<p>You need to compare it with the spectrum of a similar galaxy at a known redshift, that would probably enable you to identify features with known rest wavelengths.</p>

<p>If you can find such a template, then the best way of estimating a redshift for a galaxy spectrum like this, consisting of mostly weak and blended absorption features, is to cross-correlate your template galaxy with this one. The cross-correlation "lag" gives you the difference in redshift.</p>

CC BY-SA 4.0
2020-04-12T17:34:03.553Z
2020-04-12T17:34:03.553Z
ProfRob
<p>How does one identify the common galaxy absorption lines (e.g. Na, Mg, K, etc.) and emission lines (H-alpha, O III, S II, etc.) by just looking at a galaxy spectrum (like the one below)? I need to know how to do this because I want to calculate the redshifts of galaxies using the emitted wavelengths from the SDSS table.</p>

<p><a href="https://i.sstatic.net/aTFWI.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/aTFWI.png" alt="enter image description here"></a></p>

CC BY-SA 4.0
2020-05-15T13:28:53.027Z
are all lines going to be shifted the same amount?
Theologos K
CC BY-SA 4.0
2020-05-15T14:37:04.350Z
@TheologosK To first order, yes, but various radiative transfer effects may cause some lines to be redshifted or blueshifted relative to the overall, "systemic", shift. These shifts are important to identify some physical processes, such as galactic outflows.
pela
<galaxy><spectroscopy><spectra>
Identifying common galaxy spectral lines
RUNN
# Identifying common galaxy spectral lines

(Asked by: username_0 on 2020-04-12)

How does one identify the common galaxy absorption lines (e.g. Na, Mg, K, etc.) and emission lines (H-alpha, O III, S II, etc.) by just looking at a galaxy spectrum (like the one below)? I need to know how to do this because I want to calculate the redshifts of galaxies using the emitted wavelengths from the SDSS table.

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

**Tags:** galaxy, spectroscopy, spectra

> **Comments:**
> **username_1**: are all lines going to be shifted the same amount?  
> **username_2**: @username_1 To first order, yes, but various radiative transfer effects may cause some lines to be redshifted or blueshifted relative to the overall, "systemic", shift. These shifts are important to identify some physical processes, such as galactic outflows.  

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## 1 answers

### Answer by username_3 on 2020-04-12. Score: 7 | Accepted answer

You need to compare it with the spectrum of a similar galaxy at a known redshift, that would probably enable you to identify features with known rest wavelengths.

If you can find such a template, then the best way of estimating a redshift for a galaxy spectrum like this, consisting of mostly weak and blended absorption features, is to cross-correlate your template galaxy with this one. The cross-correlation "lag" gives you the difference in redshift.


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