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Mass-to-light ratio

Ratio of total mass of a galaxy or cluster to its luminosity From Wikipedia, the free encyclopedia

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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]

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