Near-infrared spectral properties of star clusters and galactic nuclei

Abstract

We present CCD spectra with 12.5 A resolution from 6300 to 9700 Å of 30 star clusters covering ranges 106 to 1.65 1010 yr in age, and -2.0 to 0.1 in metallicity, [Z/Zʘ]. As well, 62 galactic nuclei have been observed in galaxies with morpholog­ ical types E to Se and intrinsic luminosities -23.3≤Mb≤-16.7. For every object in the present sample, the visible spectrum has been discussed in earlier papers. We describe a powerful method for correcting CCD fringes in the near-infrared. We measure the near-infrared continuum distribution and the equivalent widths (W) of 13 absorption features. Analysis of the star cluster sample indicates that in the near-infrared spectral range, metallicity is the dominant parameter. Age produces second order effects of various types: (i) enhancement of molecular bands in certain phases of a blue cluster evolution by the accumulation ofluminous red stars, (ii) contamination of metallic !ines with Paschen absorption !ines and, (iii) slight steepening of the continuum slope for young clusters. In view of population synthesis of galactic nuclei using the star clusters, we present grid predictions as a function of metallicity and age for 5 metallic features and for the continuum distribution. Spiral galaxies and luminous elliptical galaxies show similar strong-lined spectra suggesting that their metallicities in the central regions are comparable. Even very blue galaxies exhibit in the near-infrared spectra quite similar to those of more classical galaxies. Evidence is found however that a large flux contribution to this range is not from their old underlying population. Strong-lined globular clusters like NGC 6528 have integrated spectra comparable to those of massive galaxies. Consequently, they are of a great help in population synthesis work, whatever their precise metallicity value.