Alarge mass-to-light ratio for a galaxy indicates that

A large mass-to-light ratio for a galaxy indicates that each solar mass of matter in gravity emits less light than our Sun.  

Explanation:

The ratio of mass (in units of the Sun’s mass) to light (in units of the Sun’s luminosity) is used to characterise the Galaxy. This number is used to identify the major types of stars present in most of the luminous population of the galaxy.  This ratio also helps us to identify amount of dark matter present in the galaxy. .

Galaxies have massive numbers of star where star formation is still taking place and their ratio of mass to light ranges between 1 to 10.

Galaxies consisting of already formed stellation and older stellar population, such as ellipticals, where most of stars have already completed their evolution and have stopped to shine will have ratio of mass-to-light ranging between 10 to 20.

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