A base of star clusters for stellar population synthesis

Abstract

Integrated spectra of 63 star clusters with 106 yr ≤age ≤ 1.65 1010 yr and -2.1 [Z/Zʘ] ≤ 0.1 are presented in this paper. The useful wavelength range 3780, 7690 Å was split into 70 consecutive windows, 24-190 Å wide. The main contributors to the absorption in each window were searched for: apart from the strong well-known absorption features, we detected a number of weaker absorptions quite often due to molecular bands arising from red stars. The equivalent widths W of the spectral features, as well as the continuous distribution are studied as a function of age and metallicity. Metallic lines and molecular bands present essentially the same behaviour with metallicity. In particular, we show the influence of younger turn-off points on W , for features in the blue part of the spectrum. Indeed, the top main sequence stars then enhance the continuum, defining in the W vs. metallicity plots several loci depending on the age. On the contrary, the W vs. metallicity relationship in the red, tends to be single-valued, regardless of age. For the windows containing Balmer lines from Hα to Hδ, the age dependence looks similar whatever the line, with a maximum Wvalue for t ~ 4 10 8 yr. If any, the effect of different metallicities is weak in these windows. For a given age, W(Hα) is always smaller than W(Hβ), W(Hy) or W(Hδ), dueto the fact that its underlying continuum is dominated by late-type stars which contribute very little to the line absorption. For He to H 10 lines, strongly blended with metallic features, a metal-dominated behaviour is observed. The continuous distribution of metal-poor globular clusters is strikingly different from that of young blue clusters of any age or metallicity, while their Balmer equivalent widths partly overlap. These results forma base of star cluster properties which will be used for stellar population synthesis in galaxies. The comparison of W for metallic features in the star clusters and in a sample of 152 nuclei of normal galaxies having types from E to Se and absolute luminosities MB from -16.6 to -23.3, shows that around 50 % of the nuclei can be population synthesized with the present cluster base. An extrapolation of the base properties to a metallicity [Z/Zʘ] = 0.6 is required to describe the entire galaxy sample.