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**Gas fraction estimation** is the use of formulae
(models) to estimate the gas (gas fraction) or kinds of gas in,
for example, a galaxy.
Such a method may be used with survey data
to perform such estimates on a large number of bodies
using available data.
Example (using Sloan Digital Sky Survey data):

log_{10}(G_{HI}/S) = - 1.73238(g - r) + 0.215182μ_{i}- 4.08451

where:

- G
_{HI}is HI gas the galaxy. - S is the stellar mass of the galaxy.
- μ
_{i}is the i-band surface brightness. - g and r are the corresponding passband magnitudes.

Methods are devised for different redshifts, in particular to identify star formation potential.

https://ui.adsabs.harvard.edu/abs/2009MNRAS.397.1243Z/abstract