3.1. Mass to Light Ratio
The mass-to-light ratio M/L is measured from virialized
systems from galaxies up to rich clusters.
The asymptotic universal value of the luminosity density,
, is estimated by
integrating the galaxy luminosity function.
The mass-to-light ratio in clusters is assumed to be the same
as outside clusters. Then
New Developments:
Better masses for clusters and groups are now available
from virial analysis of galaxies, from X-ray gas
with resolved temperature structure, and from gravitational lensing.
Pro:
This is a simple, straightforward method.
The result is independent of H0.
Con:
The basic hypothesis is unjustified. It rests on the implicit assumption that
galaxies form in an unbiased way. Clusters and groups may have extended
dark halos that are not traced by the galaxies or by the X-ray gas, so
M/L
may vary within the clusters as a function of radius. This problem is
even worse within galactic halos.
Current Results: From
virial analysis of galaxy velocity dispersions in several clusters
stuck together, and a corresponding estimate of
Masses for extended galaxy halos are also becoming available
from satellite galaxies and from lensing.
The total mass of clusters may be underestimated in the virial analysis based
on galaxy velocities because of possible velocity antibiasing, and because of
applying a spherical analysis to elongated systems.
The mass based on X-ray measurements may be underestimated by the
assumption that the X-ray gas is in hydrostatic equilibrium.
The data allows significant freedom in the mass profile,
which translates to an uncertainty of factor two
within the Abell radius, and a larger error at larger radii
[18].
The luminosity density may not be contributed by the same stars
that dominate the cluster light.
There are factors of 2-3 uncertainties in
stemming, for example, by differences in overall metallicity
and extinction by dust.
:
m
0.25 ± 0.05
[19].