The Luminosity Function of Star Clusters in 20 Star-Forming Galaxies Based on Hubble Legacy Archive Photometry

Abstract

Luminosity functions have been determined for star cluster populations in 20 nearby (4-30 Mpc), star-forming galaxies based on ACS source lists generated by the Hubble Legacy Archive. These cluster catalogs provide one of the largest sets of uniform, automatically-generated cluster candidates available in the literature at present. Comparisons are made with other recently generated cluster catalogs demonstrating that the HLA-generated catalogs are of similar quality, but in general do not go as deep. A typical cluster luminosity function can be approximated by a power-law, dN/dL ∝ L α , with an average value for α of −2.37 and RMS scatter = 0.18 when using the F814W ("I") band. A comparison of fitting results based on methods which use binned and unbinned data shows good agreement, although there may be a systematic tendency for the unbinned (maximum likelihood) method to give slightly more negative values of α -2for galaxies with steeper luminosity functions. We find that galaxies with high rates of star formation (or equivalently, with the brightest or largest numbers of clusters) have a slight tendency to have shallower values of α. In particular, the Antennae galaxy (NGC 4038/39), a merging system with a relatively high star formation rate, has the second flattest luminosity function in the sample. A tentative correlation may also be present between Hubble Type and values of α, in the sense that later type galaxies (i.e., Sd and Sm) appear to have flatter luminosity functions. Hence, while there do appear to be some weak correlations, the relative similarity in the values of α for a large number of star-forming galaxies suggests that, to first order, the LFs are fairly universal. We examine the bright end of the luminosity functions and find evidence for a downturn, although it only pertains to about 1% of the clusters. Our uniform database results in a small scatter (≈0.4 to 0.5 mag) in the correlation between the magnitude of the brightest cluster (M brightest ) and log of the number of clusters brighter than M I = −9 (log N). We also examine the magnitude of the brightest cluster vs. log SFR for a sample including both dwarfs galaxies and ULIRGS. This shows that the correlation extends over roughly six orders of magnitudes but with scatter that is larger than for our spiral sample, probably because of the high levels of extinction in many of the LIRG galaxies.

Probing the role of the galactic environment in the formation of stellar clusters, using M83 as a test bench

“…Some previous works in the past have found evidence of a possible truncation and/or variation of the ICMF within the same galaxy (e.g., Larsen 2009;Bastian et al 2012;Konstantopoulos et al 2013) or steepening of the cluster luminosity function (recently Whitmore et al 2014). In our work we observe a clear steepening of the ICMF, well represented by a Schechter type distribution of slope −2, and a variable Mc as function of distance from the centre of the galaxy.…”

“…The M 83 brightest young cluster of each the four bins of equal area are plotted as purple circles. The orange diamonds show the sample from Whitmore et al (2014). The red solid lines show Monte Carlo simulations of cluster populations formed with a Schechter CMF with different Mc and a cluster formation efficiency of 8%.…”

“…Table B1. a) Adamo et al (2011) (blue star symbols in Figure 6); b) Larsen (2002) (black triangles in Figure 6), only a fraction of the original catalogue is listed while the values of the omitted ones are listed with the most recent estimated values available in the literature; c) The values used for each binning (purple filled dots in Figure 6) are reported in Table 2 of the main text; d) Bastian (2008) (black squares in Figure 6); e) Whitmore et al (2014) (orange diamonds in Figure 6), some of the galaxies in the original list have been omitted because Larsen (2002) presented data with a larger coverage of the galaxy than in this paper. The dwarf galaxies (green star symbols in Figure6 Complete compilation of the data available in the literature and plotted in Figure 6 and 4.…”

Little Blue Dots in the Hubble Space Telescope Frontier Fields: Precursors to Globular Clusters?

“…For high SFR, M c can be larger than 10 6 M ⊙ (e.g., Zhang & Fall 1999) and the distribution function is essentially a power law up to a globular cluster mass. Considering this limit, or the case of a pure power law (Whitmore et al 2014), integrals over the distribution function are analytic, and the total mass of stars that accompanies a single massive cluster with M GC = 10 6 M ⊙ is:…”

Global survey of star clusters in the Milky Way

“…Larsen 2002;de Grijs et al 2003;Gieles et al 2006;Anders et al 2007;Mora et al 2009;Whitmore et al 2014). However, studies of the CLF in the Milky Way are more informative, since local clusters have a number of advantages over those in extragalactic systems.…”