Disentangling age and metallicity effects in the blueish nuclear population of m 33, ngc 278 and ngc 404

Abstract

In this study, we have applied to the blueish nuclei of the spiral galaxies M33 and NGC 278 and to the dwarf lenticular NGC 404, a population synthesis method relying upon a base of star clusters, over the range 3700-9700 Ă. We make use of a grid of star cluster spectral features as a function of age and metallicity. Whenever necessary, we refine this first approach by using the entire spectrum points directly. The algorithms employed are, in the first case a multiple minimization procedure with a statistical treatment of the acceptable solutions, and, in the second case a classical minimization procedure. The near-infrared spectrum of the nucleus in M33 is very different from that of a metal poor globular cluster. Our synthesis indicates that the early type spectrum in the blue arises from young components and that the old, very metal-poor contributions at [Z/Z0] ≤ - 1.5 are negligible in this object. The old component with metallicity in the range -1.0 ≤ [Z/Z0 ] ≤ - 0.5, as well as the intermediate age component, are important. The metallicity of the stellar population in the nucleus, at most, has reached up to the solar value. The nucleus in NGC 278 is dominated by young components, at t ≤ 5 x 10 8 yr. The emission-line spectrum suggests that the star formation process is still operating. As for M33, the maximum metallicity in the nucleus is, at most, solar. In NGC 404 the old population with [Z/Z0] ~~ -0.5 is dominant in the nucleus. The young component is weaker than in M33 and in NGC 278.