<p>Disclaimer: not an expert on this area. However, I'm writing an answer because there's not currently one, and because the answer below works for the number density of supernovae with redshift and I suspect it's similar for galaxies.</p>
<p>The discrepancy arises from how telescopes work. If you're working at low redshift, you can scan large swathes of the sky because the objects you're interested in are bright. On the other hand, if you're interested in high redshift, you need to point your telescope at a certain portion of the sky for a long time. You can't do both, and moreover in the case of supernovae, the telescope can't do (because specific telescopes are optimized for low-z or high-z).</p>
<p>In the case of SDSS, all the data comes from the same telescope, so I suspect the difference stems from their observing runs. When SDSS takes low-z data, it creates a large sample of galaxies at low-z, but doesn't find any at high-z. Presumably at the time they got the data in the plot, they had done significantly more low-z observing runs than high-z ones.</p>

CC BY-SA 4.0
2024-08-22T11:50:01.830Z
Telescopes are in general not "optimized for low-z or high-z", except in the sense that high-z observation require larger mirrors (and, in some cases, better infrared sensitivity).
Peter Erwin
CC BY-SA 4.0
2024-07-22T03:54:23.630Z
2024-07-22T03:54:23.630Z
Allure
<p>I am currently analyzing the SDSS data especially in the extremely low redshift area, in order to measure the cosmological statistics. What I mean by extremely low is z&lt;0.1. One peculiarity that I noticed was a sharp peak of number density in about z~0.02, z~0.08.</p>
<p>I reckon that it may be due to the CFA2 great wall, and am quite curious of whether this could possibly bias the mean number density of galaxies (or matter) as a function of redshift. Is there a reason for such massive peak of number density? Would it be plausible to measure the cosmological statistics within the low redshift regime?</p>
<p>Edit:
I added the plot to clarify my question. I have used the raw data of NYU-VAGC/LSS catalog (Blanton et al, 2005), without making user-defined luminosity/color cuts.</p>
<p><a href="https://i.sstatic.net/n23we.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/n23we.png" alt="NYU-VAGC LSS catalog survey" /></a></p>
<p><a href="https://i.sstatic.net/rbFJ0.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/rbFJ0.png" alt="Number density in Logscale" /></a></p>

CC BY-SA 4.0
2022-03-17T20:30:57.713Z
The bulk of this effect is due to the fact we can't see low luminosity galaxies when they are far away.
Connor Garcia
<galaxy><cosmology><sky-survey>
Reasons for extremely high number density of galaxies at low redshift
Jun-Young Lee
# Reasons for extremely high number density of galaxies at low redshift

(Asked by: username_0 on 2022-03-11)

I am currently analyzing the SDSS data especially in the extremely low redshift area, in order to measure the cosmological statistics. What I mean by extremely low is z<0.1. One peculiarity that I noticed was a sharp peak of number density in about z~0.02, z~0.08.

I reckon that it may be due to the CFA2 great wall, and am quite curious of whether this could possibly bias the mean number density of galaxies (or matter) as a function of redshift. Is there a reason for such massive peak of number density? Would it be plausible to measure the cosmological statistics within the low redshift regime?

Edit:
I added the plot to clarify my question. I have used the raw data of NYU-VAGC/LSS catalog (Blanton et al, 2005), without making user-defined luminosity/color cuts.

[![NYU-VAGC LSS catalog survey](https://i.sstatic.net/n23we.png)](https://i.sstatic.net/n23we.png)

[![Number density in Logscale](https://i.sstatic.net/rbFJ0.png)](https://i.sstatic.net/rbFJ0.png)

**Tags:** galaxy, cosmology, sky-survey
> **Comments:**
> **username_1**: The bulk of this effect is due to the fact we can't see low luminosity galaxies when they are far away.  

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## 1 answers

### Answer by username_2 on 2024-07-22. Score: 0

Disclaimer: not an expert on this area. However, I'm writing an answer because there's not currently one, and because the answer below works for the number density of supernovae with redshift and I suspect it's similar for galaxies.

The discrepancy arises from how telescopes work. If you're working at low redshift, you can scan large swathes of the sky because the objects you're interested in are bright. On the other hand, if you're interested in high redshift, you need to point your telescope at a certain portion of the sky for a long time. You can't do both, and moreover in the case of supernovae, the telescope can't do (because specific telescopes are optimized for low-z or high-z).

In the case of SDSS, all the data comes from the same telescope, so I suspect the difference stems from their observing runs. When SDSS takes low-z data, it creates a large sample of galaxies at low-z, but doesn't find any at high-z. Presumably at the time they got the data in the plot, they had done significantly more low-z observing runs than high-z ones.

> **Comments:**
> **username_3**: Telescopes are in general not "optimized for low-z or high-z", except in the sense that high-z observation require larger mirrors (and, in some cases, better infrared sensitivity).  

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