Testing Newtonian gravity with AAOmega: mass-to-light profiles and metallicity calibrations from 47 Tuc and M55

Abstract

Globular clusters (GCs) are an important test bed for Newtonian gravity in the weak‐acceleration regime, which is vital to our understanding of the nature of the gravitational interaction. Recent claims have been made that the velocity dispersion profiles of GCs flatten out at large radii, despite an apparent paucity of dark matter (DM) in such objects, indicating the need for a modification of gravitational theories. We continue our investigation of this claim, with the largest spectral samples ever obtained of 47 Tucanae (47 Tuc) and M55. Furthermore, this large sample allows for an accurate metallicity calibration based on the equivalent widths of the calcium triplet lines and K‐band magnitude of the Tip of the Red Giant Branch. Assuming an isothermal distribution, the rotations of each cluster are also measured with both clusters exhibiting clear rotation signatures. The global velocity dispersions of NGC 121 and Kron 3, two GCs in the Small Magellanic Cloud, are also calculated. By applying a simple dynamical model to the velocity dispersion profiles of 47 Tuc and M55, we calculate their mass‐to‐light profiles, total masses and central velocity dispersions. We find no statistically significant flattening of the velocity dispersion at large radii for M55, and a marked increase in the profile of 47 Tuc for radii greater than approximately half the tidal radius. We interpret this increase as an evaporation signature, indicating that 47 Tuc is undergoing, or has undergone, core‐collapse, but find no requirement for DM or a modification of gravitational theories in either cluster.

A Chemical Composition Survey of the Iron-complex Globular Cluster NGC 6273 (M19)*

“…Many globular clusters have been shown to rotate with amplitudes of the order of a few kms 1 (e.g., Côté et al 1995;Lane et al 2009Lane et al , 2010aBellazzini et al 2012;Bianchini et al 2013;Kacharov et al 2014;Kimmig et al 2015;Lardo et al 2015). In Figure 4, we investigated net rotation in NGC 6273 by following a standard technique in which the average radial velocity is calculated for stars on either side of an imaginary line passing through the cluster center.…”

TheGaia-ESO Survey: Kinematics of seven Galactic globular clusters

“…To do this, we used the same method as adopted by Cote et al (1995), Pancino et al (2007), Lane et al (2009Lane et al ( , 2010a, and B12. Rotations were measured by halving the cluster by position angle (PA) 8 and calculating the mean radial velocity of each half.…”

“…The samples we analyse were collected for a completely different scientific purpose, therefore they present intrinsic limitations for the characterisation of the cluster kinematics. The most recent dedicated studies used up to several hundred radial velocity determinations (see e.g., Lane et al 2009Lane et al , 2010a, in some cases complemented with proper motions McLaughlin et al 2006;Watkins et al 2013), while we have V r determinations for fewer than 100 stars for some clusters (i.e., NGC 2808, NGC 4833, NGC 5927). Furthermore, the cluster members are unevenly distributed with radius within each cluster, with the large majority of the stars lying at distances greater than the half-light radius, because it is difficult to allocate fibers in the very crowded central regions.…”

Deriving metallicities from calcium triplet spectroscopy in combination with near-infrared photometry

“…However, the scales of Zinn & West (1984) and Carretta et al (2009) are not correlated properly by a simple linear relation, but by at least a quadratic expression. Previous works that used NIR photometry to correct for the CaT are Lane et al (2010) and Warren & Cole (2009), but the former used the K s magnitude of the tip of the RGB as reference level, while the latter took the value of K s at the RR Lyrae instability strip for the GCs.…”