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As the header stated, what exactly is the 'emission line ratio'? Like, [O III]/Hβ or [Ne V]/[Ne II]. Recently I've been reading some research papers in astronomy and astrophysics pertaining to galactic astronomy, and they have been using the method of emission line ratio as an indicator of emission line strength and relate it to the bolometric luminosity of the galaxy. However, I can't seem to find any source giving a concrete and concise definition of this quantity.

I've been reading Kurt T. Soto 2012 ApJ 757:86, Bren E. Backhaus ApJ 962:195, and Veilleux and Osterbrock 1987 ApJ 63:295-310. However, none of them give the definition of emission line ratio. Although, I'm still an undergrad, if there's no simple way to explain this quantity, any materials that could lead me to an answer will be greatly appreciated. Thank you in advance. : D

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It is as you might expect a measurement of the ratio of the fluxes that are present in the named emission lines.

One construct ratios, as opposed to absolute fluxes in the individual lines, because you hope to eliminate some things that are either uncertain or not particularly interesting when it comes to the properties of the gas doing the emitting. For example, a line ratio does not generally depend on the distance and, if the lines are reasonably close together in wavelength, will not depend on either local or more general extinction. A line ratio will also not depend on how well-calibrated your instrument is with regard to the absolute values of the fluxes being measured.

Line ratios have all sorts of uses in astronomy but principally, an appropriate choice of ratios using several sets of line diagnostics from the same element and between forbidden and allowed transitions can yield all of: the density, temperature and chemical composition of the emitting gas.

The classic reference for these methods is undoubtedly "Astrophysics of Gaseous Nebulae and Active Galactic Nuclei" by Osterbrock (1989), which is colloquially known as AGN$^2$.

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  • $\begingroup$ Thank you a lot! I will give the book a read. <3 : D $\endgroup$
    – ZenithalizeSquads
    Commented Jun 19 at 17:21

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