H4 - mass-to-light ratio

the total luminosity of the galaxy can be estimating by measuring the brightness profile and fitting the velocity profile
the mass-to-light ratio (M/L) is the ratio between the mass and the luminosity, expressed in $M_{⊙} / L_{⊙}$
it depends on:
- initial mass function (IMP)
- mass-luminosity relation for stars $L \propto M^{3.5}$
- star formation rate (SFR)
- gas and dust
- dark matter mass
from the first four, the expected M/L $\sim 1 - 10 M_{⊙} / L_{⊙}$
typically, M/L $\approx 50 M_{⊙} / L_{⊙}$
generally, M/L $> 20 M_{⊙} / L_{⊙}$ is considered a very strong evidence for dark matter