[Paper Review] ACS imaging of star clusters in M51. I. Identification and radius distribution

R. A. Scheepmaker, M. Haas+4 more|ArXiv.org|Apr 26, 2007

Stellar, planetary, and galactic studies62 references60 citations

TL;DR

This study uses Hubble Space Telescope/ACS imaging to identify 7,698 star clusters in M51 and measure their effective radii with high precision. It finds that cluster radii range from 0.5 to 21.6 pc, with a median of 2.1 pc, and reveals a non-power-law radius distribution, suggesting non-uniform early dynamical evolution; clusters in spiral arms are more compact, and radius increases with age (as traced by colour), while no strong mass-radius correlation is observed.

ABSTRACT

We use HST/ACS observations of the spiral galaxy M51 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 M51. We present the dataset and study the radius distribution and relations between radius, colour, arm/interarm region, galactocentric distance, mass and age. We select a sample of 7698 (F435W), 6846 (F555W) and 5024 (F814W) slightly resolved clusters and derive their effective radii by fitting the spatial profiles with analytical models convolved with the point spread function. The radii of 1284 clusters are studied in detail. We find cluster radii between 0.5 and ~10 pc, and one exceptionally large cluster candidate with a radius of 21.6 pc. The median radius 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 M51. The radius distribution can not be fitted with a power law, but a log-normal distribution provides a reasonable fit to the data. This indicates that shortly after the formation of the clusters from a fractal gas, their radii have changed in a non-uniform way. We find an increase in radius with colour as well as a higher fraction of redder clusters in the interarm regions, suggesting that clusters in spiral arms are more compact. We find a correlation between radius and galactocentric distance which is considerably weaker than the observed correlation for old Milky Way GCs. We find weak relations between cluster luminosity and radius, but we do not observe a correlation between cluster mass and radius.

Motivation & Objective

• To identify and measure the effective radii of a large sample of star clusters in the spiral galaxy M51 using Hubble Space Telescope/ACS data.
• To investigate the radius distribution of young and intermediate-age star clusters and compare it to theoretical models of star-forming clouds.
• To examine correlations between cluster effective radius and physical parameters such as galactocentric distance, luminosity, colour, age, and environment (spiral arm vs. interarm regions).
• To search for evidence of dynamical evolution in cluster size by analyzing trends with colour and spatial location.
• To identify potential old globular cluster candidates and faint fuzzy clusters in the M51 disc.

Proposed method

• Acquired deep ACS F435W, F555W, and F814W imaging of the M51 disc from the Hubble Space Telescope archive.
• Identified 7,698 slightly resolved star clusters using photometric and morphological criteria across three filters.
• Fitted cluster surface brightness profiles with analytical models convolved with the instrument's point spread function (PSF) to derive effective radii ($R_{ ext{eff}}$) for 1,284 clusters with high accuracy.
• Used colour-colour and colour-magnitude criteria to estimate cluster ages and metallicities, and to classify old globular cluster candidates and faint fuzzy clusters.
• Analyzed the radius distribution and tested for power-law fits, comparing results to those of star-forming clouds and old Galactic globular clusters.
• Performed statistical analysis of $R_{ ext{eff}}$ correlations with galactocentric distance, luminosity, mass, and environment (spiral arm vs. interarm regions), using power-law fitting and error propagation.

Experimental results

Research questions

• RQ1 What is the distribution of effective radii among star clusters in the M51 disc, and how does it compare to the radius distribution of star-forming molecular clouds?
• RQ2 Is there a correlation between cluster effective radius and galactocentric distance, and how does it compare to the relation observed in old Milky Way globular clusters?
• RQ3 Do clusters in spiral arms differ in size from those in interarm regions, and is this related to environmental pressure or formation history?
• RQ4 Is there a measurable correlation between cluster mass and effective radius, and what does its absence imply about early cluster evolution?
• RQ5 How do cluster radii correlate with colour (as a proxy for age), and what does this imply about dynamical expansion over time?

Key findings

• The effective radius distribution of M51 star clusters spans 0.5 to ~10 pc, with a median of 2.1 pc, and one exceptionally large cluster candidate at $R_{ ext{eff}} = 21.6$ pc.
• The radius distribution does not follow a power law like that of star-forming clouds, indicating non-uniform early dynamical evolution after cluster formation.
• Clusters in spiral arms have a smaller median radius (1.9 pc) than those in interarm regions (2.7 pc), suggesting that clusters in high-pressure environments are more compact.
• A weak correlation is found between $R_{ ext{eff}}$ and galactocentric distance, scaling as $R_{ ext{eff}} ightarrow R_{ ext{G}}^{0.12 /pm 0.02}$, much weaker than the relation seen in old Galactic globular clusters.
• No significant correlation is found between cluster mass and effective radius, despite accurate mass estimates for 271 clusters.
• Weak luminosity-radius relations are observed: $R_{ ext{eff}} ightarrow L^{0.15 /pm 0.02}$ in interarm regions and $R_{ ext{eff}} ightarrow L^{-0.11 /pm 0.01}$ in spiral arms, suggesting environment-dependent size evolution.