Constraining the cosmic abundance of stellar remnants with multi-TeV gamma-rays

If galactic halos contain stellar remnants, the infra-red flux from the remnant progenitors would contribute to the opacity of multi-TeV gamma-rays. The multi-TeV gamma-ray horizon is established to be at a redshift z>0.034 by the observation of the blazar Mkn501 . By requiring that the optical depth due to \gamma \gamma ---> e+ e- be less than one for a source at z=0.034 we limit the cosmological density of stellar remnants, Omega_rm \le (2-4) x 10^-3 h_70^-1 (h_70 is the Hubble constant in units of 70 km sec-1 Mpc-1) and thus strongly constrain stellar remnants as a cosmologically significant source of dark matter.