High Resolution Iras Maps Nnd Ir Emission of M31 —-1. Morphology and Sources A13s'i'llact

The morphology of the infrared (11?) emission and the properties of discrctc far-infrared (10 R) sources in M31 disk are studied using the high resolution (I IiRes) maps fro]n 1 RAS. Very thin and bright FIR. arm segments are shown ill tllmc maps, which have similar structure as the 111 gas but with much c]lhanccd arn]/inter-arm contrast, typically a factor of N 5 on the 60pTrL image. We identify 39 unconfused sources (excluding the nucleus) and 14 confused sources (7 pairs) at 60 p,], by direct Gaussian fittings to the image. lRAS colors of 10 bright isc)latcd sources are studied, which are presumably representative of the cliscrete sources in general. 'i'he colors follow the well known anticorrelation between ffjop /flool, and f] zP/fzsl,. All sources cc)incide with optical 1111 regions. A comparison with 11a observatio]ls shows that the tcjtal luminosity (in the wavelength range 8-1000pm) associated with HI1 regions is 7.2:! 2.9 1081~@ , namely 304 14yo of the total llt cmissio]l of M31. l[alf of the IR lu]]linosity of this coxnpone]lt. emerges fro]ll the sources. 'J'his lulninosity translates into a prcsc]]t-c]ay star fc)rlnation rate of 0.36:10.14 MO /yr, about an order of nlagllitlldc lower tllal] t}lat of the Milky Way Galaxy. TWO characteristics make M31, the Andromeda Galaxy, unique for a far-infrared (FIR) study. '1'hc first is its proximity: being the nearest spiral galaxy outside the Milky. Way, it oflcrs the best linear resolution for observations of spirals with a given angular resolution. 'l'his is a significant advantage for k'] R obseJ vations, given the relatively poor angular resolution of the current generation of FIR instruments. Second, it is known to have a very low current star formation rate, perhaps an order of nlagnitude lower than that of the Milky Way (Walterbos 1988). ?lecause of their very low FIR luminosities, not many such quiescent spirals can be studied in the FIR, and none except M31 with such high spatial resolution. The FIR emission of M31 has been studied by IIabing et al. (1 984), Soifer et al. (1986), and very extensively by Walterbos and Schwcling (1987, hereafter WS87) using IRAS maps. The main results from these early studies can bc summarized as follows: (1) (2) (3) M31 emits only a few percent of its total lumillosity at wavc]cngth longer than 10pm. The rest is mainly the optical radiation from stars (Jlabing et al. 1984; WS87). In the central part …