I need to find metal lines in the Lyman-alpha forest of quasar spectra, without the red side of lyman alpha emission line.
Can you guide me how to do this?


1 Answer 1


I'm a bit unsure what you mean: Your spectrum doesn't include the red side of the quasar (seems weird), your Ly$\alpha$ line doesn't have a red side (even weirder), or you're only interested in the metal lines on the blue side?

Anyway, metals on the blue side are somewhat difficult: At high redshifts, they're blended with the $\mathrm{Ly}\alpha$ forest (or erased by the Gunn-Peterson trough), while at low redshifts they're observed in the UV which is absorbed in Earth's atmosphere.

So, you really need to know where to look. But if you know the position of the quasar's Ly$\alpha$ line, then you can look e.g. for

  • Fe II + Fe III at 1123 Å
  • O VI doublet at 1032/1037 Å (the strongest one, but unfortunately very close to Ly$\beta$
  • Semi-forbidden C III] at 988 Å (close to Ly$\gamma$)
  • O III at 831 Å
  • Ne VIII and O IV at 772 Å

where all wavelengths are given here in the rest frame.

Except for the O VI line, they're all rather weak. But if you have many spectra, you can try to stack them so that the noise and the Ly$\alpha$ forest lines cancel out, enhancing the metal lines. This has been done e.g. by Lusso et al. (2015).

  • $\begingroup$ I have red side but I need to remove metal lines in lyman alpha forest. $\endgroup$
    – Sara
    Commented Jan 15, 2020 at 7:57

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