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

  • $\begingroup$ are all lines going to be shifted the same amount? $\endgroup$ Commented May 15, 2020 at 13:28
  • $\begingroup$ @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. $\endgroup$
    – pela
    Commented May 15, 2020 at 14:37

1 Answer 1


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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