Mass-to-light ratio

In astrophysics and physical cosmology the mass-to-light ratio, normally designated with the Greek letter upsilon, ϒ,^[1] is the quotient between the total mass of a spatial volume (typically on the scales of a galaxy or a cluster) and its luminosity.

These ratios are calculated relative to the Sun as a baseline ratio which is a constant ϒ_☉ = 5133 kg/W: equal to the solar mass M_☉ divided by the solar luminosity L_☉, ⁠M_☉/L_☉⁠. The mass-to-light ratios of galaxies and clusters are all much greater than ϒ_☉ due in part to the fact that most of the matter in these objects does not reside within stars and observations suggest that a large fraction is present in the form of dark matter.^[2]: 368

Luminosities are obtained from photometric observations, correcting the observed brightness of the object for the distance dimming and extinction effects. In general, unless a complete spectrum of the radiation emitted by the object is obtained, a model must be extrapolated through either power law or blackbody fits. The luminosity thus obtained is known as the bolometric luminosity.^{[citation needed]}

Masses are often calculated from the dynamics of the virialized system or from gravitational lensing. Masses can also be measured through CMB backlighting.^[3] Typical mass-to-light ratios for galaxies range from 2 to 10 ϒ_☉ while on the largest scales, the mass to light ratio of the observable universe is approximately 100 ϒ_☉, in concordance with the current best fit cosmological model.^{[citation needed]}

Mass-to-light ratios in application can be used to gain insight into the dark matter content and dust extinction in a galaxy.^[4] Historically, rotation curves for spiral galaxies have been used to study galaxies, but mass-to-light ratios prove more accurate as a method of measuring mass.^[5]