Is there a size difference between red and blue globular clusters?

Abstract

Blue (metal-poor) globular clusters are observed to have half-light radii that are ∼20 per cent larger than their red (metal-rich) counterparts. The origin of this enhancement is not clear and differences in either the luminosity function or in the actual size of the clusters have been proposed. I analyse a set of dynamically self-consistent Monte Carlo globular cluster simulations to determine the origin of this enhancement. I find that my simulated blue clusters have larger half-light radii due to differences in the luminosity functions of metal-poor and metal-rich stars. I find that the blue clusters can also be physically larger, but only if they have a substantial number of black holes dynamically heating their central regions. For some combinations of metallicity and initial conditions the difference in luminosity function is sufficient to explain the observed effect while for other combinations the size enhancement due to black holes is necessary to match the observations. I conclude that the observed difference in half-light radii between red and blue globular clusters is due to a combination of differences in size and luminosity function. I find an additional corollary: the half-light radius is not a straightforward proxy for cluster size.Key words: methods: numerical -methods: statistical -binaries: close -globular clusters: general -galaxies: star clusters: general. I N T RO D U C T I O NGlobular cluster (GC) systems are common in disc and elliptical galaxies, and in the past decade significant observational progress has been made in understanding them. GCs are frequently used to help understand the formation and evolution of their host galaxies but the observational properties of GCs are not fully understood. A case in point is the half-light radius (r hl ). Several models show that the half-light radius of a cluster should remain fairly constant during its lifetime (Spitzer & Thuan 1972;Aarseth & Heggie 1998) so it is often used to compare the structures of GCs of different ages. However, several observational studies show that metal-poor (blue) GCs have half-light radii that are systematically larger (by ∼20 per cent) than their metal-rich (red) counterparts (Kundu & Whitmore 1998Kundu et al. 1999;Puzia et al. 1999;Larsen et al. 2001a;Larsen, Forbes & Brodie 2001b;Barmby, Holland & Huchra 2002;Harris et al. 2002;Jordán et al. 2004;Harris 2009).The origin of this discrepancy is not clear and it is not even certain that it truly represents a difference in the sizes of the clusters as measured by their half-mass radii (r hm ). Larsen & Brodie (2003) proposed that the observed enhancement in the size of the blue clusters could be due to a projection effect. They note both that blue and red GCs follow different radial distributions and that in the Milky Way there is a relationship between cluster size and galactocentric distance (van den Bergh, Morbey & Pazder 1991). They argue that E-mail: downin@ari.uni-heidelberg.de if such a relationship exists in all galaxies, the differing radial distribut...

Impact of metallicity on the evolution of young star clusters

“…According to Jordán (2004), the observed difference in the half-light radii does not imply a difference in the half-mass radii. Recent Monte Carlo (Downing 2012) and N -body (Sippel et al 2012; see also Hurley et al 2004) simulations of GCs confirm the results by Jordán (2004), finding that the difference in the half-light radii arises from the dependence of luminosity and stellar lifetime on metallicity. According to Downing (2012), there may be even a difference in the half-mass radii, but only as a consequence of dynamical interactions, mainly due to the presence of massive stellar black holes (BHs).…”

“…Recent Monte Carlo (Downing 2012) and N -body (Sippel et al 2012; see also Hurley et al 2004) simulations of GCs confirm the results by Jordán (2004), finding that the difference in the half-light radii arises from the dependence of luminosity and stellar lifetime on metallicity. According to Downing (2012), there may be even a difference in the half-mass radii, but only as a consequence of dynamical interactions, mainly due to the presence of massive stellar black holes (BHs). Finally, Schulman, Glebbeek & Sills (2012) ran N -body simulations of young intermediatemass (10 3 − 10 4 M⊙) SCs with different metallicities.…”

The mass spectrum of compact remnants from the parsec stellar evolution tracks

“…BHs with mass 10 2 − 10 5 M ). Finally, compact remnants are also expected to affect the overall dynamical evolution of star clusters (Downing 2012;Sippel et al 2012;Trani et al 2014).…”

N-body models of globular clusters: metallicities, half-light radii and mass-to-light ratios

“…Recently, Downing () has published a set of Monte Carlo models exploring the origin of the observed size difference between metal‐rich and metal‐poor GCs, which provides an excellent comparison for our work. This follows from the N ‐body models of Schulman, Glebbeek & Sills (), who investigated the evolution of half‐mass radius with metallicity in small‐ N clusters.…”