APOD: NGC 253: The Sculptor Galaxy (2011 Dec 20)

[APOD Robot]

NGC 253: The Sculptor Galaxy

Explanation: NGC 253 is not only one of the brightest spiral galaxies visible, it is also one of the dustiest. Discovered in 1783 by Caroline Herschel in the constellation of Sculptor, NGC 253 lies only about ten million light-years distant. NGC 253 is the largest member of the Sculptor Group of Galaxies, the nearest group to our own Local Group of Galaxies. The dense dark dust accompanies a high star formation rate, giving NGC 253 the designation of starburst galaxy. Visible in the above photograph is the active central nucleus, also known to be a bright source of X-rays and gamma rays.

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[Beyond]

For those of you that prefer the black and white photos, click on the 4th NGC 253 link.

[Ann]

Today's APOD looks lovely indeed, not least to me, the lover of blue things. I'd like to point out, nevertheless, that NGC 253 is a relatively yellow galaxy overall. It is extremely dusty, which reddens the light of it. Also, although NGC 253 is a starburst galaxy, there is relatively little star formation in the disk of the galaxy, which means that the disk isn't particularly blue. The starburst takes place near the nucleus, where it is hidden from our view.

M82. Credit: Pablo Rodríguez-Gil and Pablo Bonet

There are some interesting similarities between NGC 253 and M82. Both are starburst galaxies. Both have disks with relatively little or no star formation. Both have starbursts near their nuclei. Both are dusty, and both are bright in infrared light. Both have outflows from their nuclear regions. Admittedly M82 is a lot more extreme as a galaxy than NGC 253.

In any case, today's APOD is lovely and beautiful, and I was glad to see it!

Ann

[islader2]

@ ANN {without an E} You beat me to the first post today. I was going to remind you that there is a link to CAROLINE H.==one of your heroines [pardon the sexism, I am not typing about Wilhelm]. As Rosanna used to say on SNL: Never mind! Thanks for your contributions.

[Ann]

@ ANN {without an E} You beat me to the first post today. I was going to remind you that there is a link to CAROLINE H.==one of your heroines [pardon the sexism, I am not typing about Wilhelm]. As Rosanna used to say on SNL: Never mind! Thanks for your contributions.

Caroline Herschel pointing out the location of NGC 253?

Actually, islader2, I was going to say something about Caroline Herschel, but it sort of slipped my mind. Thanks for reminding me!
(And thank you for liking my contributions! )


Ann

[Frosty Topp]

In this picture like most (all maybe) pictures of distance galaxies there are numberous stars which do not appear to be associated with a particular galaxy. Are there orphan stars between galaxies or are we looking through a closer galaxy in which these stars belong?

[Ben Lassing]

Hello All,
Wouldn't it be nice to display some more information in the APOD's, like the velocity of the circumference of a galaxy-system, or how long it -assumed- takes that a member-star on the outskirts of a galaxy will complete one revolution.
Is it possible to display the density in particles per cubic centimeter of a nebula? And how do we experience that if such a dustcloud would be (partially) in our solar system. What would we see then?

Greetings, Ben

[starchaser]

In this picture like most (all maybe) pictures of distance galaxies there are numberous stars which do not appear to be associated with a particular galaxy. Are there orphan stars between galaxies or are we looking through a closer galaxy in which these stars belong?

Correct me if I'm wrong, but I think that those are the stars from our own galaxy that are just in the line of view.

[12zsawer]

In this picture like most (all maybe) pictures of distance galaxies there are numberous stars which do not appear to be associated with a particular galaxy. Are there orphan stars between galaxies or are we looking through a closer galaxy in which these stars belong?


Correct me if I'm wrong, but I think that those are the stars from our own galaxy that are just in the line of view.

I'm no expert but I think yes, most of the stars that appear to be drifting in this picture are indeed inside our own galaxy, or they are other entire galaxies at a much greater distance. So glad we have access to this resource, a great wealth of knowledge and information!

[orin stepanek]

I like the reference to it being called the silver dollar galaxy! http://en.wikipedia.org/wiki/Sculptor_Galaxy I think that fits NGC 253 very well!

[BMAONE23]

Every time I look at images like this one, my mind automatically sees it rotating

[drollere]

i appreciated the link to the "starburst" paper on ADS, quite a nice example of galaxy dissection and analysis.

it is striking to note the number of raggedy ("young") and OB rich galaxies in the sculptor group, which seems to be a new galactic suburb under construction. by comparison clusters with predominantly elliptical galaxies seem like stodgy places to live. since the sculptor cluster is closest to our small group i wonder if they might merge in the future ... as i recall we're actually moving in the opposite direction, toward virgo.

[TNT]

I like the reference to it being called the silver dollar galaxy! http://en.wikipedia.org/wiki/Sculptor_Galaxy I think that fits NGC 253 very well!

I half agree and half disagree with you, Orin. Sure the galaxy does look like a silver dollar, but it seems rusty to me! Maybe it should be called the rusty silver dollar galaxy.

Oh, yeah, silver doesn't rust. Oh, well.

[orin stepanek]

Silver dollars aren't made of silver anymore though! http://wiki.answers.com/Q/What_is_a_sil ... ar_made_of

[jimsaruff]

So what's it's age; (BTW how old is our's?)

And why doesn't it have arms?

[orin stepanek]

And why doesn't it have arms?

If you look at it closely; it does have arms! It also looks like it may be a bar galaxy! It just isn't face on enough to see the spiral arms very well!

[jimsaruff]

I believe you're right, Orin.

What's the life cycle of a galactic arm anyway?

[Ann]

So what's it's age; (BTW how old is our's?)

And why doesn't it have arms?

It is likely that a large galaxy like NGC 253 is very old. It may well have started forming in the early childhood of the universe, and the universe is estimated to be, if I remember correctly, 13.7 billion years old. (But you shouldn't take my word for it. Google it if you are really interested.)

If we assume, however, that the universe really is 13.7 billion years old, then a galaxy like NGC 253 is probably at least 12 billion years old, if you ask me. But you can be sure that it hasn't always looked the way it does now. For one thing, it must of course have been much, much smaller when it started forming 12 billion years ago or more. It must also have undergone violent episodes of star formation, when its shape must have been chaotic and its light must have been dominated by bright blue stars and pink emission nebulae.

Personally I'd say that if we could see NGC 253 more face on, we still wouldn't think that its arms were very regular and dominant. Not all galaxies possess large and dominant arms. Please follow this link to see a picture by Jyri Näränen and Kalle Torstensson of almost face-on galaxy NGC 7217. The galaxy has intricate "swirling" dust structures, but it doesn't really have arms.

On the other hand, galaxies can be seen to have obvious arms even if they are seen at a high inclination. Follow this link to see a picture of galaxy NGC 3981. Even though the galaxy is almost edge-on to us, we can clearly see its two dominant spiral arms.

Also take a look at NGC 4100 by Jack Burgess and Adam Block here. Even though the general outline of that galaxy is very regular, the arm structure of its disk is very obvious.

As to why some galaxies have dominant arms and other don't, well, I'd say it has everything to do with how the individual galaxy grew and evolved, how many mergers it had and how messy they were, and whether they enhanced or braked the rotation of the galaxy's disk, and how big the black hole in the center of the galaxy is, and so on and so on.

Ann

[orin stepanek]

Ann I can see at least 2 and maybe a 3rd arm in the APOD photo and also what looks like a bar! I think that if the galaxy was more face on they would be quite dominant! But maybe that is only my perception!

[jimsaruff]

Hmmm....I can't tell if my 'Quick reply' was posted or not.

So, Ann!!!

Are most galaxies of a similar age as NGC 253?

And, BTW....hahaha...were is all the 'expansion' taking place in the visible universe? Between galaxies, generally speaking?

Thanks for taking the time to share the information.

[Chris Peterson]

And, BTW....hahaha...were is all the 'expansion' taking place in the visible universe? Between galaxies, generally speaking?

Mostly, between galaxy clusters. Gravity easily overcomes the expansion of space, so you need a long distance between massive objects in order to have expansion.

[Ann]

Ann I can see at least 2 and maybe a 3rd arm in the APOD photo and also what looks like a bar! I think that if the galaxy was more face on they would be quite dominant! But maybe that is only my perception!

Orin, you're right, NGC 253 obviously does have arms. I still don't think those arms would be exactly dominant if we saw the galaxy face on. I agree they would be a lot more pronounced and definitely more arm-like than the dust structures of NGC 7217, however.

Jimsaruff, I think that very many galaxies can trace their origins back to the first few billion years of the universe. In fact, if you bear in mind that the universe has been expanding ever since it was born, but no new matter is believed to have been created after the universe became cold enough for ordinary matter to "condense" out of the "energy soup" of the primordial universe, then it makes sense that the matter that exists must have been a lot more crowded in the distant past than it is now. Therefore it must have interacted more than it does now. My understanding of interactions of clumps of matter is that it encourages clustering and concentrations of mass, i.e., it encourages the formation of star clusters and gas clouds that can grow into galaxies.

So yes, I believe that very many and perhaps most galaxies are very old, but I also believe that they have changed very much since they first began forming. That is certainly particularly true of large galaxies. But in fact, many and perhaps most small galaxies probably also started forming in the distant past. Many of the galactic midgets have certainly disappeared because they have been "eaten" by large galaxies like the Milky Way, but those that remain likely started forming many billion years ago. A seemingly youthful galaxy like the Large Magellanic Cloud nevertheless contains stars and globular clusters that may be as old, or almost as old, as the oldest stars in the Milky Way.

Nevertheless, small galaxies are still forming today, as a result of violent mergers. You can see an example here. The galaxy is called the Tadpole Galaxy, and you can see that there are bluish "clumps" in its long tail. These clumps will likely evolve into dwarf galaxies. Similar clumps can be seen in one of the tails of the Antenna Galaxies. You can see a picture of these newborn "proto-dwarf galaxies" here.

Ann

[Jim Leff]

Every time I look at images like this one, my mind automatically sees it rotating

Does anyone have any idea how long it takes large galaxies like this to complete a full rotation? I know they vary; I'm just wondering the normal range.

[Chris Peterson]

Does anyone have any idea how long it takes large galaxies like this to complete a full rotation? I know they vary; I'm just wondering the normal range.

It's a tricky question, because a galaxy doesn't rotate as a single entity. Stars orbit at different rates, depending on their distance from the center. Central stars have orbits of a few million years or less (really central stars, around a supermassive black hole, might orbit in just months). Outer stars have orbits of hundreds of millions of years. The spiral arm pattern may appear to rotate every 50-100 million years. A bar might rotate in only 10 or 20 million years.

[Jim Leff]

Thanks.

So... galaxies really swirl quite a bit over their billions-of-years lifetimes. Interesting!