Lois & Clark Forums Forums General Discussion Off Topic The Big Bang and the Universe, Part 5: Where are the li'l galaxies?

So Hubble continued observing galaxies through his magnifying glass, eh, through his telescope...



And he got all these spectra of galaxies, and after ten years or so of studying them and thinking about them, he pronounced this LAW (I'm stealing the wording from Wikipedia):

Or, in other words, the farther away a galaxy is from us, the faster it is receding from us.

But wait a minute. How did Hubble know that? How did he measure the distance to galaxies independent of their redshift? How did he know how far away they really were? He had no really reliable way of knowing that. Wasn't he just saying that, hey, this galaxy has a large redshift and therefore it is far away? Isn't this like saying that a mountain will look bluer the farther away it is? On the Earth, that is a correct conclusion, but on the Earth it is always more or less possible to travel to the mountains we were looking at (watching them become ever less blue as we approach them) and then we can measure the actual distance to them. We can find out if our inference that the mountains look bluer the farther away they are was correct.



Knowing the distance to blue-looking earthly mountains isn't that hard, but what about redshifted galaxies?

Do you remember Stephan's Quintet? Well, open this link and take a look:

NGC 7331 and Stephan\'s Quintet

At the bottom left of the picture you should see if you open the link, you can see Stephan's Quintet. At the top right, however, you will find a much larger-looking galaxy, NGC 7331.

Guess what? The redshift of NGC 7331 is pretty much the same as it is for the "odd-man-out" of Stephan's Quintet, NGC 7320. Both NGC 7331 and NGC 7320 have a redshift corresponding to a velocity of about 800 kilometers per second. Does that mean that these two galaxies are at the same distance from us? Yes, virtually all astronomers are confident that they are. But the other galaxies belonging to Stephan's Quintet, those whose "recession velocity" is around 6600 kilometers per second, are they much farther away than NGC 7331 and NGC 7320, then? Yes, that is what modern astronomy argues.

Take a look at another image of NGC 7331:

[img]http://www.dailygalaxy.com/.a/6a00d8341bf7f753ef0120a5270e6e970c-500wi[/img]

The colors here are a bit weird, but never mind. What I want you to concentrate on in this picture is actually the group of smaller galaxies "above" NGC 7331. This is the so-called Deer Lick group of galaxies. If you take a look at them, you may see that they are more or less the same apparent size as the galaxies making up Stephan's Quintet. (You should be able to see that by clicking on the link above once again, because that will show you a picture where you can see both Stephan's Quintet and the Deer Lick group.) Amazingly, the galaxies of the Deer Lick group aren't just more or less the same apparent size as the galaxies of Stephan's Quintet, but they have about the same redshift, too, minus the "interloper" NGC 7320, of course. Conclusion? If redshift has anything to say about it, the galaxies in the Deer Lick Group and Stephan's Quintet (minus NGC 7320) are at about the same distance from us, and they are receding from us at the same speed. They are all pretty mcuh the same size as the large "foreground galaxy", NGC 7331 - all of them, except for tiny little NGC 7320, which has just been proved to be a true dwarf galaxy!

Okay, right now I have to run, but I'll be back!

Ann

To summarize my previous post, we can say that the galaxies of Stephan's Quintet (minus NGC 7320) and the galaxies of the Deer Lick group are more or less the same apparent size, and they have pretty mcuh the same redshift. We can conclude from this that the galaxies of these two galaxy groups are at more or less the same distance from us, and they are more or less the same size.

But NGC 7331 and NGC 7320 are most certainly not the same size, even though they have pretty much the same redshift and are at pretty much the same distance from us. This means that NGC 7320 is a dwarf galaxy.

We can draw two conclusions from this. First, that the apparent size of a galaxy is often a fairly good indicator of how distant it is. And two, that the apparent size of a galaxy is not always a good indicator of how distant it is.

Hubble used the apparent size of galaxies when he judged how distant they were. He classified small galaxies as distant and large galaxies as nearby, or pretty much so anyway. But the small galaxies aren't always distant, as NGC 7320 shows us. All galaxies are not the same size.

Is there a way to judge from a galaxy's general appearance, not just its size, how distant it is? Yes, actually, the general appearance of individual galaxies do give us clues about how big they are. Take a look at Stephan's Quintet again:



Note that NGC 7320 doesn't have an obvious spiral structure. Instead its star formation regions are scattered more or less randomly across its disk. Compare the disk of NGC 7320 with the dramatic spiral arms of the galaxy below it. Galaxies with well-formed spiral arms are almost always big. Take a look at the picture of NGC 7331 and the Deer Lick group in my post above. You can see that the small galaxy on the left has very well-formed spiral arms. That is a very clear indicator that this is an intrinsically big galaxy.

Let's return to Stephan's Quintet and NGC 7320. Note the relative faintness of NGC 7320's central region, its so-called bulge. This bulge is really quite small and faint. In the middle of the bulge sits a tiny, rather faint nucleus.

Big galaxies almost never have such faint bulges and nuclei. Compare the bulges of other galaxies of Stephan's Quintet with that of NGC 7320. The other galaxies have almost blindingly bright bulges, so bright that we can't see the nuclei in there. I have to admit that the bulge of NGC 7331 looks faint in the picture in my post above, but that is because the picture has been processed to reveal features deep into the center of the galaxy. In more "normal" pictures of NGC 7331, its bulge looks much brighter:



So we can really tell from the general appearance of NGC 7320 that it is indeed probably a dwarf galaxy. Similarly, Hubble probably learnt to tell big galaxies from dwarfs by their looks alone.

And Hubble picked galaxies and galaxy groups because of the sizes of their individual members and took their redshifts to determine the distance to them. This it what it could look like when Hubble found different galaxy clusters at different redshifts:



And nowadays, when astronomy has better ways of judging the actual distance of individual galaxies and clusters - for example by identifying Cepheid stars, Henrietta Leavitt's stars, in fairly distant galaxies - astronomers find that Hubble's Law, which states that the redshift of a galaxy is proportional to its distance from us, holds remarkably true. When astronomers plot the velocities of galaxies (which correspond to redshift) against their known or as-good-as-known distances, they come up with nice straight diagonal lines like this one:



So the redshift of galaxies, which is comparatively easy to measure, has turned out to be enormously useful when it comes to determining the distances to these far-away cities of stars.

Ann

I need to state more clearly what Hubble's Law really says. The law says that v = Ho X D. Here v means velocity, D is distance, X is of course "multiplied by" and Ho - which I didn't manage to write correctly - is the Hubble constant. The latest measurements by the Hubble Telescope gives the value of Ho as 74.2 +/- 3.6 kilometers per second per megaparsec. A megaparsec is about 30 million light years. Hubble's law means that the recession velocity of a galaxy (the speed with which is it moving away from us) increases by 74.2 +/- 3.6 kilometers per second for every 30 million light years that its distance from us increases. Thus the relationship between the distance of a galaxy and the speed with which it recedes from us is perfectly linear, at least according to Hubble's Law.

How should we understand Hubble's Law? The best explanation by far - indeed the one that the vast majority of astronomers insist on - is that it is space itself that it increasing and growing larger. As it is growing larger, it is carrying the galaxies it contains along with it as flotsam on its surface, carrying all galaxies further and further apart.



We are being carried away by the groundswell of the universe as flotsam on its surface. (Hope you don't mind the Japanese whisky ad, by the way.)

Ann

There is just one little flaw with Hubble's law: Far away objects like the Andromeda galaxy move. Some will move away from us, others - like the cited Andromeda galaxy - move towards us. Which will undoubtedly have an effect on redshift/blueshift. Besides, if distance is indicated by redshift, how can you explain blueshift? Only by movement. Not distance.

Only nearby galaxies are blueshifted, at least as a general rule. The Andromeda galaxy is blueshifted because it is moving toward us - and indeed, astronomers firmly believe that it is going to collide with us in a few billion years' time!

The Milky Way and the Andromeda galaxy are the two major members of our Local Group of galaxies. These two major players of the Local Group are gravitationally bound, as are the rest of the galaxies in our own smallish galaxy cluster.

But galaxies farther away from us are not gravitationally bound to us. Their movement is dominated by the expansion of the universe, which may or may not be called "the Hubble flow" (I don't have the energy to google it). There is actually one big galaxy in the Virgo cluster, circa 60 million light years away, which is blueshifted, but that is because the galaxies in this large and rather massive cluster are "flung about" by their mutual gravity.

For more distant galaxies, but not for galaxies very far away, redshift does seem to correspond very closely to actual distance.

Ann