Structure and Internal Kinematics of Nine Inner Milky Way Globular Clusters

Abstract

This study constitutes part of a larger effort aimed at better characterizing the Galactic globular clusters (GGCs) located towards the inner Milky Way bulge and disk. Here, we focus on internal kinematics of nine GGCs, obtained from space-based imaging over time baselines of >9 years. We exploit multiple avenues to assess the dynamical state of the target GGCs, constructing radial profiles of projected stellar density, proper motion dispersion, and anisotropy. We posit that two-thirds (6/9) of our target GGCs are in an advanced dynamical state, and are close to (or have recently undergone) core collapse, supported by at least two lines of evidence: First, we find relatively steep proper motion dispersion profiles, in accord with literature values for core-collapsed GGCs. Second, we find that our sample is, in the mean, isotropic even out to their half-light radii, although one of our target clusters (NGC 6380) is tangentially anisotropic at >1σ beyond its half-light radius, in accord with theoretical predictions for clusters evolving in strong tidal fields. Our proper motion dispersion and anisotropy profiles are made publicly available.

Relative Ages of Nine Inner Milky Way Globular Clusters from Proper Motion Cleaned Color-Magnitude Diagrams

“…To measure relative proper motions, we obtained a second epoch of deep imaging with ACS/WFC onboard HST (GO-15065, PI:Cohen), using the F606W and F814W filters to produce homogenous photometry with an identical observing setup as that used for the ACS Galactic Globular Cluster Treasury Survey (Sarajedini et al 2007), on which several recent MWGC age compilations are based (Marín-Franch et al 2009, Dotter et al 2010. The details of both epochs of observations for all target clusters, separated by time baselines of 9 yr, are given in Table 1 of Cohen et al (2020) 1 .…”

“…The tools and techniques we used to produce astrophotometric catalogs including PSF photometry and relative proper motions for our target clusters are identical to those used in other recent studies (Bellini et al 2014(Bellini et al , 2017a(Bellini et al , 2018Libralato et al 2018), and further details, including photometric quality cuts and extensive artificial star tests, are given in Cohen et al (2020). The resulting catalogs yield proper-motion-cleaned, Vegamag-calibrated PSF photometry extending several magnitudes faintward of the main sequence turnoff (MSTO) of all target clusters, and as far brightward as the red giant branch (RGB), and in most cases the horizontal branch (HB), limited only by the archival first epoch observations 2 .…”

“…Here, we exploit multi-epoch, homogenous space-based imaging of an ensemble of nine inner Milky Way globular clusters to measure their ages. We have already demonstrated the potential of this dataset from both a photometric and astrometric perspective in Cohen et al (2020), where we analyzed radial density and proper motion dispersion profiles of our target clusters, finding that they are preferentially highly dynamically evolved. Here, we use the propermotion-cleaned color-magnitude diagrams (CMDs) to gain additional insight on the evolutionary history of these clusters to place them on the MWGC age-metallicity relation in the context of their more well-studied counterparts from Sarajedini et al (2007).…”

APOGEE discovery of a chemically atypical star disrupted from NGC 6723 and captured by the Milky Way bulge

“…With the new improved data from Gaia EDR3 (Gaia Collaboration et al 2020), and the available Hubble Space Telescope (HST) imaging data within 1.1 from the cluster center (Sarajedini et al 2007), we can determine updated structural parameters with better precision. To construct number density profiles, we make use of star counts by adopting the same technique as in Cohen et al (2020) to combine both surveys. Briefly, the stellar density is found in annuli of varying radii around the centre of the cluster, and these density values are fitted by a King profile (King 1966).…”