<p>Catalogs have mistakes in them. Misidentified objects, misidentified lines, foreground or background contamination of spectra, ...</p>
<p>This is difficult work, impossible to do perfectly.</p>

CC BY-SA 4.0
2024-07-10T13:45:47.333Z
2024-07-10T13:45:47.333Z
John Doty
<p>I've crossmatched quasars from DESI DR9 and from SDSS DR16. When I plot the spectroscopic redshift from each of the individual catalogs for each source, I get:</p>
<p><a href="https://i.sstatic.net/jtJtrUaF.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/jtJtrUaF.png" alt="enter image description here" /></a></p>
<p>Naively, I'd expect the <span class="math-container">$z_{spec}$</span> to be the same from each catalog, so my question is <strong>why are there outliers, some of them catastrophic?</strong></p>

CC BY-SA 4.0
2024-07-10T12:27:42.853Z
You might want to look at these outliers in regard to the stated reliability of the values given in the respective catalogues and repeat the plot with errorbars included. Maybe also the source types matter and some are hard or difficult to characterize.
planetmaker
<redshift><quasars><star-catalogues><sdss><crossmatching>
Why would some sources in a cross-matched DESI/SDSS catalog have different spectroscopic redshifts?
Jim421616
# Why would some sources in a cross-matched DESI/SDSS catalog have different spectroscopic redshifts?

(Asked by: username_0 on 2024-07-09)

I've crossmatched quasars from DESI DR9 and from SDSS DR16. When I plot the spectroscopic redshift from each of the individual catalogs for each source, I get:

[![enter image description here](https://i.sstatic.net/jtJtrUaF.png)](https://i.sstatic.net/jtJtrUaF.png)

Naively, I'd expect the $z\_{spec}$ to be the same from each catalog, so my question is **why are there outliers, some of them catastrophic?**

**Tags:** redshift, quasars, star-catalogues, sdss, crossmatching

> **Comments:**
> **username_1**: You might want to look at these outliers in regard to the stated reliability of the values given in the respective catalogues and repeat the plot with errorbars included. Maybe also the source types matter and some are hard or difficult to characterize.  

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## 1 answers

### Answer by username_2 on 2024-07-10. Score: 1

Catalogs have mistakes in them. Misidentified objects, misidentified lines, foreground or background contamination of spectra, ...

This is difficult work, impossible to do perfectly.


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