ar X iv : a st ro - p h / 06 05 02 2 v 1 1 M ay 2 00 6 The radii of thousands of star clusters in M 51 with HST / ACS

1 M51 – a star cluster laboratory The young (< 1 Gyr) star cluster population of M51 (NGC 5194, Hubble-type Sc) is, due to its relative proximity and high number of star clusters, an excellent candidate for studies to the formation and evolution of young star clusters (YCs) in spiral disks. M51 has many star clusters because of its high star formation rate, which may have been caused by its interaction with NGC 5195, and because the disk is almost face-on, we can observe the clusters throughout the disk without being hampered by extinction too much. The new HST/ACS mosaic image of M51, taken as part of the Hubble Heritage program [13], covers the entire disk in 6 pointings in F435W (∼ B), F555W (∼ V), F814W (∼ I) and F658N (Hα), with a resolution of ∼2 pc per pixel. Previous studies on the star cluster population of M51 used lower resolution WFPC2 data that did not cover the entire disk (see e.g. [10, 1, 11]). In the study of [1], clusters were selected based on their fit to SSP models, which led to a possible contamination of individual stars. In a different study clusters were selected based on their sizes, which made it difficult to select a large sample due to the limited resolution [11]. However, by exploiting the superb resolution of the ACS camera and the large field of view of ∼ 17.5 × 24.8 kpc, we now can select a large sample of clusters based on their sizes. This sample can then be used to study the formation history and evolution of the star cluster population and to derive infant mortality rates and study how these depend on the environment. Since this is work in progress, we here select a smaller preliminary sample of clusters and we present some first results that can already be derived by looking at the sizes and the positions of the star clusters in the disk of M51.