Star count density profiles and structural parameters of 26 Galactic globular clusters

TL;DR

This study provides a comprehensive catalog of star count density profiles and structural parameters for 26 Galactic globular clusters, challenging previous findings of central cusps and analyzing their dynamical states.

Contribution

It presents the largest homogeneous catalog of star count profiles for globular clusters, comparing King and Wilson models, and explores their structural and dynamical properties.

Findings

Central cusps previously detected are not confirmed.

Most clusters fit both King and Wilson models equally well.

Cluster size correlates with galactocentric distance.

Abstract

We used a proper combination of high-resolution HST observations and wide-field ground based data to derive the radial star density profile of 26 Galactic globular clusters from resolved star counts (which can be all freely downloaded on-line). With respect to surface brightness (SB) profiles (which can be biased by the presence of sparse, bright stars), star counts are considered to be the most robust and reliable tool to derive cluster structural parameters. For each system a detailed comparison with both King and Wilson models has been performed and the most relevant best-fit parameters have been obtained. This is the largest homogeneous catalog collected so far of star count profiles and structural parameters derived therefrom. The analysis of the data of our catalog has shown that: (1) the presence of the central cusps previously detected in the SB profiles of NGC 1851, M13 and M62 is not confirmed; (2) the majority of clusters in our sample are fitted equally well by the King and the Wilson models; (3) we confirm the known relationship between cluster size (as measured by the effective radius) and galactocentric distances; (4) the ratio between the core and the effective radii shows a bimodal distribution, with a peak at ~ 0.3 for about 80% of the clusters, and a secondary peak at ~ 0.6 for the remaining 20%. Interestingly, the main peak turns out to be in agreement with what expected from simulations of cluster dynamical evolution and the ratio between these two radii well correlates with an empirical dynamical age indicator recently defined from the observed shape of blue straggler star radial distribution, thus suggesting that no exotic mechanisms of energy generation are needed in the cores of the analyzed clusters.