Star formation history and chemical enrichment in the nuclear regions of m31 and its dwarf companions m32 and ngc 205

Abstract

We present a comparative discussion about the stellar population and chemical enrichment in the nuclear regions of M 31 and of its dwarf companions M 32 and NGC 205, from a new set of long slit CCD spectra in the range 3500-10 000 Å. We have performed the population syntheses using a grid of star cluster spectral features as a function of age and metallicity. The algorithm is a multiple minimization procedure, including a statistical treatment of the acceptable solutions and for which we discuss as well the question of the uniqueness of the solution. It provides a more significant result with respect to classical minimization methods relying on a single optimal solution. We find that the semistellar nucleus (SSN) and the bulge of M 31 are dominated by old metal-rich populations. The overall enrichment in heavy elements is however larger in the SSN ([Z/Zʘ ] ~ 0.6 at least) than in the inner bulge ([Z/Zʘ ] = 0.3). In the visible, flux fractions of 10 to 20% arise from an intermediate age component. The old age components span the entire metallicity range, with a predominant contribution from the highly metallic ones. The young age component remains inconspicuous in the visible range. In the nucleus of M 32, the metal enrichment comes out to be around solar and the light is dominated by old components in the range -0.5 < [Z/Zʘ ] <O. As much as 30% ofthe flux at 5870 Å arises from an intermediate age component corresponding to a 15% mass fraction. Both the old metal-poor and the young components in M 32 contribute very little to the visible flux. Finally, our synthesis shows that the dominant population in the nucleus of NGC 205 is young, in the range 10⁸ to 510⁸ yr. The old component with metallicity -1 < [Z/Zʘ ] < -0.5 and the intermediate age components provide each around 20% of the visible light. The maximum metallicity attained in this object is [Z/Zʘ ] = -0.5.