Integrated spectral properties of star clusters in the near-ultraviolet

Abstract

We study integrated spectra of 37 star clusters in the range 3100-4200 Å. The sample includes Galactic globular and open clusters, as well as Magellanic Cloud clusters. Feature equivalent widths (W) and continuum distribution are analyzed as a function of age and metallicity. In particular we discuss the behaviour of the Balmer Jump and of the 4000 Å Break, which are measured by the same continuum ratio C3660 Å / C4020 Å. For ages t < 500 Myr the Balmer Jump is dominant over the 4000 Å Break and for t < 50 Myr it is an excellent age discriminator. The effects of blue Horizontal Branch morphology on globular cluster spectra are also discussed: they increase the Balmer line W, but do not dilute substantially the Near­ UV metallic features. The present data will provide a means to connect optical to UV integrated star cluster spectra, with im­ portant applications in stellar population analyses, in particular for distant galaxies at redshift up to 1.