The interpretation of the UV light of elliptical galaxies.

Abstract

The UV spectra of the dwarf elliptical M 32 and the two Virgo giant ellipticals NGC 4472 and NGC 4649 are discussed using the spectrum synthesis technique in the whole wavelength region from 1500 A to 2.3 microns. For M 32, the UV spectrum may be explained in terms of evolved blue stars from a minor metal-poor component together with a small contribution from white dwarfs and post-AGB stars from the dominant metal-rich component. For the Virgo ellipticals NGC 4472 and NGC 4649, the above mentioned UV bright stars are not able to explain the UV spectra unless one invokes nonstandard behavior for these stars (e.g., longer lifetimes of post-AGB stars and the existence of metal-rich blue HB stars). The most likely explanation for the UV light of these galaxies seems to be bursts of young stars. The difference in the UV spectra of NGC 4472 and NGC 4649 is then explained in terms of different burst ages.