The UV continuum spectrum of M81

Abstract

From IUE data the absolute continuum spectrum for the nucleus of M81 in the 1200-3200 A range is presented. The spectrum can be fitted by an old stellar population with a small contribution from horizontal-branch stars. It is found that the number of stars with m less than 0.3 solar mass is negligible. An upper limit for the nonthermal contribution to the UV spectrum is obtained, which together with the radio and X-ray fluxes yields a power law spectrum with alpha(ux) ranging from 0.7 to 1.3. The power-law radiation field produces from 5 percent to 50 percent of the photons needed to maintain the nuclear regions of M81 ionized.