ACSimaging of star clusters in M 51

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 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 eff = 21.6 pc. The median R 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 eff with colour as well as a higher fraction of clusters with B−V > ∼ 0.05 in the interarm regions. We find a correlation between R eff and galactocentric distance (R G ) of the form R eff ∝ R 0.12±0.02 G , which is considerably weaker than the observed correlation for old Milky Way GCs. We find weak relations between cluster luminosity and radius: R eff ∝ L 0.15±0.02 for the interarm regions and R eff ∝ L −0.11±0.01 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.

Inverse dynamical population synthesis

“…(7)). This relation implies that the radii of birth clusters have a weak stellar mass dependence, r h ∝ M 0.13 ± 0.04 ecl , that is consistent with other observational data (Zepf et al 1999;Larsen 2004;Scheepmaker et al 2007). King et al (2012) compare the binary properties of five young clusters analysed here (except USco-A).…”

“…Zepf et al (1999) and Scheepmaker et al (2007) similarly demonstrated that there is a mass -radius relation for clusters that is shallow for both the galaxy merger NGC 3256 and young clusters in M 51. In Fig.…”

The Age, Mass, and Size Distributions of Star Clusters in M51

“…Previously, Scheepmaker et al (2007) noted an increase in size for redder clusters in M51, although they were unable to fully quantify the evolution in cluster size because they did not have age estimates for their full cluster sample. Bastian et al (2008) compiled information on clusters in M51, the Magellanic Clouds and M82, and found a similar trend in the age-size relationship as found here.…”

“…M51 is a nearby, grand-design spiral with a rich population of star clusters that has played a critical role in understanding cluster formation and evolution (e.g., Boutloukos & Lamers 2003;Bastian et al 2005;Gieles et al 2005;Scheepmaker et al 2007;Hwang & Lee 2010;Chandar et al 2011). In this work we present a catalog of compact star clusters selected from the beautiful mosaic images taken with the WFC camera on the Advanced Camera for Surveys (ACS).…”

The effect of primordial mass segregation on the size scale of globular clusters

“…Observations of MW GCs, however, give the empirical relation of r h ∝ R 1/2 G (VandenBerg 1991;McLaughlin 2000). This is closer to the Roche lobe filling relation, which is less steep with r h ∝ R 2/3 G (Scheepmaker et al 2007). …”