# How many galaxies are there?

1. Jun 24, 2009

### TalonD

rough estimate, how many galaxies in the visible portion of the universe? Just curious.

2. Jun 24, 2009

### Wallace

Edit: Sorry, I did post an incomplete answer, but I stuffed up somewhere. I eneded up calculting there are about 10 thousand galaxies. Which is just a little bit too small...!

Anyone else want to give this a shot? I will try and work out where I went wrong?

Last edited: Jun 24, 2009
3. Jun 24, 2009

### nicksauce

From Wikipedia: 'There are probably more than 100 billion galaxies in the observable universe", with the reference: http://astronomy.swin.edu.au/~gmackie/billions.html

But we can do an order of magnitude calculation to estimate this.
Assumptions:
1) All the baryonic matter in the universe is in galaxies, in the form of stars
2) The universe is flat with $\Omega_Bh^2 = 0.02$
3) Each galaxy has 100 billion stars, averaging 1 solar mass
4) The size of the observable universe is about 4 * 10^32 ly^3 (http://en.wikipedia.org/wiki/Observable_universe)

This gives about 6 * 10^11 galaxies, close to the 100 billion quoted by Wikipedia.

Last edited: Jun 24, 2009
4. Jun 24, 2009

### Staff: Mentor

You can also reach a similar estimate by taking the Hubble Ultra Deep Field (which contains around 10,000 galaxies) and extrapolating it across the whole sky.

5. Jun 24, 2009

### negitron

Well, we can try counting them. Um, one...

6. Jun 25, 2009

### Chronos

Agree with nick, over 100 billion galaxies, so far. The universe is really gigantic, relatively speaking.

7. Jun 25, 2009

### TalonD

Does that mean that hubble can see all the way to the earliest forming galaxies ?

8. Jun 25, 2009

### Wallace

Hubble, being the best telescope we have ever had for this sort of thing, has seen the most distant galaxies ever observed (measured by their redshift). There is a fair bit of uncertainty over exactly what redshift the furthest Hubble Ultra Deep Field galaxies are, since all you really get is a hazy blob seen in a few different filters (e.g. in different colours) from which you make an informed guess about the redshift, based on what colours you expect galaxies to be in their rest frame.

I believe (if anyone could provide a reference that would be good) that Hubble has seen galaxies as distant (in terms of redshift) that it might be expected to be able to. Therefore an even more powerful telescope could in principle see even more distant galaxies, if indeed galaxies existed at such high redshifts, which is not a sure thing.

In terms of a ball park estimate of the total number of galaxies in the observable universe though, these extra ones that we can't yet see (if they exist) won't change the figures people have quoted in this thread by much.

9. Jun 25, 2009

### Blenton

One thing, when we look at the hubble deep field, how do we know there arent any galaxies behind the galaxies we see there. Id assume that light wouldnt pass through.

10. Jun 25, 2009

### Wallace

To some extent that it is true, although there are enough gaps through which we can see nothing in the way. On the other hand, if the seperation in redshifts between the galaxies fits a certain criteria, it is actually better if the further one is behind the first, since the gravity of the foreground galaxy can act as a graviational lens, in fact allowing us to see the more distant galaxy with a greater intensity than otherwise. Some of the most distant objects seen have been discovered this way.

11. Jun 25, 2009

### negitron

Huh? I do not think you understand the process here. Redshift measurement doesn't involve guesswork except to the extent that it's difficult to get readable spectra from such distant, faint objects. Once a clean spectrum has been obtained, however, redshift can be measured precisely because it doesn't involve guessing about colors, it looks for specific signature spectral emission and/or absorption lines, such as the ones for hydrogen and measures exactly how far it has been shifted towards the red (or blue, as the case may be). The color of the galaxy is irrelevant.

12. Jun 25, 2009

### Wallace

Hi Negitron, what you are saying is true in some cases, so it is good to have it pointed out. There are indeed two main ways to measure redshift, spectroscopic and photometric. Spectroscopic is very very accurate (if you get a signal at all, you know the redshift to 2 or 3 decimal places at least), but is expensive in terms of telescope time, since making a spectrum needs many more photons than an image. Photometric redshift measurements on the other hand is a technique to estimate the redshift based on in image in only a few colour filters, based on the knowledge of the major features in the spectrum. Photometric redshifts are used in, for instance, weak lensing surveys to get many many redshifts of background galaxies near gravitational lenses. While this is less accurate than spectroscopic, it is impractical (would take more telescope time than is available) to get spectroscopic redshifts.

Specifically in the case of the HDF and the HUDF, photmetric redshifts are the only ones possible since these surveys were done using the Hubble wide field camera in a number of colour bands. This means that photmetric, rather than spectroscopic, redshifts must be used. It simply isn't possible using current technology to get spectra of such distant objects.

13. Jun 25, 2009

### negitron

Ah, it wasn't immediately clear to me that you were speaking of photometric analysis rather than spectroscopic. If I'd given it some thought, it would have occurred to me that, of course, Hubble can't do spectroscopic analysis of such distant objects. Why? not because the technology doesn't exist, it's simply that at the time Hubble was deployed, it wasn't known that there were such distant objects!

14. Jun 25, 2009

### Wallace

Hubble has a spectrograph, it did get broken, but has been replaced in a recent servicing mission. Hubble spectra are pretty crappy though, because the Hubble mirror is so small (only ~2m). Ground based telescopes are actually better than Hubble for spectra because the distortion effects of the atmosphere are not a problem, so it is a case of bigger is better. The highest redshift spectra have been obtained from the ground, not from space.

The reason Hubble has a spectrograph is to get spectra in the UV, which can't be taken from the ground because of the ozone layer. This generally means objects either in our own galaxy or relatively nearby, since UV emission from high redshift galaxies gets shifted into the visible (or infra-red in extreme cases).

15. Jun 25, 2009

### Staff: Mentor

I was under the impression there was a different type of telescope (a radio telescope maybe?) that could see galaxies further away, but can't remember where I heard that. In any case, yeah, Hubble probably gets us on the right order of magnitude.

16. Jun 25, 2009

### TalonD

Correct me if I'm wrong, and I very well may be considering I'm just a layperson. But wouldn't the most distant galaxies we can see be limited by the surface of last scattering, that is, the microwave background radiation. so the radius of the visible universe is around 45 billion light years? So is hubble's resolution high enough to image galaxies or protogalaxies that far away?

17. Jun 25, 2009

### matt.o

Yes, the http://www.stsci.edu/hst/cos" [Broken] which will hopefully be taking spectra within the next few months.

Speaking from experience (mainly at the AAT...), bad seeing is most definitely a problem for collecting spectra, in particular for higher redshift objects, where bad seeing will mean you'll have to significantly increase your exposure time!

Last edited by a moderator: May 4, 2017
18. Jun 26, 2009

### Wallace

Ah okay. I was comparing Hubble spectra to Keck spectra that I worked on years ago, the Keck ones were waaaay better than Hubble. So seeing is still a problem for getting spectra (which makes sense) but not so bad such that a 10m ground based telescope still beats (by a fair way) a 2m space telescope, which is not the case for imaging?

19. Jun 26, 2009

### matt.o

Yes, Keck will definitely do a better job than Hubble as it has way more collection area, as you correctly pointed out!

20. Jun 26, 2009

### TalonD

6*10^11 is 600 trillion, thats not quite the same as 100 billion