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I am trying to find out line intensity and FWHM of H$_\alpha$6565 emission line from a quasar spectrum obtained from SDSS DR-14Q. Visual inspection of spectrum shows H$_\alpha$ being contaminated with [NII] 6549,6585 emission lines. I tried fitting voigt profile on the emission line to extract FWHM and line intensities. But the problem is voigt profile is not being fitted properly onto the data as shown here data fitted with voigt profile

I tried fitting 3 gaussian profiles on the data as there are 3 peaks visible. While fitting multiple gaussians it is clear that the gaussian profile doesn't fit over the left and right wings of the data. 3 gaussians I have already subtracted the underlying continuum and tried to normalize the flux as well.

Can anyone comment if I am doing it correctly because in both cases it seems that there is some sort of data loss? Or can anyone suggest a better way to do that?

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    $\begingroup$ Have you considered rotational line broadening? A quasar is like integrating over a whole rotating galaxy. Maybe you can even reverse the problem and derive the rotational speed $\endgroup$ – planetmaker Sep 26 '20 at 7:55
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I would've posted this as a comment, but I lack sufficient reputation.

I like your idea of fitting three Gaussians, but I think you should try fitting with one large positive Gaussian and two small negative Gaussians.

My physical justification for this is that I would expect a lot of self-absorption from the H-alpha line, which should be well-modeled using the larger negative Gaussian. I'm not sure if the other notch is absorption from another line, but it also looks like a negative Gaussian would fit well there.

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  • $\begingroup$ Quasars have emission spectra. $\endgroup$ – ProfRob Sep 28 '20 at 6:36
  • $\begingroup$ I admit I don't know anything about quasars. Don't we expect the ISM or other hydrogen along the line-of-sight to absorb a non-negligible component of this line? $\endgroup$ – Roy Smart Sep 28 '20 at 17:34
  • $\begingroup$ They are emission lines. Gaussians don't fit because the lines aren't Gaussian. $\endgroup$ – ProfRob Sep 28 '20 at 18:53

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