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Galactic & cosmological numbers

by pellikkan
Tags: cosmological, galactic, numbers
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Aug2-13, 05:07 PM
P: 6
Can anyone point me to some references?
Greetings All. I am searching for a couple of numbers, but my books are packed away and stored somewhere due to moving. I was hoping someone could provide the best accepted observational values and the reference sources for those numbers.

First I am searching for the best number for the observed number density of all galaxies within about redshift 0.5, ...that is, ..the total number of galaxies per unit volume. I found one publication online that seems to imply that there are about 6 or 12 per cubic Mpc, but not sure about it. I suppose one could find a histogram of galaxies vs redshift and integrate out to say z=0.4, and then divide by the volume therein, but I'm not sure if those histograms are over the total angular sky or just a section of it. Also there's probably galaxies that don't show up on the histogram maybe because they're not big or bright enough or something.

Secondly I am looking for the best accepted average cross sectional size, ( or equivalently the diameter) for all the galaxies implied in the above paragraph, ..indeed if there is such a calculation somewhere. There are surely several ways to do this. I think over half of those galaxies are irregulars, maybe a third spirals, and the rest ellipticals, etc. Also I think the diameters of those are supposed to be maybe 1 to 10 kpc for irregulars, 5 to 50 kpc for spirals, and 1 to 200 kpc for ellipticals. Maybe the geometric mean or something for each type could be taken and then multiplied times the respective percentage of that type, and then all types averaged together or something like that. Also, maybe the total semi-local mass density could be divided by the average mass of all galactic types, or mass of each type averaged individually, and then multiplied by the percentage of the type, and then by the average size But I digress......maybe the best average cross section is already done and published somewhere, ...and so that's what I am asking. Wikipedia says that the diameters range from about 1,000 to about 100,000 parsecs, and so from that I'd guess that an average would end up being about 10,000 pc or less.

Thirdly, but less importantly, I could use the reference to the best histogram of all types of quasars vs redshift, ...out to maybe z=4, or around there. It would also be helpful to know the total local number density of all quasars semi-locally within redshift 0.3 or 0.4. Again, I'm not sure if those counts have been done over the total angular sky or not. Maybe they've been done over part of the sky and then extrapolated to the entire sky or something.

Thanks much in advance if you can help finding any of those references, and best regards.
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Aug2-13, 07:34 PM
PF Gold
P: 1,882
As for your third request regarding a histogram of quasars vs redshift, this paper may have the information you’ve asked for:

“SDSS quasars in the WISE preliminary data release and quasar candidate selection with optical/infrared colors” Wu, Xue-Bing et al. Astron.J. 144 (2012) 49, and at arXiv:1204.6197 [astro-ph.CO]

For a review and some significant graphs from that paper, see:

I notice that you’ve asked the same questions at researchgate as well:
Gregory Newton
University of Washington Seattle
Aug2-13, 09:38 PM
PF Gold
P: 1,882
As for your first request regarding “observed number density of all galaxies within about redshift 0.5”, see:

The Astrophysical Journal, 620:564-583, 2005 February 20
2005. The American Astronomical Society. All rights reserved. Printed in U.S.A.
The Luminosity, Stellar Mass, and Number Density Evolution of Field Galaxies of Known Morphology from z = 0.5 to 3
Christopher J. Conselice ,1,2 Jeffrey A. Blackburne ,1,3 and Casey Papovich 4

The Astrophysical Journal Supplement Series, 172:434-455, 2007 September
2007. The American Astronomical Society. All rights reserved. Printed in U.S.A.
The Evolution of the Number Density of Large Disk Galaxies in COSMOS1

“Interpreting the Cosmic Infrared Background: Constraints on the
Evolution of the Dust Enshrouded Star Formation Rate”

“Galaxy Cluster Virial Masses and Omega”

Aug4-13, 02:30 PM
P: 6
Galactic & cosmological numbers

Yes, thanks very much. The qso histogram is excellent.

As for the number density of galaxies, I am searching through
those references, and thanks so much for relating those.
I have found a quite direct answer in a publication at,
which summarizes by stating that there are about 12 galaxies
per cubic mpc, but was published in 1946. I guess I could
trace the trail of publications that cite that document
to find more current such estimates of the neighbourhood
within z=0.5 when I get some extra time. Can anyone comment
on the 12 galaxies/cubic Mpc given by that publication?
Does it sound reasonable estimate by today's methods?

Still can't find anything that summarizes an average galactic
diameter within redshift 0.5.

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