Halo globular clusters observed with AAOmega: dark matter content, metallicity and tidal heating

Abstract

Globular clusters (GCs) have proven to be essential to our understanding of many important astrophysical phenomena. Here, we analyse spectroscopic observations of 10 halo GCs to determine their dark matter (DM) content, their tidal heating by the Galactic disc and halo, describe their metallicities and the likelihood that Newtonian dynamics explains their kinematics. We analyse a large number of members in all clusters, allowing us to address all these issues together, and we have included NGC 288 and M30 to overlap with previous studies. We find that any flattening of the velocity dispersion profiles in the outer regions of our clusters can be explained by tidal heating. We also find that all our GCs have M/LV≲ 5, therefore, we infer the observed dynamics do not require DM, or a modification of gravity. We suggest that the lack of tidal heating signatures in distant clusters indicates the halo is not triaxial. The isothermal rotations of each cluster are measured, with M4 and NGC 288 exhibiting rotation at a level of 0.9 ± 0.1 km s−1 and 0.25 ± 0.15 km s−1, respectively. We also indirectly measure the tidal radius of NGC 6752, determining a more realistic figure for this cluster than current literature values. Lastly, an unresolved and intriguing puzzle is uncovered with regard to the cooling of the outer regions of all ten clusters.

TheGaia-ESO Survey: Kinematics of seven Galactic globular clusters

“…For NGC 6752, we found 159 stars in common with the sample presented by A115, page 4 of 13 Lane et al (2010b), and for these stars we measured a mean difference V r (this paper) − V r (Lane) of -0.95, σ = 1.90 km s −1 . For NGC 1851 we have 104 stars in common with Carretta et al (2010).…”

“…For example, while M 15 has been extensively studied presented a detailed analysis of this cluster based on nearly two thousand V r and proper motions), for NGC 5927 no velocity dispersion profile and no estimate of the central velocity dispersion are available in the literature (Simmerer et al 2013 provided only an estimate of the overall dispersion). For several clusters the samples presented in the literature are smaller than (NGC 6752, NGC 1851; Lane et al 2010b;Scarpa et al 2011;Carretta et al 2010Carretta et al , 2011 or similar to (NGC 4833, NGC 4372;Carretta et al 2014;Kacharov et al 2014) those considered here. An independent check of the Upper panels: we show the distribution of velocity differences with respect to the velocity measured with HR 10 for all the stars observed (from left to right, with HR 21, HR 11 and HR 13) and estimated uncertainties on V r ≤ 1 km s −1 for each measurement.…”

The properties of energetically unbound stars in stellar clusters

“…This flattening has been observed in an increasing number of clusters (Drukier et al 1998;Scarpa et al 2007;Lane et al 2010), although there are many cases where self-consistent models (King 1966;Wilson 1975) have accurately fit the observed velocity dispersion profile of Milky Way and local group clusters (McLaughlin & van der Marel 2005;Barmby et al 2009). It is not understood why some E-mail: i.claydon@surrey.ac.uk (IC); m.gieles@surrey.ac.uk (MG); a.zocchi@surrey.ac.uk (AZ) clusters show this feature and others do not, or how many clusters would be expected to display it.…”

Differences in the rotational properties of multiple stellar populations in M13: a faster rotation for the ‘extreme’ chemical subpopulation

“…Geyer et al 1983;Meylan & Mayor 1986;White & Shawl 1987;Gebhardt et al 1994;Peterson & Cudworth 1994;Meylan & Heggie 1997;van Leeuwen & Le Poole 2002;Anderson & King 2003;Chen & Chen 2010;Lane et al 2010;Bellazzini et al 2012;Bianchini et al 2013;Fabricius et al 2014;Kacharov et al 2014;Lardo et al 2015). That said, the link between rotation and flattening is not necessarily obvious, because other factors such as pressure anisotropy and Galactic tides can lead to flattening (e.g.…”