APOD: Elliptical M60, Spiral NGC 4647 (2012 Sep 14)

[APOD Robot]

Elliptical M60, Spiral NGC 4647

Explanation: Giant elliptical galaxy M60 and spiral galaxy NGC 4647 do look like an odd couple in this sharp cosmic portrait from the Hubble Space Telescope. But they are found in a region of space where galaxies tend to gather, on the eastern side of the nearby Virgo Galaxy Cluster. About 54 million light-years distant, bright M60's simpler egg-like shape is created by its randomly swarming older stars, while NGC 4647's young blue stars, gas and dust are organized into winding arms rotating in a flattened disk. Spiral NGC 4647 is estimated to be more distant than M60, some 63 million light-years away. Also known as Arp 116, the pair of galaxies may be on the verge of a significant gravitational encounter, though. M60 (aka NGC 4649) is about 120,000 light-years across. The smaller NGC 4647 spans around 90,000 light-years, about the size of our own Milky Way.

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[/b]

[bystander]

http://asterisk.apod.com/viewtopic.php?t=29493
http://asterisk.apod.com/viewtopic.php?p=183079#p183079

[Ann]

It is of course a great image, and there can't be many other portraits of this galaxy pair that show the same resolution.

Nevertheless, a lot of color information has been needlessly lost. According to this Hubble Heritage page, four different filters have been used to create this image: one blue filter at 475W, one green one at 555W, and two infrared ones at 814W and 850LP. I question the decision to use two infrared filters, but I must point out that a lot of interesting color information could have been extracted if the image taken through the green filter had been shown as green. As it is, apparently both the blue and the green filter images were colored blue, while the infrared filter images were shown as red. This means that a lot of color information was lost, and the colors of the picture look "flat".

Ann

[bystander]

The image is a composite of two separate images, an ACS/WFC image of M60 (using two filters) and a WFPC2 image of NGC 4647 (using two filters).

[J Johnson]

How can the stars gravitationally swarm in an elliptical galaxy? Wouldn't they tend to all fall in toward the center after a while, just as gravity pulls the dust in a dust cloud into a whirling star-forming center?

Do ellipticals have an axis of rotation about which all the stars rotate, or are the stellar trajectories just random?

[Sinan İpek]

Why almost all galaxies are about the same size? (Around 100,000 light years in diameter.)

[nstahl]

How can the stars gravitationally swarm in an elliptical galaxy? Wouldn't they tend to all fall in toward the center after a while, just as gravity pulls the dust in a dust cloud into a whirling star-forming center?

Do ellipticals have an axis of rotation about which all the stars rotate, or are the stellar trajectories just random?

I've pondered and asked similar questions myself. The pertinent difference between a "cloud" of stars and a cloud of dust is that the stars only very rarely collide. It's counter-intuitive to me, too, but elliptical clusters and elliptical galaxies obviously survive for billions of years.

An elliptical galaxy must have a theoretical axis of rotation, which you could calculate if you knew all the trajectories and had a big fast computer, but it doesn't seem to be obvious observationally and I too wonder how it affects the dynamics of the galaxy or beyond; if you knew that axis is there anything physically important you could predict? The stars orbit around the center of mass of the galaxy, as is the case with spiral galaxies, but with spirals far the biggest component of velocity is around the axis of rotation. But stars cross the plane of the spiral twice per revolution like planets cross the plane of the solar system. Of course spirals don't rotate like a solid body; the stars far out take many times as long to orbit the center as the stars close in; again like the solar system.

[starsurfer]

It is of course a great image, and there can't be many other portraits of this galaxy pair that show the same resolution.

Nevertheless, a lot of color information has been needlessly lost. According to this Hubble Heritage page, four different filters have been used to create this image: one blue filter at 475W, one green one at 555W, and two infrared ones at 814W and 850LP. I question the decision to use two infrared filters, but I must point out that a lot of interesting color information could have been extracted if the image taken through the green filter had been shown as green. As it is, apparently both the blue and the green filter images were colored blue, while the infrared filter images were shown as red. This means that a lot of color information was lost, and the colors of the picture look "flat".

Ann

I heartily agree with you Ann! While many professional images have stunning resolution, a few suffer from a lack of colour information due to the number of filters used or assignment of colours. There is a nice amateur image of M60 by the Capella Observatory team that I feel shows the colours better: http://www.capella-observatory.com/Imag ... es/M60.htm

[Ann]

I would like to recommend this picture of M60 and NGC 4647 by Adam Block. The colors are superb. You can see a number of small pink emission nebulae in NGC 4647, and you can see that the bright nucleus of NGC 4647 appears brilliantly blue-white. You can also see a large number of faint whitish points of light that might be globular clusters belonging to giant elliptical galaxy M60.

Ann

[neufer]

Why almost all galaxies are about the same size? (Around 100,000 light years in diameter.)

One answer might be that dwarf galaxies don't get much noticed (unless they are really nearby):

<<A dwarf galaxy is a small galaxy composed of up to several billion stars, a small number compared to our own Milky Way's 200–400 billion stars. The Milky Way has more than 20 known dwarf galaxies orbiting it, and recent observations have also led astronomers to believe the largest globular cluster in the Milky Way, Omega Centauri, is in fact the core of a dwarf galaxy with a black hole in its center, which was at some time absorbed by the Milky Way.>>

Another answer might be that back when galaxies first started to form a few billion years after the Big Bang the average spacing between galaxies was probably only about 200,000 light years so that there simply wasn't room for galaxies much larger than about 200,000 light years.

[Chris Peterson]

How can the stars gravitationally swarm in an elliptical galaxy? Wouldn't they tend to all fall in toward the center after a while, just as gravity pulls the dust in a dust cloud into a whirling star-forming center?

The stars are in individual orbits around the common center of mass of the galaxy. There is no mechanism for them to fall in towards the center. In a dense dust cloud, you have fluid dynamic mechanisms: the dust interacts with itself, through collisions and electromagnetic effects, resulting in the transfer of angular momentum and in the dissipation of energy, which serve to reduce the size of dust orbits and steadily bring material in towards the center. A galaxy, however, is essentially empty space. The only way stars can interact with each other is gravitationally, and such interactions are as likely to push a star in to a larger orbit as a smaller one.

Do ellipticals have an axis of rotation about which all the stars rotate, or are the stellar trajectories just random?

They are essentially random, although you'd expect a statistical bias, which would give the entire galaxy some net, non-zero angular momentum.

[Sinan İpek]

Another answer might be that back when galaxies first started to form a few billion years after the Big Bang the average spacing between galaxies was probably only about 200,000 light years so that there simply wasn't room for galaxies much larger than about 200,000 light years.

Is the estimated average size of the universe (i.e. average galaxy size * number of all galaxies in the universe) consistent with the estimated start of galaxy formation process?

[Chris Peterson]

I heartily agree with you Ann! While many professional images have stunning resolution, a few suffer from a lack of colour information due to the number of filters used or assignment of colours.

"Color" is rarely of scientific interest or value. In fact, most images made with a pair of filters contain more information than images made with broad RGB filters. The latter will present an image that approximates what the eye might see were it more sensitive, but that type of color tends to be very ambiguous with respect to the mechanisms that produce it. Professional images are very commonly made using just two filters, because the intensity ratio between properly chosen bands provides critical information about physical mechanisms. Sometimes more filters are used, of course, but given the value of observational time on large instruments, the minimum number of filters necessary to extract the intended information will normally be used.

[neufer]

[b][color=#0000FF]Black hole mass plotted against velocity dispersion of stars in the galaxy bulge. The M-\sigma relation is shown in blue.[/color] (Note: NGC 4486 = [url=http://en.wikipedia.org/wiki/Messier_87]M87[/url])[/b]

---

<<The M-sigma (or M-σ) relation is an empirical correlation between the stellar velocity dispersion σ of a galaxy bulge and the mass M of the supermassive black hole at the galaxy's center. The M-σ relation was first presented in 1999 during a conference at the Institut d'astrophysique de Paris in France. Publication of the relation in a refereed journal, by two groups, took place the following year. One recent study, based on a complete sample of published black hole masses in nearby galaxies, gives



Earlier work had demonstrated a possible relationship between galaxy luminosity and black hole mass, but that relationship had a large scatter. The much smaller scatter of the M-σ relation is generally interpreted to imply some source of mechanical feedback between the growth of supermassive black holes and the growth of galaxy bulges, although the source of this feedback is still uncertain.

Discovery of the M-σ relation was taken by many astronomers to imply that supermassive black holes are fundamental components of galaxies. Prior to about 2000, the main concern had been the simple detection of black holes, while afterward the interest changed to understanding the role of supermassive black holes as a critical component of galaxies. This led to the main uses of the relation to estimate black hole masses in galaxies that are too distant for direct mass measurements to be made, and to assay the overall black hole content of the Universe.

The tightness of the M-σ relation suggests that some kind of feedback acts to maintain the connection between black hole mass and stellar velocity dispersion, in spite of processes like galaxy mergers and gas accretion that might be expected to increase the scatter over time. One such mechanism was suggested by Joseph Silk and Martin Rees in 1998. These authors proposed a model in which supermassive black holes first form via collapse of giant gas clouds before most of the bulge mass has turned into stars. The black holes created in this way would then accrete and radiate, driving a wind which acts back on the accretion flow. The flow would stall if the rate of deposition of mechanical energy into the infalling gas was large enough to unbind the protogalaxy in one crossing time. The Silk and Rees model predicts a slope for the M-σ relation of α=5, which is somewhat larger than observed, but does predict approximately the correct normalization of the relation.
Importance

Before the M-σ relation was discovered in 2000, a large discrepancy existed between black hole masses derived using three techniques. Direct, or dynamical, measurements based on the motion of stars or gas near the black hole seemed to give masses that averaged ~1% of the bulge mass (the Magorrian relation). Two other techniques—reverberation mapping in active galactic nuclei, and the Soltan argument, which computes the cosmological density in black holes needed to explain the quasar light—both gave a mean value of M/Mbulge that was a factor ~10 smaller than implied by the Magorrian relation. The M-σ relation resolved this discrepancy by showing that most of the direct black hole masses published prior to 2000 were significantly in error, presumably because the data on which they were based were of insufficient quality to resolve the black hole's dynamical sphere of influence. The mean ratio of black hole mass to bulge mass is now believed to be approximately 0.1%, i.e., a bulge of one billion solar masses contains a black hole of approximately one million solar masses.

A common use of the M-σ relation is to estimate black hole masses in distant galaxies using the easily-measured quantity σ. Black hole masses in thousands of galaxies have been estimated in this way. The M-σ relation is also used to calibrate so-called secondary and tertiary mass estimators, which relate the black hole mass to the strength of emission lines from hot gas in the nucleus or to the velocity dispersion of gas in the bulge.

The tightness of the M-σ relation has led to suggestions that every bulge must contain a supermassive black hole. However, the number of galaxies in which the effect of the black hole's gravity on the motion of stars or gas is unambiguously seen is still quite small. It is unclear whether the lack of black hole detections in many galaxies implies that these galaxies do not contain black holes; or that their masses are significantly below the value implied by the M-σ relation; or that the data are simply too poor to reveal the presence of the black hole.

The smallest supermassive black hole with a well-determined mass has M≈10⁶ solar masses (see figure). The existence of black holes in the mass range 10⁴ - 10⁶ solar masses ("intermediate-mass black holes") is predicted by the M-σ relation in low-mass galaxies, and the existence of intermediate mass black holes has been reasonably well established in a number of galaxies which contain active galactic nuclei, although the values of M in these galaxies are highly uncertain. No clear evidence has been found for ultra-massive black holes with masses above 10¹⁰ solar masses, although this may be an expected consequence of the observed upper limit to σ.>>

[zbvhs]

Is that fog we see in the image actually billions or trillions of unresolved individual stars? If individual stars can be picked out in other distant galaxies, why not here?

[Ann]

Is that fog we see in the image actually billions or trillions of unresolved individual stars? If individual stars can be picked out in other distant galaxies, why not here?

You can't pick out the individual stars in M60 due to the nature of this galaxy: it is a giant elliptical galaxy, made up almost entirely of very old stars. We can't rule out the presence of a few young stars, which may be as bright as Vega or maybe even as bright as Regulus. But at the distance of M60, these stars blend into the "background".

The brightest stars of M60 are typically red giants. These may well be brighter than Vega or Regulus, but they, too, are too faint to stand out. To get an idea of how faint these stars typically are as individuals but how bright their combined light is, take a look at this picture of the bulge of the Andromeda Galaxy. The yellowish stars in the bulge of M31 are the same kind of stars that totally dominate elliptical galaxy M60. You can see, however, that although the Andromeda Galaxy is really quite close to us - about two million light-years away - the only stellar resolution you can see in this picture is a general graininess of the yellowish population. (The bright points of light are probably either foreground Milky Way stars or M31 globular clusters.)

In fact, even in spiral galaxy NGC 4647 only a very few individual stars are likely to stand out, even though this galaxy contains a relatively large population of young bright stars. Most of the bright points of light in NGC 4647 are likely to be clusters instead of individual stars.

Ann

[Chris Peterson]

Is that fog we see in the image actually billions or trillions of unresolved individual stars? If individual stars can be picked out in other distant galaxies, why not here?

The majority of stars in other galaxies are not resolvable. We detect individual stars when they are significantly brighter than the stars surrounding them, are somewhat isolated (as towards and edge), and when the galaxies are quite close. The structural uniformity of elliptical galaxies reduces the opportunity to detect individual stars, but I'm not at all sure it can't be done.

An elliptical galaxy is something like a spiral galaxy with no disc- and in the bulge of a spiral galaxy, we normally see all the stars as a fog like this, unresolvable individually.

[Emphyrio]

This reply will probably demonstrate my abysmal ignorance but I have a question for you all:
In the APOD for today showing M60 and NGC 4647, there is a link in the text "simpler egg like shape". If you click on that, you get what is supposed to be another shot of M60. But, in that shot, there's a jet coming out of the thing and NGC 4647 is nowhere to be seen. That sorta got to me.
Any explanation that I can place in my brain without terminal overload?

[Boomer12k]

This reply will probably demonstrate my abysmal ignorance but I have a question for you all:
In the APOD for today showing M60 and NGC 4647, there is a link in the text "simpler egg like shape". If you click on that, you get what is supposed to be another shot of M60. But, in that shot, there's a jet coming out of the thing and NGC 4647 is nowhere to be seen. That sorta got to me.
Any explanation that I can place in my brain without terminal overload?

I think they are trying to show you a similarity about Elliptical Galaxies....SHAPE in this instance...AND the difference between the Spiral (more complex than a simple egg) Galaxy....a note on comparison...

:---[===] *

[bystander]

In the APOD for today showing M60 and NGC 4647, there is a link in the text "simpler egg like shape". If you click on that, you get what is supposed to be another shot of M60.

That's not M60, but M87, another elliptical galaxy similar in size to M60.

[nstahl]

Or maybe it was taken from a different angle.

[Wadsworth]

Or maybe it was taken from a different angle.

Got my vote.

[Lordcat Darkstar]

This reply will probably demonstrate my abysmal ignorance but I have a question for you all:
In the APOD for today showing M60 and NGC 4647, there is a link in the text "simpler egg like shape". If you click on that, you get what is supposed to be another shot of M60. But, in that shot, there's a jet coming out of the thing and NGC 4647 is nowhere to be seen. That sorta got to me.
Any explanation that I can place in my brain without terminal overload?

The other shot is of M87 not M60. It's there to give a better view of the shape of elliptical galaxies. Great pic by the way

[Emphyrio]

The other shot is of M87 not M60. It's there to give a better view of the shape of elliptical galaxies. Great pic by the way

Gah!!! Told you I was ignorant. I didn't even read the part about the other picture being M87.

[quigley]

If earth was a planet in either galaxy M60 or NGC4647, what would the night sky look like? What percentage of the night sky would be filled with the dense stars of the neighboring galaxy, assuming that your attitude of observation would be toward that galaxy?