Astronomy & Astrophysics

Issue		A&A Volume 469, Number 3, July III 2007
Page(s)		925 - 940
Section		Extragalactic astronomy
DOI		10.1051/0004-6361:20077511

A&A 469, 925-940 (2007)
DOI: 10.1051/0004-6361:20077511

ACS imaging of star clusters in M 51

I. Identification and radius distribution

R. A. Scheepmaker¹, M. R. Haas¹, M. Gieles¹, N. Bastian², S. S. Larsen¹, and H. J. G. L. M. Lamers¹

¹ Astronomical Institute, Utrecht University, Princetonplein 5, 3584 CC Utrecht, The Netherlands
e-mail: scheepmaker@astro.uu.nl
² Department of Physics and Astronomy, University College London, Gower Street, London, WC1E 6BT, UK

(Received 20 March 2007 / Accepted 23 April 2007)

Abstract
Context.Size measurements of young star clusters are valuable tools to put constraints on the formation and early dynamical evolution of star clusters.
Aims.We use HST/ACS observations of the spiral galaxy M 51 in F435W, F555W and F814W to select a large sample of star clusters with accurate effective radius measurements in an area covering the complete disc of M 51. We present the dataset and study the radius distribution and relations between radius, colour, arm/interarm region, galactocentric distance, mass and age.
Methods.We select a sample of 7698 (F435W), 6846 (F555W) and 5024 (F814W) slightly resolved clusters and derive their effective radii $(R_{\rm eff})$ by fitting the spatial profiles with analytical models convolved with the point spread function. The radii of 1284 clusters are studied in detail.
Results.We find cluster radii between 0.5 and ~10 pc, and one exceptionally large cluster candidate with $R_{\rm eff}$ = 21.6 pc. The median $R_{\rm eff}$ is 2.1 pc. We find 70 clusters in our sample which have colours consistent with being old GC candidates and we find 6 new "faint fuzzy" clusters in, or projected onto, the disc of M 51. The radius distribution can not be fitted with a power law similar to the one for star-forming clouds. We find an increase in $R_{\rm eff}$ with colour as well as a higher fraction of clusters with $B-V \ga$ 0.05 in the interarm regions. We find a correlation between $R_{\rm eff}$ and galactocentric distance (R_G) of the form $\mbox{$R_{\rm eff}$ }\propto R_{\mathrm^{0.12\pm0.02}$ , which is considerably weaker than the observed correlation for old Milky Way GCs. We find weak relations between cluster luminosity and radius: $\mbox{$R_{\rm eff}$ }\propto L^{0.15\pm0.02}$ for the interarm regions and $\mbox{$R_{\rm eff}$ }\propto L^{-0.11\pm0.01}$ for the spiral arm regions, but we do not observe a correlation between cluster mass and radius.
Conclusions.The observed radius distribution indicates that shortly after the formation of the clusters from a fractal gas, the radii of the clusters have changed in a non-uniform way. We find tentative evidence suggesting that clusters in spiral arms are more compact.

Key words: galaxies: individual: M 51 -- galaxies: star clusters

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