Images in the rocket ultraviolet : photometry of M 101.

Abstract

Three ultraviolet images of the Sc I galaxy M101 (NGC 5457) were obtained by a rocket-borne telescope, using a microchannel plate image intensifier coupled to IIa-O film, giving an effective wavelength of 2250 Å, a bandpass of 970 Å, and about 10arcsec resolution over an 80arcmin field. Extreme Population I dominates in the UV in the form of OB/H II complexes. The 57 OB/H II sources measured here account for half the UV flux of the galaxy. Large-scale spiral structure was investigated using azimuthal and radial profiles. A comparison with optical results shows that the colors of the disk and spiral arm components do not vary significantly with radius, and that the arm-to-disk ratio is a factor ≡3 higher in the UV than in the visible.