X-ray and ultraviolet observations of extragalactic HII regions.

Abstract

The dwarf galaxies NGC 5204, NGC 5474, and NGC 5585 have been shown by Sandage and Tammann (1974) to be physically associated with M101 (distance 7.2 Mpc). The present investigation is concerned with observations of the dwarf galaxies in the M101 group conducted with the aid of the Einstein satellite. Upon detection of X-ray emission from NGC 5204, it was found possible to observe this galaxy also with the International Ultraviolet Explorer (IUE). It is now feasible to compare the predictions on the stellar population of the galaxy inferred from the X-ray data with a more direct measure concerning this population. It is found that the hypothesis regarding the responsibility of binary X-ray sources of Population I progenitors for most of the X-ray emission is strengthened by the IUE short-wavelength spectrum of NGC 5204.