Globular cluster number density profiles using Gaia DR2

Abstract

Using data from Gaia DR2, we study the radial number density profiles of the Galactic globular cluster sample. Proper motions are used for accurate membership selection, especially crucial in the cluster outskirts. Due to the severe crowding in the centres, the Gaia data is supplemented by literature data from HST and surface brightness measurements, where available. This results in 81 clusters with a complete density profile covering the full tidal radius (and beyond) for each cluster. We model the density profiles using a set of single-mass models ranging from King and Wilson models to generalised lowered isothermal LIMEPY models and the recently introduced SPES models, which allow for the inclusion of potential escapers. We find that both King and Wilson models are too simple to fully reproduce the density profiles, with King (Wilson) models on average underestimating(overestimating) the radial extent of the clusters. The truncation radii derived from the LIMEPY models are similar to estimates for the Jacobi radii based on the cluster masses and their orbits. We show clear correlations between structural and environmental parameters, as a function of Galactocentric radius and integrated luminosity. Notably, the recovered fraction of potential escapers correlates with cluster pericentre radius, luminosity and cluster concentration. The ratio of half mass over Jacobi radius also correlates with both truncation parameter and PE fraction, showing the effect of Roche lobe filling.

The peculiar kinematics of the multiple populations in the globular cluster Messier 80 (NGC 6093)

“…In light of the results of Dalessandro et al (2018b), who used artificial star tests to infer that their star counts are complete at a > 95% level, no incompleteness corrections were applied. The resulting surface density profiles for the four populations are shown in Fig 2. As the photometry of Dalessandro et al (2018a) is limited to the central region of the cluster, we complemented our global number density profile with the Gaia data recently presented by de Boer et al (2019). After accounting for a vertical offset, the two profiles were stitched together.…”

“…While the ellipticities of most clusters are still poorly known, Gaia nowadays provides us with the data necessary to improve the situation. de Boer et al (2019) provided source lists of cluster members for all of the objects included in their study. We obtained their source list for NGC 6093 and determined the eigenvalues and eigenvectors of the two-dimensional cluster member distributions in radial bins around the centre (similar to Fabricius et al 2014;Kamann et al 2018).…”

“…3. Note that we restricted our analysis of the Gaia data to the region outside the incompleteness limit of 2.59 determined by de Boer et al (2019) and used the results from the HST star counts presented in Kamann et al (2018) to complement it in the central region. The uncertainties shown in Fig.…”

High-resolution Optical Spectroscopy of Stars in the Sylgr Stellar Stream*

“…I19 note that the Sylgr stream has similar energy and angular momentum to the GCs M10 (NGC 6254) and M12 (NGC 6218). Neither cluster has an obvious excess of extra-tidal stars (de Boer et al 2019;Kundu et al 2019). I19 excluded these GCs as the progenitor based on their less radial orbits with smaller apocenters (< 5 kpc; Baumgardt et al 2019) and the much higher metallicities of M10 and M12 ([Fe/H] ≈ −1.6 and −1.3; Carretta et al 2009a) relative to the SSPP metallicity estimates.…”

The loss of the intracluster medium in globular clusters

“…gas accumulation) or an asymptotic behaviour indicating a steady state. We choose this radius as it is a good approximation for the half-mass radius of globular clusters in general (Baumgardt & Hilker 2018;de Boer et al 2019). Indeed, most of the observations to detect the ICM in GCs are focused on their core region and are done within this radius.…”