High Resolution Iras Maps and Ir Emission of M31 | I. Morphology and Sources

The morphology of the infrared (IR) emission and the properties of discrete farinfrared (FIR) sources in M31 disk are studied using the high resolution (HiRes) maps from IRAS. Very thin and bright FIR arm segments are shown in these maps, which have similar structure as the HI gas but with much enhanced arm/inter-arm contrast, typically a factor of 5 on the 60 m image. We identify 39 unconfused sources (excluding the nucleus) and 14 confused sources (7 pairs) at 60 m by direct Gaussian ttings to the image. IRAS colors of 10 bright isolated sources are studied, which are presumably representative of the discrete sources in general. The colors follow the well known anticorrelation between f60 =f100 and f12 =f25 . All sources coincide with optical HII regions. A comparison with H observations shows that the total luminosity (in the wavelength range 8 | 1000 m) associated with HII regions is 7:2 2:9 108L , namely 30 14% of the total IR emission of M31. Half of the IR luminosity of this component emerges from the sources. This luminosity translates into a present-day star formation rate of 0.36 0:14 M /yr, about an order of magnitude lower than that of the Milky Way Galaxy. Subject headings: galaxies: individual { galaxies: interstellar matter { galaxies: photometry { interstellar: grains { interstellar: HII regions