. Ann

Ann, you are describing the colors of various regions of stars as foretelling their age. I know that colors of stars reveal their temperatures. Also, colors in certain wavelengths reveal types of elements and molecules. A beige color region of stars may reveal that they are K and M-type stars, but how does this fact indicate their age ? These low mass stars may have been born one billion years ago or 10 billion years ago. I know that the ratio of Population I to Population II stars can predict the age of a star cluster or perhaps a galaxy; are you referring to this case ? Perhaps you are referring to the ratios of large size stars to mid size stars to small size stars. If there are no large or mid size stars in a region then the region is very old becuase it has exhausted all its opportunities to create larger-mass Population I stars.

Doug Ettinger, Pittsburgh, PA 02/22/2011

[dougettinger]

Hello Ann, thank you for expressing your wll illustrated ideas about flocculent galaxies and their plumes. You also made another quotable statement about barred galaxies. I liked your hypothesis about the absence of central bars in galaxies which prevents thorough mixing of interstellar materials and allows plumes to be stable, and may also be the possible reason for ring galaxies.

Allow me to give my idea of plumes seen in galaxies which corresponds with your idea. These plumes are the boundaries of the envelops of supernova remnants from super massive stars. The gravity waves normally created by a central bar of a galaxy do not exist to help mix and disperse these recognizable plumes. As an observer inside our own galaxy's disk we see through these remnants of supernovas and cannot visual their intersecting envelops; also, as you suggest starburst activity is not as active in our galaxy as is the case in these flocculent galaxies for whatever reasons.

Returning to barred galaxies, I believe they are created by two large collections of masses, probably two large black holes, that are rotating about each other similar to two equal-sized binary stars. In fact, this barred phenomenom probably creates spiral galaxies as you have postulated. Also, these bar-like central regions may be triggered by collisions or close encounters of two galaxies.

So this discussion leads to two more basic questions. In your opinion of which I value, which came first - elliptical or spiral galaxies? Is one derived from the other or are they separate phenomenom ?

Doug Ettinger, Pittsburgh, PA 02/22/2011

[Chris Peterson]

The gravity waves normally created by a central bar of a galaxy do not exist to help mix and disperse these recognizable plumes.

Gravity waves are too weak by many, many, many orders of magnitude to influence mass structures on any macroscopic scale.

[dougettinger]

Thank you for the correction. I really meant the influences of a rapidly moving gravity field (in cosmic time) and fluid dynamics.

Doug 02/22

[Ann]

Doug Ettinger wrote:

Ann, you are describing the colors of various regions of stars as foretelling their age. I know that colors of stars reveal their temperatures. Also, colors in certain wavelengths reveal types of elements and molecules. A beige color region of stars may reveal that they are K and M-type stars, but how does this fact indicate their age ? These low mass stars may have been born one billion years ago or 10 billion years ago.

It is not so much the presence of red dwarfs and red giants that shows that the stellar population in a galaxy, or in a part of a galaxy, is old. It is the absence of bluer stars that proves that stars are old.

Like you said, any galaxy at all will contain red dwarf stars. To the best of our knowledge, red dwarf stars are always created where stars are being formed at all. Compare this with the situation in our own solar system. We have one Sun, two gas giants, two ice giants, four rocky planets, a large number of minor planets, and a bewildering number of moons, plus an even greater number of rocks and rubble orbiting the Sun. In our solar system, the larger a body is, the more unusual it is, generally speaking. And the smaller it is, the more objects of a similar size exist in our solar system. Smaller bodies are more common than bigger ones, and this is true among the stars, too.

We keep being told that our Sun is an average star. That is not true at all! The average star in the Sun's neighbourhood is about half as massive than the Sun. And because the Sun is about twice as massive as the average star, it is many times as luminous as the average star. The nearest star apart from the Sun, Proxima Centauri, is a true little runt. I'm not sure how massive it is, and don't have the energy to try to find some information about it, but let's say that it weighs no more than 20%, or at best 30%, as much as the Sun. Okay, but do you know how faint it is? It is about ten thousand times fainter than the Sun, or rather, the Sun is about ten thousand times brighter than Proxima Centauri in visual light! That's pretty amazing! That means that it takes 10,000 stars like Proxima Centauri to shine as bright in visual light as just one star like the Sun!

All right, but now consider Rigel, the blue supergiant in Orion. Rigel shines about 60,000 times brighter than the Sun in visual light. That means that one star like Rigel shines as brightly as 600,000,000 stars like Proxima Centauri!

This is my point. Blue stars are never very numerous, but as long as they are main sequence stars (like the Sun) or giants (like Rigel) they are so bright that they outshine huge numbers of fainter and redder stars. That is why so many galaxies look bluish. It is not because they lack red dwarfs, because no galaxies lack red dwarfs. No, it is because they have a good or fair supply of bright blue stars, whose light outshines the light from the redder stars.

But the bluer and brighter a star is, the more massive it is. And the more massive it is, the faster it will evolve and die. Put simply, the blue stars don't last long, and the bluer they are, the faster they are going to die. On the other hand, the redder and fainter a star is, the less massive it is, and the less massive it is, the longer it will live. A typical red M-type dwarf is expected to live many times longer than the current age of the universe. Therefore, no red dwarf that has ever formed in our universe is supposed to have died, except perhaps through some incredibly weird accident (like being eaten by a black hole or something). But the blue stars always die quickly.

Take a look at NGC 1313 again:

Click to view full size image

NGC 1313 has undergone a recent massive starburst. A respectable number of highly luminous blue stars were formed during this starburst. I'm talking about stars like Theta1c Orionis, the O6 star that is ionizing the Orion Nebula. A larger number of not-so-luminous blue stars were also formed, stars like bright-looking B7 star Regulus, the alpha star in the constellation Leo. Many more blue-white A-type stars were formed, stars like Vega and Sirius. Still more white F-type stars were formed, stars like Procyon. Still more G-type stars were formed, like the Sun. Very large numbers of K-type stars were formed, like Epsilon Eridani. And huge numbers of M-type stars were formed, but they are so faint that they don't contribute much light, despite their prodigious numbers.

When a galaxy has undergone a recent starburst, it produces a lot of blue light, because of its highly luminous young blue stars. But it doesn't take long until the blue stars begin to die (and before they die they turn red, or at least yellow-orange, too. Famous red supergiant Betelgeuse was originally a blue star.)

As the bluest stars die, the slightly less blue stars hang on a bit longer - I'm talking about stars like Vega and Sirius. An O star can live no more than ten or, at most, twenty million years, but an A star like Vega or Sirius can live for 300 million years or more. But no A star can last for a billion years. After a billion years, all the Vegas and Siriuses that were produced in the starburst are gone. Some F stars are left, but after two or three billion years the F stars are gone, too. After five to ten billion years the G stars are gone. So, after ten billion years, what is left of the original starburst? Well, all the K stars are left. They are faint, and much fainter than the Sun, but at least they are not as ridiculously faint as the M dwarfs. They do produce a bit of orange light. But also a lot of G stars have turned into red giants, and they are quite bright, though not as bright as the brightest blue stars. Still, they can shine a few hundred times as bright as the Sun shines today - and they can do that even though they don't necessarily contain more mass than the Sun. They are therefore very bright for their masses. Red giants don't last very long, but as long as there is a good supply of G-type stars like the Sun, there is a good supply of stars that can turn into red giants. The Sun is destined to end its days as a red giant.

Take alook at the picture of NGC 1313 again. This galaxy contains a lot of bright blue and pink regions. The light output from the blue regions is dominated by not very large numbers of highly luminous blue stars. There are white, yellow, orange and red stars there too, but they are so faint compared with the bright blue stars that the total light ouput from these "starburst regions" is blue.

But the blue stars don't last long. Bright blue stars are always young. A part of a galaxy that appears to shine with blue light contains young stars. That is always true.

Similarly, a galaxy that is all yellow does not contain any bright O stars or any bright B stars. It may possibly contain very small numbers of A-type blue straggler stars, but these stars will be so few in number and so scattered that they will not contribute noticeably to the light output of any part of the galaxy. A galaxy that shines with an all yellow light is always old. Admittedly it is not possible to say just how old it is just because it is yellow. A galaxy dominated by eight-billion-year-old stars will be as yellow as a galaxy dominated by ten- to twelve-billion-year-old stars. But in any case, the stars of an all-yellow galaxy (or an all-yellow part of a galaxy) will be many billion years old.

Take a look at NGC 1313 again. Look at the beige "envelope" of stars surrounding the bright blue and pink regions. Beige is a shade of yellow - you can say that it is a very faint shade of yellow, sort of mixed with a bit of black. Therefore, the stars in this beige envelope are basically all yellow. There are not huge numbers of them, which is why their light is faint. Still, the fact that their light is beige proves that they are old, many billions of years old. Because if there were younger stars among them, these younger stars would shine relatively brightly with a considerably whiter light, which would affect the overall color of this part of the galaxy. And if they were even younger stars there, the overall light would be bluish.

Take a look at this picture of galaxy M106 - and the picture is so large that I can only post it as a link:

http://images.astronet.ru/pubd/2009/05/ ... KC2048.jpg

The overall color balance here is too bluish, which means that there are parts of this galaxy which are really quite yellow, but which don't look very yellow here.

Anyway, you can see that the very center of the galaxy is bright yellow. That is because there are truly huge numbers of old reddish and yellow stars here. The stars here are typically M dwarfs, K dwarfs, a few G dwarfs like the Sun, and a relatively large population of red giants. No stars have formed here for billions of years.

Outside the very center is the bulge, which is really quite yellow, although it doesn't look that way here. There are few if any young stars here. But the stars are not as densely packed in the bulge as they are in the very center, so the light from the bulge is fainter. (There are red outflows of ionized hydrogen from the center, which may be slightly similar to the gigantic outflows from the center of nearby relatively small galaxy M82.)

The bulge of M106 appears to be "encircled" by two arms, which contain quite a bit of star formation. You can see young blue star clusters here, and a few bright pink emission nebulae. But I want you to pay attention to a stretch of almost aqua-color light at a position of about one o'clock, below and to the left of a string of pink emission nebulae. This stretch of aqua light is a post-starburst region. All the brightest, bluest stars that formed in this starburst have died, but all the A stars are still left. In combination with the red giants that have formed out of what used to B-stars, this part of M106 shines with an aqua-colored light.

Finally, take a look at the very blue outer arms. Why are they so blue? I'd say that the reason is that there are not very many stars out there, and therefore there aren't very many red giants out there. Therefore the bright blue stars that have formed out there don't have a lot of "competition". Therefore the blue light from a few highly luminous stars dominate the outer arms completely.

So in short, this is how you can tell from the color of a galaxy, or from the color of a part of a galaxy, how old the stars of it are. If the stellar population is all yellow, then the lack of blue stars means that the stars are invariably old.

Ann

[dougettinger]

Ann, thank you for your very descriptive explanation of the colors and mixture of colors and over-powering brightnesses found in various parts of galaxies. So summarizing, the comparative oldest age of part of a galaxy reveals its overall age, although it is hard to nail-down the precise number of billions of years.

Please checkout my previous response to your answer on plumes. I will be asking more about the pants and puffs of red giants and O-type stars later.

How did you access images at cosmo.nyu.edu website ? You know your galaxies very well. Thanks again for sharing.

Very excited about O-type stars,

Doug

[Ann]

Doug, I'll try to answer some of your questions about about bars and elliptical galaxies. I'm afraid I don't have much to say about plumes. Please note that what I say about bars and elliptical galaxies is strictly speculations on my part.

You asked:

In your opinion of which I value, which came first - elliptical or spiral galaxies? Is one derived from the other or are they separate phenomenom ?

I discussed the number of elliptical galaxies versus the number of spiral and irregular galaxies with Chris Peterson here some time ago, and Chris had access to information according to which only 30% of galaxies are elliptical. That surprised me. I had thought that elliptical galaxies constituted the majority of galaxies in the universe.

Interestingly, I'm not absolutely sure whether that figure, 30%, represents all the galaxies whose type can be determined in the observable universe, or if it refers to relatively nearby galaxies (say, galaxies within a billion light-years or so). Interestingly, I'm sure that elliptical galaxies were more uncommon in the past than they are now. When Hubble looks back in time at more and more distant galaxies, it finds fewer and fewer ellipticals and more and more irregular galaxies.

So ellipticals were more uncommon in the distant past. But ellipticals did exist in the very distant past, nevertheless. How did the earliest ellipticals form? What separates them from the galaxies that turned into spirals? Why have the elliptical galaxies grown more and more common over time? Why do they still apparently constitute a minority of the total number of galaxies?

I think the "original ellipticals" may have started out as ultra-luminous infrared galaxies:

Click to view full size image

Ultra-luminous infrared galaxies are, of course, absolutely extremely luminous in the infrared. These galaxies are undergoing incredibly violent starbursts. These starbursts create huge amounts of dust, which may all but shroud the galaxies in heavy dust clouds, preventing our view of them in visible light. The thing to remember is that they are undergoing absolutely extreme starbursts.

But such totally extreme starbursts will use up the galaxy's available gas supply quite efficiently. Much of the gas that is left can be driven out of the galaxy by the fierce stellar winds of abnormal numbers of O stars, and then these O stars will set off incredible numbers of supernovae that go off pretty much simultaneously. This, too, will drive a lot of gas out of the galaxy. In short, incredibly violent starbursts may rob a galaxy of nearly all its available gas. And when the gas is gone, the galaxy can't make any new stars.

The way I see it, you need two things to make an elliptical galaxy. You need to drive the gas away and rob the galaxy of its starmaking ability. But you also need to put the stars in the galaxy in chaotic orbits in order to give the galaxy its well-known elliptical shape. Personally I believe that a violent enough primordial starburst may create enough havoc in a galaxy to ruin any flat disk and throw the stars into chaotic orbits. Otherwise, though, I think violent collisions and messy galactic mergers may be the best way to throw the stars of a galaxy into chaotic orbits. Such merges can set off violent starbursts, too, and use up most of the gas supply of the of the galaxy and set off fireworks of supernovae that can drive out most of the rest of the gas. So I think that ultra-luminous starbursts and early messy mergers created the original ellipticals. But mergers keep happening, and they may create new elliptical galaxies out of previous spiral ones, although I am certain that mergers can just create larger spiral galaxies, too. But very often mergers both use up and expel much of the gas of the constituent galaxies. Therefore, over time, as you have more and more galactic mergers, you will get more and more ellipticals and also more and more large spirals with less and less star formation.

When a galaxy is inside a large cluster it will interact with other galaxies in such a way that most of the individual galaxies have their gas driven out of them. This is a typical large cluster of galaxies, the Coma Cluster, which is full of yellow ellipticals:

Click to view full size image

The color balance in this picture is slightly yellow. Let's take a look, however, at another picture of this galaxy cluster where the overall color balance of the picture is excessively blue:

Click to view full size image

Here a number of the galaxies in the cluster look blue, but as I said, the color balance of the picture is excessivley blue. You shouldn't believe in all the blue things you see here. The sharply blue foreground stars are probably white-to-yellowish stars of, say, late class G, slightly yellower than our Sun. (Edit: I just checked them with my software. They are mid F-type main sequence stars, a bit bluer than the Sun.)

But take a look at a prominent spiral in the lower left part of the image. That spiral is part of the Coma cluster, and it appears to have blue spiral arms that are good at forming stars, doesn't it?

Well, it may look that way in this excessively blue image, but the truth about that spiral galaxy is another:

Click to view full size image

This is a Hubble Space Telescope image of that spiral galaxy in the coma Cluster, NGC 4921. The Hubble Telescope often takes pictures that aren't very good at revealing the overall color of a galaxy, and this looks like a picture that is a combination of exposures through just two filters, probably a blue and an infrared filter. Or if we are unlucky, the Hubble people may have used an orange and an infrared filter! This creates a skewed and muted color balance here. Nevertheless, you can see how the color of this galaxy is pretty much "the same all over". Hubble exaggerates this effect, because the arms really are bluer than the central part of the galaxy, but it is nevertheless true that there is very little star formation and also very little gas and dust in NGC 4921. The outer arms are extremely smooth, and they are about to lose their identity as individual arms, too. They are about to merge with one another and form one smooth outer featureless disk. Eventualy, this outer featureless disk will get thicker, too, and it is possible that the galaxy could slowly become more elliptical "by itself", even if it undergoes no mergers.

Compare the appearance of the Coma Cluster with the appearance of the Hercules Cluster:

Click to view full size image

The Hercules cluster is clearly younger than the Coma Cluster. There are several blue spiral galaxies here, many of them in states of mergers. Note, too, that some of the all-yellow galaxies are also merging. These merging yellow galaxies have "thrown out" long yellow tidal tails entirely composed of old yellow stars. These yellow merging galaxies have used up all their gas long ago, but they are still undergoing new mergers.

It is definitely true that dense galactic clusters inhibit star formation and tend to turn all its constituent members into more and more elliptical-like galaxies. It is also true that mergers favor the creation of elliptical galaxies, so where there are many mergers, you may eventually end up with an all-yellow population of ellipticals.

My point is that the greatest difference between ellipticals and spirals is that spirals almost always have a gas supply and at least some star formation, and they have a flat disk with some spiral structure. Ellipticals lack gas and star formation, and they also lack a flat disk. But there are some dwarf ellipticals which may indeed have some gas and a bit of star formation. And spiral galaxies can definitely turn into elliptical galaxies through messy mergers, but I think it is much, much harder (or perhpas impossible) for an elliptical galaxy to "change back" into a spiral. When it comes to the really monstrously large elliptials, they have monstrous black holes in their centers, and these can have violent outbursts which in themselves inhibit star formation. Therefore, giving an elliptical galaxy a new gas supply may not lead to the creation of new stars, but rather to chatoically whirling gas clouds in the galaxy and possibly to more outburst from the black hole in the center of the galaxy, if it gets to swallow some of this new gas.

Hmm, my posts tend ot be long. Let's see if I can find the time to write something about galactic bars in another post.

Ann

Doug, I didn't really asnwer your question about which type of galaxy came first, spiral or elliptical. I think that neither may have come first, but that the irregular galaxies came first. Then the irregular galaxies evolved into either spirals or ellipticals. On the other hand, astronomers have really found extremely early ellipticals, so some ellipticals must have been created very early on, probably after having undergone a monstrously large starburst and spent some time as ultra-luminous infrared galaxies.

[Ann]

Let me say a few things about galactic bars, too.

If you ask my completely non-professional opinion, the key to bars is Kepler's law about the elliptical orbits of planets around stars. Kepler said that the planets in our solar system follow elliptical orbits around the Sun. But this seems to be a universal law about the shape of orbits: they are basically always elliptical.

I think we can envision galactic bulges as enormously enlarged versions of our solar system. The central supermassive black hole at the center of large galaxies corresponds to the Sun at the center of our solar system. The billions of stars that orbit the black hole in the bulges of large galaxies correspond to the planets that orbit the Sun in our solar system. And like the planets in our solar system, the billions of stars in galactic bulges follow elliptical orbits around the central mass concentration of their galaxies.

How do bars form? This is what I think. I think that the stars in bulges have synchronized their orbits around the central black hole in their galaxies. The Hubble Space Telescope has shown that barred galaxies were less common in the past. If I have understood things correctly, bars appear to become progressively more common over time. If we imagine that the synchronization of elliptical stellar orbits around a central black hole is a process that takes time, then it makes sense that bars form gradually, and that they become more common over time.

If I have understood things correctly, bars rotate like a rigid body. I think they tend to sweep the area they cover as they rotate free of gas and dust, so that there is generally no star formation in or along bars. However, there are often two dust lanes that emerge from the center of barred spiral galaxies and which follow the bars more or less "straight outwards" until these dust lanes often merge with the spiral arms that begin at the ends of the bar. In fact, in several barred galaxies it may well be these dust lanes in the bulge that rotate like rigid bodies. These bars, or the dust lanes associated with the bars, tend to "sweep up gas" like a snowplow and deposit a lot of it at the ends of the bar. Typically, barred galaxies often have an enhanced rate of star formation at the ends of their bars:

Click to view full size image

NGC 1300 is a barred galaxy with a very long bar. It has two dust lanes that emerge from an inner ring, and it has enhanced star formation at the ends of its bar.

This is a hugely interesting galaxy, NGC 4314:

Click to view full size image

As you can see, this galaxy is almost all yellow. Star formation has ceased long ago everywhere in the galaxy, except in a small circumnuclear ring! I believe that bar ends sometimes sweep up and concentrate gas not only at the outer ends of its bar, but also at the inner ends of its bar.

You can see the central bar structure, which is quite bright and obvious. You can see two well-formed but all-yellow and dust-free spiral arms that emerge form the outer ends of the bar. You can also see a faint yellow disk between the bar and the arms. I would say that the strong bar structure in NGC 4314 has played a hugely important role in shaping the overall morphology of NGC 4314.

I don't want you to think that barred galaxies typically have only a small amount of star formation. Therefore I want you to take a look at barred and richly star forming galaxy M61:

Click to view full size image

Ann

P.S. Doug, you said:

Returning to barred galaxies, I believe they are created by two large collections of masses, probably two large black holes, that are rotating about each other similar to two equal-sized binary stars. In fact, this barred phenomenom probably creates spiral galaxies as you have postulated. Also, these bar-like central regions may be triggered by collisions or close encounters of two galaxies.

It is an interesting thought that bars could get started when two black holes rotate around each other in the center of a galaxy. Yes, that sounds like a definite possibility.

[BMAONE23]

snip

Click to view full size image

This is a Hubble Space Telescope image of that spiral galaxy in the coma Cluster, NGC 4921. The Hubble Telescope often takes pictures that aren't very good at revealing the overall color of a galaxy, and this looks like a picture that is a combination of exposures through just two filters, probably a blue and an infrared filter. Or if we are unlucky, the Hubble people may have used an orange and an infrared filter! This creates a skewed and muted color balance here. Nevertheless, you can see how the color of this galaxy is pretty much "the same all over". Hubble exaggerates this effect, because the arms really are bluer than the central part of the galaxy, but it is nevertheless true that there is very little star formation and also very little gas and dust in NGC 4921. The outer arms are extremely smooth, and they are about to lose their identity as individual arms, too. They are about to merge with one another and form one smooth outer featureless disk. Eventualy, this outer featureless disk will get thicker, too, and it is possible that the galaxy could slowly become more elliptical "by itself", even if it undergoes no mergers.

snip
Ann

Ann,
Looking closely at the Hubble image, I notice that there are numerous other galaxies in the background that are similarly generally white all over. The ghostly appearance of NGC4921 may have been unintentionally created by the filters used to produce the image. The only colors that my eyes discern are white and shades of muddy yellow.

In this zoomable image you will note that even some of the closer ellypticals are white in color and the majority of the more distant reddened objects are displayed as a muddy yellow. The zoomed image is really something though as there are a number of irregular galaxies in the background along the left side of the image.

[BMAONE23]

I just noticed, in the lower left corner of the zoomable image, there is an "S" shapped galaxy with a diffraction spike centered on it's core. Does anyone know if this is a chance alignment with a local star, a chance capture of a distant supernova, or a highly unlikely stitching artifact?

[rstevenson]

I just noticed, in the lower left corner of the zoomable image, there is an "S" shapped galaxy with a diffraction spike centered on it's core. Does anyone know if this is a chance alignment with a local star, a chance capture of a distant supernova, or a highly unlikely stitching artifact?

The few other diffraction spikes are 8-pointed, but that one is a four-pointed one. I'm not sure what conclusions can be drawn from that observation, however.

Rob

[BMAONE23]

I just noticed, in the lower left corner of the zoomable image, there is an "S" shapped galaxy with a diffraction spike centered on it's core. Does anyone know if this is a chance alignment with a local star, a chance capture of a distant supernova, or a highly unlikely stitching artifact?

The few other diffraction spikes are 8-pointed, but that one is a four-pointed one. I'm not sure what conclusions can be drawn from that observation, however.

Rob

The two yellower stars at the top do show 8 spikes while the white star near the "S" shaped galaxy only shows 4. It could be something that only shows up with one specific filter but not the other of the two that were used while the 2 yellower stars show in both filters giving 8 spikes when combined

[Ann]

BMAONE23 wrote:

Ann,
Looking closely at the Hubble image, I notice that there are numerous other galaxies in the background that are similarly generally white all over. The ghostly appearance of NGC4921 may have been unintentionally created by the filters used to produce the image. The only colors that my eyes discern are white and shades of muddy yellow.

I just found some information about the picture and the filters used on ESA's homepage. This is what it says:

This very deep image taken with the NASA/ESA Hubble Space Telescope shows the spiral galaxy NGC 4921 along with a spectacular backdrop of more distant galaxies. It was created from a total of 80 separate pictures through yellow and near-infrared filters.

I know that near-infrared isn't a visual color. Anyway, my point is that all galaxies, spirals and ellipticals alike, emit a lot of yellow and near-infrared light, so yellow and infrared is a bad choice of filters if you want to bring out a galaxy's color properties.

It is true, nevertheless, that NGC 4921 is bluer than the two giant Coma Cluster ellipticals, NGC 4884 and NGC 4874. For NGC 4921 the color indexes are, U-B = 0.390 and B-V = 0.870. For NGC 4884 the values are, U-B = 0.520 and B-V is 1.040. The values of NGC 4874 are similar to those of NGC 4884.

So there is no doubt that the disk of NGC 4921 contain a younger population of stars than what you can find in the giant Coma Cluster ellipticals. There are even a few young clusters in NGC 4921. But by and large, OB stars are extremely rare in this galaxy, and the light of the disk of NGC 4921 is probably dominated by A- and F-type stars.

Ann

[Ann]

I just noticed, in the lower left corner of the zoomable image, there is an "S" shapped galaxy with a diffraction spike centered on it's core. Does anyone know if this is a chance alignment with a local star, a chance capture of a distant supernova, or a highly unlikely stitching artifact?

Thanks for the zoomable image, BMAONE23! Very nice. As for the S-shaped galaxy with the diffraction spike centered on its core, it might possibly be a galaxy with an active and highly luminous, starlike core. Such galaxies exist.

Ann

[Ann]

Doug, I haven't responded to your idea about plumes, so I will try to do that now:

Allow me to give my idea of plumes seen in galaxies which corresponds with your idea. These plumes are the boundaries of the envelops of supernova remnants from super massive stars. The gravity waves normally created by a central bar of a galaxy do not exist to help mix and disperse these recognizable plumes. As an observer inside our own galaxy's disk we see through these remnants of supernovas and cannot visual their intersecting envelops; also, as you suggest starburst activity is not as active in our galaxy as is the case in these flocculent galaxies for whatever reasons.

I agree with you that supernovae explosions create all sorts of havoc in galaxies. They may certainly pile up a lot of gas and dust and create noticable dust structures. To me (and to James D. Wray, from whom I've got my idea about plumes) plumes are, however, longer, preferentially relatively straight structures that don't follow the curvature of the arms, and I doubt that supernovae generally create such very long and oblong features. So I sometimes feel that you and I don't talk about the same thing when we talk about plumes.

But indeed, supernovae can create a lot of dust. I have just talked about ultra-luminous infrared galaxies. The copious amount of dust in them is supposed to have been created by supernovae. So I must definitely agree with you that various dust structures in a galaxy may have been created by supernovae.

You also said that starburst activity is not as active in our galaxy as it often is in flocculent galaxies. You are certainly right that the Milky Way is not a starburst galaxy by any means, but that doesn't mean that it doesn't have a respectable amount of star formation. We have a very unfavorable view of our galaxy - it's like sitting inside an opaque table top and trying to see what is on the table - but there are several ambitious attempts to map our galaxy and determine its properties. One such study, GLIMPSE - I think the acronym refers to an infrared study of the mid-plane of our galaxy - found many more star forming sites than the researchers had expected. So I don't think it is right to regard the Milky Way as a galaxy that is poor in star formation.

Also, I don't think it is true that flocculent galaxies are generally very rich in star formation. If anything, I think the situation may be the opposite. NGC 2841, the flocculent galaxy that started off all these discussions, isn't forming very many stars at all. I'm sure it is less proficient at making new stars than the Milky Way is. The color indexes of this galaxy is B-V = 0.870 and U-B = 0.340. That is not particularly blue, especially not the U-B value which is particularly sensitive to the presence of hot blue stars. In the case of NGC 2841, we can actually be sure that dust structures inside it are not hiding a lot of star formation. The reason for that is the galaxy's infrared magnitude. A galaxy that contains a lot of dust-shrouded star formation will be very bright in the far infrared. But in the case of NGC 2841, it is actually fainter in the far infrared than in blue light. This strongly suggests that its rate of star formation is low.

This is another flocculent galaxy, NGC 2613:

Click to view full size image

This galaxy is situated at very low galactic latitude, and we see it through a veil of foreground stars, which reddens the galaxy. My impresion is that this galaxy is slightly bluer than NGC 2841 and has more and slightly larger star forming sites. You can actually see a row of star forming sites at the upper "rim" of the galaxy near the bluest foreground star. NGC 2613 is also slightly brighter in the far infrared than in blue light, which suggests a much higher rate of star formation than what is present in NGC 2841. However, NGC 2613 is more inclined to us than NGC 2841, which means that more of the galaxy's light will be filtered through its dust, which in itself will make the galaxy brighter in the far infrared. I think that NGC 2613 probably forms fewer stars than the Milky Way, but that it is better at star formation than NGC 2841.

Generally speaking, though, flocculent galaxies are not very efficient at star formation. This is another relatively flocculent galaxy, Messier 88, which is not vigorously starforming:

http://thebigfoto.com/wp-content/upload ... ier-88.jpg

The best-known of all flocculent galaxies is M63, the "Sunflower Galaxy":

Click to view full size image

The color balance here appears to be pretty good. The bright foreground star is just a bit bluer than the Sun, which is exactly what it looks like in the picture. Anyway, M63 is clearly forming stars relatively efficiently. It's U-B value is 0.030, much bluer than any of the other flocculent galaxies we have looked at, and its infrared magnitude is considerably brighter than its blue magnitude. M63 proves that flocculent galaxies can form a lot of stars. Nevertheless, M63 is absolutely no starburst galaxy, and it can't compete with the starforming vigor of many of the starforming non-flocculent spirals.

Ann

[Ann]

I'll shut up very shortly, but Doug, you asked me about red giant stars that puff and pant and enrich the interstellar medium with heavy elements. A textbook example of a red giant that puffs and pants on the brink of death is Mira, a red giant that is not going to end its days as a supernova, but as a planetary nebula and a white dwarf. Here are excerpts of what Professor Emeritus Jim Kaler says about Mira (the full text is here: http://stars.astro.illinois.edu/sow/mira.html):

Mira, a class M7 red giant 420 light years away, is the brightest and nearest of the red class M "long period variables," thousands of which are known. The star varies from about third magnitude (though sometimes it can reach second) all the way down to tenth, 40 or so times fainter than the human eye can see alone, and then back again over a 330 day period (the spectral class varying as well between M5 and M9 coolest when faintest).

So since Mira changes from third magnitude down to tenth, it is certainly highly variable.

The light variations are caused by pulsation, changes in size that also affect the star's temperature and thus the amount of light that leaks out at visual wavelengths (the infrared variation nowhere near so large).

The star not only pulsates in light output, it also changes in size.

These long period variables help enrich the interstellar gases, out of which new stars condense, with dust and chemical elements formed in their nuclear cauldrons. Most of the carbon in the Universe seems to have come from them.

Jim Kaler doesn't say that most of the iron in our galaxy has come from red giants like Mira, but I'm sure I have read something like that some time. I read the Wikipedia article about supernova nucleosynthesis, and it said this about nucleosynthesis in stars that don't explode as supernovae:

A neutron capture process known as the s process which also occurs during stellar nucleosynthesis can create elements up to bismuth with an atomic mass of approximately 209. However, the s process occurs primarily in low-mass stars that evolve more slowly.

Those low-mass stars in which the s-process occurs are certainly red giants like Mira.

The Wikipedia article about the S-process says:

The S-process produces approximately half of the isotopes of the elements heavier than iron, and therefore plays an important role in the galactic chemical evolution.

Ann

[dougettinger]

Ann, thank you for your very descriptive explanation of the colors and mixture of colors and over-powering brightnesses found in various parts of galaxies. So summarizing, the comparative oldest age of part of a galaxy reveals its overall age, although it is hard to nail-down the precise number of billions of years.

Please checkout my previous response to your answer on plumes. I will be asking more about the pants and puffs of red giants and O-type stars later.

How did you access images at cosmo.nyu.edu website ? You know your galaxies very well. Thanks again for sharing.

Ann, thank you for supplying your overall hypothesis for the creation of different galaxies. I am very impressed. I was away for two weeks studying atolls and have now once again returned to the forum. You have postulated that after spiral galaxies become quiet they turn into elliptical galaxies, and displayed an example of this possible occurrence. I suppose as the orbits become more compressed that perturbations throw many stars outside the disk plane creating the elliptical/spherical shapes. Your ideas about galaxy clusters was fantastic. Do astronomers have actual evidence of black holes inside elliptical galaxies ?

Doug Ettinger, Pittsburgh, PA 3/19/2011