Globular Cluster Systems in external galaxies

People involved at OAB: Cacciari, Federici, Galleti

Spectroscopy of extragalactic globular clusters provides a wealth of information on the formation and evolution of their parent galaxies. The aim of this project is to study the globular cluster systems in galaxies of different morphological types (E/S0 and spirals), in order to investigate the existence of stellar sub-populations with different chemical and/or dynamical characteristics, to estimate the galaxy mass and to probe the existence of a dark matter halo.

In this framework, we have obtained deep MOS spectroscopy using FORS1 at the VLT of about 75 globular clusters candidates in the Sombrero galaxy (NGC4594), an early-type spiral with a dominant bulge.

Wide field imaging (BVR) of NGC 4594 was taken with the ESO/MPG 2.2m telescope. The sample of highly probable globular cluster candidates selected using color and shape criteria shows a bimodal color distribution and has been observed with VIMOS in April 2003 during Period 71. Fifty-seven candidates out to $\sim 40$ kpc in the halo of the galaxy were confirmed to be bona-fide globular clusters, 27 of which are new. The metallicities, derived from absorption line indices, confirm a bimodal [Fe/H] distribution, in agreement with the results of the photometry (Moretti et al. 2003). The distribution was fitted with two gaussians peaking at $[Fe/H] \approx -1.7$ and $[Fe/H] \approx -0.7$, very similar to those of globular clusters in our Galaxy. Preliminary results have been presented by Held et al. (2003).

Since efficient spectroscopic observations require a previous identification of a sample of bona-fide globular cluster candidates, and in preparation for a systematic spectroscopic study at the VLT with FLAMES or VIMOS, we have undertaken a wide-field multicolor imaging survey of galaxies of different morphological types (E/S0) as far as the Virgo cluster: NGC 3115, NGC 4526, NGC 4406, NGC 253, NGC 5128, NGC 4594. For one of these galaxies, NGC 253, we have already analysed our 2.2m-WFI BVI images (FoV $=33\times34$ arcmin) and identified about 400 globular cluster candidates (Galleti et al. 2004). This number of candidates is much larger than the numbers found in all previous surveys of this galaxy, and even taking into account a significant degree of contamination it represents a major improvement over all previous studies.

This research is in collaboration with Held, Moretti, Rizzi (INAF - Padova Obs.) and Testa (INAF - Rome Obs.)