The Stellar Content of the Nuclei of Nearby Galaxies. I. M31, M32, and M81

Photoelectric scanner observations with spectral resolutions of 16 and 32 A have been obtained for the nuclei of the nearby galaxies M31, M32, and M81. The objective is to synthesize model galaxies from individual local stars observed with the same equipment. In the models many groups of stars representing different segments of the H-R diagram are blended together to match the galaxy observations suitably-the scanner 1(X) measures made from X3300 to Xl 0700 and infrared colors to 3.4 . Both lines and continua are included. The spectral features measured (molecular bands of TiO, CN, CH, CaH, and MgH; plus atomic lines of Na I, Ca I and Ca ii, Mg I and the Balmer lines of hydrogen) were chosen so as to maximize the likellhood of achieving almost-unique models to represent the stellar content of the nuclei of galaxies. Given a few astrophysical constraints (mainly a continuous luminosity function), good-fitting models which are relatively unique can be obtained for M31. The solutions for M32 and M81 are somewhat less satisfactory. Mternative solutions are resented, mainly to illustrate some of the difficulties with previous models. ur main conclusions for the center of M31 are as follows: a) There is a strongly dwarf-enriched lower main sequence, leading to a stellar M/L = 44 for the best models. Dwarfs contribute 30 percent of the V light in these models and somewhat more in the near-infrared. b) The main-sequence turnoff point is at a slightly hotter temperature than the Sun, an observation implying a rather old stellar population. However, some star formation continued until 4 billion years ago. c) The stars in the nucleus are very strong-lined, with an average metal abundance much higher than the Sun's and probably somewhat in excess of that found in old super-metal-rich open clusters like M67. No weak-lined stars are allowed in the M31 model; i.e. Population II plays no role. The best M32 models are quite different. They require a mildly metal-poor globular-cluster-like population as a base. The solution for the M81 nucleus is complicated by the presence of many emission lines. They could be associated with the small radio source there. The best models for M81 have more gM's than the M31 model; this is required to match the stronger M81 infrared continuum flux.