The WAGGS project-III. Discrepant mass-to-light ratios of Galactic globular clusters at high metallicity

Abstract

Observed mass-to-light ratios (M/L) of metal-rich globular clusters (GCs) disagree with theoretical predictions. This discrepancy is of fundamental importance since stellar population models provide the stellar masses that underpin most of extragalactic astronomy, near and far. We have derived radial velocities for 1,622 stars located in the centres of 59 Milky Way GCs -twelve of which have no previous kinematic information -using integral-field unit data from the WAGGS project. Using N-body models, we determine dynamical masses and M/L V ratios for the studied clusters. Our sample includes NGC 6528 and NGC 6553, which extend the metallicity range of GCs with measured M/L up to [Fe/H] ∼ −0.1 dex. We find that metal-rich clusters have M/L V more than 2 times lower than what is predicted by simple stellar population models. This confirms that the discrepant M/L-[Fe/H] relation remains a serious concern. We explore how our findings relate to previous observations, and the potential causes for the divergence, which we conclude is most likely due to dynamical effects.

MUSE narrow field mode observations of the central kinematics of M15

“…Integral field spectroscopy (e.g. Lanzoni et al 2013;Kamann et al 2014;Dalgleish et al 2020) allows spectra to be observed even in dense fields, especially when used with techniques to reliably extract spectra from blended stars (Kamann, Wisotzki & Roth 2013). With wide field integral field spectrographs, large numbers of spectra can be obtained at once, with Kamann et al (2018a) measuring the radial velocities of over 200 000 stars in 22 GCs using MUSE (the Multi-Unit Spectroscopic E-mail: chris.usher@astro.su.se Explorer, Bacon et al 2010) on the European Southern Observatory Very Large Telescope (VLT).…”

“…Dotter et al 2010;VandenBerg et al 2013), metal-poor ([Fe/H] =−2.33, Carretta et al 2009) GC with a mass of ∼ 5 × 10 5 M (e.g. Dalgleish et al 2020). According to the 2010 edition of the Harris (1996) catalogue, M15 lies 10.4 kpc from the Sun as well as 10.4 kpc from the Galactic centre.…”

Dynamical masses and mass-to-light ratios of resolved massive star clusters -- II. Results for 26 star clusters in the Magellanic Clouds

“…Since the observed 𝑀/𝐿 𝑉 of the clusters in our sample are smaller than SSP model predictions, the argument above implies that a bottom light IMF is needed to reconcile the data and models in the absence of any other effects-such as dynamical evolution as discussed above-which may alter 𝑀/𝐿 𝑉 . We follow the discussion of Dalgleish et al (2020) who defined two simple power-law mass functions (MF). In both cases, 𝛼 3 = 2.3 (the same as the Kroupa IMF), but for lower stellar masses between 0.08 and 1.0 M , 𝛼 1 = 𝛼 2 = 1.3 for their so-called 'bottom-light' MF, and 𝛼 1 = 𝛼 2 = 0.3 for their 'extreme bottom-light' MF.…”

“…Some of these studies have revealed variations of 𝑀/𝐿 ratios with respect to metallicity and cluster masses that do not appear to conform to model expectations (e.g. Strader et al 2009Strader et al , 2011Kimmig et al 2015;Baumgardt 2017;Dalgleish et al 2020).…”

“…Numerous studies have compared predictions from such models with empirical 𝑀/𝐿 ratios, mostly for old globular clusters either in the Milky Way (e.g. Pryor & Meylan 1993;McLaughlin & van der Marel 2005;Kimmig et al 2015;Baumgardt 2017;Baumgardt & Hilker 2018;Dalgleish et al 2020) or local group (LG) galaxies (e.g. Larsen et al 2002;Strader et al 2009Strader et al , 2011.…”