Light-to-Mass Variations with Environment

Large and well-defined variations exist between the distribution of mass and the light of stars on extragalactic scales. Mass concentrations in the range 1012-1013 M☉ manifest the most light per unit mass. Group halos in this range are typically the hosts of spiral and irregular galaxies with ongoing star formation. On average M/LB ~ 90 M☉/L☉ in these groups . More massive halos have less light per unit mass. Within a given mass range, halos that are dynamically old as measured by crossing times and galaxy morphologies have distinctly less light per unit mass. At the other end of the mass spectrum, below 1012 M☉, there is a cutoff in the manifestation of light. Group halos in the range 1011-1012 M☉ can host dwarf galaxies but with such low luminosities that M/LB values can range from several hundred to several thousand. It is suspected that there must be completely dark halos at lower masses. Given the form of the halo mass function, the low relative luminosities of the high-mass halos have the greatest cosmological implications. Of order half the clustered mass may reside in halos with greater than 1014 M☉. By contrast, only 5%-10% of clustered mass would lie in entities with less than 1012 M☉.