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Sandoval
2009-Jan-27, 12:30 AM
A few years ago, astronomers at the Space Telescope Science Institute unveiled the Hubble Ultra Deep Field (HUDF) (http://hubblesite.org/newscenter/archive/releases/2004/07/) pictures, showing a extremely tiny patch of the sky to contain about 10,000 galaxies. Has this finding led to a reassesment of the estimated number of galaxies in the known universe? If so, by how many orders of magnitude? And finally, what is the best educated guess for such a number today?

Tim Thompson
2009-Jan-27, 05:10 AM
How curious that you should ask, only a few days after I have already done exactly that. The Hubble Ultra Deep field is 3x3 arcminutes in size. There are 16,501,200 HUD fields on the whole sky. There are about 10,000 galaxies cataloged in the HUDF. So, if the HUDF is a typical or average field of view, then there should be about 1.65012x1012 galaxies in the universe that are in principle visible to the HST under HUDF circumstances. However, in practice it took 10 days of observing time to get the HUDF image. To repeat that exposure over the whole sky would take 452,087.67 years of telescope time (not counting procedural overhead & etc.). So that's not going to happen.

I don't think this has motivated any real reassessment of the number of galaxies in the universe, if only because nobody ever really had anything you could call a serious estimate on the number of galaxies in the universe anyway. Besides, there are lots of galaxies that are visible only in radio waves. If there are any of those in the HUDF (I am sure there are but I have not checked), they are invisible to the optical & infrared HST image.

One interesting sideline is estimating the mass of the universe. If we take the average mass of a galaxy at redshift 1 (1.5x1011 solar masses) as the representative mass of a galaxy in the HUDF, then the 1.65012x1012 galaxies weigh in at 2.47518x1023 solar masses or 4.92313x1053 kilograms. But interestingly, the average mass density of the universe is about 2.4848x10-36 gm/cm3. If we assume a spherical universe with a radius of 12 billion parsecs (Key, et al., 2007 (http://adsabs.harvard.edu/abs/2007PhRvD..75h4034K)), than we get a total mass of 5.27629x1053 kg. The agreement between these two independent methods of estimating the mass of the universe strikes me as quite surprising, and I also note that the latter mass is slightly larger than the former mass, as we would have expected should be the case. Luck, perhaps, but still it seems that 5x1053 kg is a reasonable estimate for the mass of the universe, partly thanks to the HUDF.

astromark
2009-Jan-27, 08:59 AM
Good grief... Just when I was thinking how significant we are... Along comes this number 16,501,200 x 10,000...give or take a million or so up or down...

Its positively daunting to ponder the number of possible Earth like environments across the whole universe. but to get back onto topic... Yes that HUDF did change the way we think and what we know of the universe. Knowledge has a tendency to change our view.

George
2009-Jan-27, 02:46 PM
How curious that you should ask, only a few days after I have already done exactly that. The Hubble Ultra Deep field is 3x3 arcminutes in size. There are 16,501,200 HUD fields on the whole sky. There are about 10,000 galaxies cataloged in the HUDF. So, if the HUDF is a typical or average field of view, then there should be about 1.65012x1012 galaxies in the universe that are in principle visible to the HST under HUDF circumstances.
I think the FOV was about 11.9 sq. arcminutes for the ACS (http://acs.pha.jhu.edu/instrument/detectors/), though Wiki claims 11.

Also, you are a factor of 10 too high in your math. [This is a rare moment. :)]

Cougar
2009-Jan-27, 02:46 PM
Has this finding led to a reassesment of the estimated number of galaxies in the known universe? .... what is the best educated guess for such a number today?

I assume you mean the visible universe. Technically, I guess you would have to specify what particular age of the universe you want an estimate for, since galaxies are merging all the time.

trinitree88
2009-Jan-27, 04:35 PM
I think the FOV was about 11.9 sq. arcminutes for the ACS (http://acs.pha.jhu.edu/instrument/detectors/), though Wiki claims 11.

Also, you are a factor of 10 too high in your math. [This is a rare moment. :)]

Yep. 1.6012 X 1011, ought to do it.

George
2009-Jan-27, 05:41 PM
Yep. 1.6012 X 1011, ought to do it. Using the 11.9 sq. arcminutes yields about 125 billion galaxies when extrapolated.

To justify such an extrapolation, I am fairly certain they did a southern field, too, and got the same results.

trinitree88
2009-Jan-27, 07:49 PM
Using the 11.9 sq. arcminutes yields about 125 billion galaxies when extrapolated.

To justify such an extrapolation, I am fairly certain they did a southern field, too, and got the same results.

George. OK. My recollection is that the Southern exposure turned up about twice as many galaxies as the Northern one, but considering the technology, and the limiting number of exposures, they're pretty close. Of course, a repeat indicating a fundamental asymmetry in matter density between the two hemispheres might be indicative of something more fundamental..pete

George
2009-Jan-27, 09:07 PM
Of course, a repeat indicating a fundamental asymmetry in matter density between the two hemispheres might be indicative of something more fundamental..pete Look for more water mass in the Southern hemisphere. ;)

mugaliens
2009-Jan-27, 09:29 PM
Yep. 1.6012 X 1011, ought to do it.

Ok... How many stars is that?

Spaceman Spiff
2009-Jan-27, 10:02 PM
George. OK. My recollection is that the Southern exposure turned up about twice as many galaxies as the Northern one, but considering the technology, and the limiting number of exposures, they're pretty close. Of course, a repeat indicating a fundamental asymmetry in matter density between the two hemispheres might be indicative of something more fundamental..pete

You're confusing the HDF-N and HDF-S (http://www.stsci.edu/ftp/science/hdf/hdf.html) done in 1995 and 1998 with the HUDF (http://hubblesite.org/newscenter/archive/releases/2004/07/) released in March 2004 with a larger format, more sensitive camera. The OP is about the HUDF.

The HDF-S (southern sky exposure) had the added benefit (compared to the HDF-N) of the near-IR camera. And given the tiny fractions of the sky (and differing sensitivities) of HDF-N and HDF-S, I am not at all surprised to see slightly differing numbers of galaxies within each piece of the sky.

George
2009-Jan-27, 10:16 PM
You're confusing the HDF-N and HDF-S (http://www.stsci.edu/ftp/science/hdf/hdf.html) done in 1995 and 1998 with the HUDF (http://hubblesite.org/newscenter/archive/releases/2004/07/) released in March 2004 with a larger format, more sensitive camera. The OP is about the HUDF. I was confused first. :) I suppose there was no HUDF-S imaging.

speedfreek
2009-Jan-27, 11:30 PM
10,000 galaxies in an area of the sky which would easily be covered by a 1mm square piece of paper, held at arms length.

ExpErdMann
2009-Jan-28, 02:04 PM
One interesting sideline is estimating the mass of the universe. If we take the average mass of a galaxy at redshift 1 (1.5x1011 solar masses) as the representative mass of a galaxy in the HUDF, then the 1.65012x1012 galaxies weigh in at 2.47518x1023 solar masses or 4.92313x1053 kilograms. But interestingly, the average mass density of the universe is about 2.4848x10-36 gm/cm3.

Tim, why is your estimate of density so far from from the textbook values (http://hypertextbook.com/facts/2000/ChristinaCheng.shtml) of around 10^-30 gm/cc?

trinitree88
2009-Jan-28, 05:22 PM
You're confusing the HDF-N and HDF-S (http://www.stsci.edu/ftp/science/hdf/hdf.html) done in 1995 and 1998 with the HUDF (http://hubblesite.org/newscenter/archive/releases/2004/07/) released in March 2004 with a larger format, more sensitive camera. The OP is about the HUDF.

The HDF-S (southern sky exposure) had the added benefit (compared to the HDF-N) of the near-IR camera. And given the tiny fractions of the sky (and differing sensitivities) of HDF-N and HDF-S, I am not at all surprised to see slightly differing numbers of galaxies within each piece of the sky.

Spaceman Spiff. Yep. You're right. I was thinking the other two. Are they planning on a southern HUDF? pete

mugaliens
2009-Jan-28, 07:59 PM
Ok... How many stars is that?

George
2009-Jan-28, 08:22 PM
Ok... How many stars is that?
See my post #7. When the results were first announced, I recall that we came up with 131 billion galaxies, which means we were using a slightly smaller field of view for the image.

Cougar
2009-Jan-28, 08:45 PM
Ok... How many stars is that?

Oh, just add 9 or 10 zeroes. :whistle:

Spaceman Spiff
2009-Jan-29, 01:45 AM
Ok... How many stars is that?

This set of pages (http://www.atlasoftheuniverse.com/universe.html) comes up with a number. A lot of estimating and extrapolating, but reasonable stuff. Comparing this number to all the grains of sand on all of the beaches, here (http://astronomy.swin.edu.au/~gmackie/billions.html).

Cougar
2009-Jan-29, 02:29 AM
This set of pages (http://www.atlasoftheuniverse.com/universe.html) comes up with a number.

That's a great site. It also notes....




The true size of the universe is probably much larger than the visible universe. The geometry of the universe suggests that it may have an infinite size and that it will expand forever. Even if the universe is not infinite, our visible universe must be a minute speck in a much larger totality.

Sandoval
2009-Jan-29, 04:47 AM
Superb replies, excellent points raised. Thank you Tim Thompson and everyone else.

John Jaksich
2009-Mar-17, 05:02 AM
Someone correct me...but I have been under the impression that the images you have been refering to in the post were subject to gravitational lensing effects and; therefore some of the aforementioned galaxies are "mirror" images within the image itself? I hope I come across clearly, here.

Thanks ...

Sandoval
2009-Mar-19, 04:20 AM
Someone correct me...but I have been under the impression that the images you have been refering to in the post were subject to gravitational lensing effects and; therefore some of the aforementioned galaxies are "mirror" images within the image itself? I hope I come across clearly, here.

Thanks ...
Jaksichj, this is a very interesting point. Unfortunately, it is also well beyond my capability to speculate about. I hope someone knowledgeable will pick up this issue.

gzhpcu
2009-Mar-19, 04:37 AM
According to Wikipedia:


The observable universe contains about 3 to 7 1022 stars (30 to 70 sextillion stars),[14] organized in more than 80 billion galaxies, which themselves form clusters and superclusters

source: http://en.wikipedia.org/wiki/Observable_universe

slang
2009-Mar-19, 08:07 AM
Someone correct me...but I have been under the impression that the images you have been refering to in the post were subject to gravitational lensing effects and; therefore some of the aforementioned galaxies are "mirror" images within the image itself? I hope I come across clearly, here.

The wiki page about gravitational lenses (http://en.wikipedia.org/wiki/Gravitational_lens) describes what I think you mean. I'm not sure if there are any in the HUDF, HDF or HDF-S, maybe a few. I don't think it has a significant influence on the total number of objects seen in the images though.