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Register a new userDynamical Masses and Mass-to-light Ratios of Resolved Massive Star Clusters. I. NGC 419 and NGC 1846
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As an introduction of a kinematic survey of Magellanic Cloud (MC) star clusters, we report on the dynamical masses and mass-to-light ($M/L$) ratios of NGC 419 (SMC) and NGC 1846 (LMC). We have obtained more than one hundred high-resolution stellar spectra in and around each cluster using the multi-object spectrograph M2FS on the $Magellan$/Clay Telescope. Line-of-sight velocities and positions of the stars observed in each cluster were used as input to an expectation-maximization algorithm used to estimate cluster membership probabilities, resulting in samples of 46 and 52 likely members ($P_{M}geq 50$%) in NGC 419 and NGC 1846, respectively. This process employed single-mass King models constrained by the structural parameters of the clusters and provided self-consistent dynamical mass estimates for both clusters. Our best-fit results show that NGC 419 has a projected central velocity dispersion of $2.44^{+0.37}_{-0.21} {rm km,s^{-1}}$, corresponding to a total mass of $7.6^{+2.5}_{-1.3}times10^4 {rm M}_{odot}$ and $V$-band $M/L$ ratio of $0.22^{+0.08}_{-0.05}$ in solar units. For NGC 1846, the corresponding results are $2.04^{+0.28}_{-0.24} {rm km,s^{-1}}$, $5.4^{+1.5}_{-1.4}times10^4 {rm M}_{odot}$ and $0.32^{+0.11}_{-0.11}$. The mean metallicities of NGC 419 and NGC 1846 are found to be $rm [Fe/H]=-0.84pm0.19$ and $-0.70pm0.08$, respectively, based on the spectra of likely cluster members. We find marginal statistical evidence of rotation in both clusters, though in neither cluster does rotation alter our mass estimates significantly. We critically compare our findings with those of previous kinematic studies of these two clusters in order to evaluate the consistency of our observational results and analytic tools.
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We present spectroscopy of individual stars in 26 Magellanic Cloud (MC) star clusters with the aim of estimating dynamical masses and $V$-band mass-to-light ($M/L_V$) ratios over a wide range in age and metallicity. We obtained 3137 high-resolution stellar spectra with M2FS on the textit{Magellan}/Clay Telescope. Combined with 239 published spectroscopic results of comparable quality, we produced a final sample of 2787 stars with good quality spectra for kinematic analysis in the target clusters. Line-of-sight velocities measured from these spectra and stellar positions within each cluster were used in a customized expectation-maximization (EM) technique to estimate cluster membership probabilities. Using appropriate cluster structural parameters and corresponding single-mass dynamical models, this technique ultimately provides self-consistent total mass and $M/L_V$ estimates for each cluster. Mean metallicities for the clusters were also obtained and tied to a scale based on calcium IR triplet metallicites. We present trends of the cluster $M/L_V$ values with cluster age, mass and metallicity, and find that our results run about 40 per cent on average lower than the predictions of a set of simple stellar population (SSP) models. Modified SSP models that account for internal and external dynamical effects greatly improve agreement with our results, as can models that adopt a strongly bottom-light IMF. To the extent that dynamical evolution must occur, a modified IMF is not required to match data and models. In contrast, a bottom-heavy IMF is ruled out for our cluster sample as this would lead to higher predicted $M/L_V$ values, significantly increasing the discrepancy with our observations.
We investigate the stellar and dynamical mass profiles in the centres of 25 brightest cluster galaxies (BCGs) at redshifts of 0.05 $leq z leq$ 0.30. Our spectroscopy enables us to robustly measure the Gauss-Hermite higher order velocity moments $h_{3}$ and $h_{4}$, which we compare to measurements for massive early-type galaxies, and central group galaxies. We measure positive central values for $h_{4}$ for all the BCGs. We derive the stellar mass-to-light ratio ($Upsilon_{star rm DYN}$), and velocity anisotropy ($beta$) based on a Multi-Gaussian Expansion (MGE) and axisymmetric Jeans Anisotropic Methods (JAM, cylindrically- and spherically-aligned). We explicitly include a dark matter halo mass component, which is constrained by weak gravitational lensing measurements for these clusters. We find a strong correlation between anisotropy and velocity dispersion profile slope, with rising velocity dispersion profiles corresponding to tangential anisotropy and decreasing velocity dispersion profiles corresponding to radial anisotropy. The rising velocity dispersion profiles can also indicate a significant contribution from the intracluster light (ICL) to the total light (in projection) in the centre of the galaxy. For a small number of BCGs with rising velocity dispersion profiles, a variable stellar mass-to-light ratio can also account for the profile shape, instead of tangential anisotropy or a significant ICL contribution. We note that, for some BCGs, a variable $beta_{z}(r)$ (from radial to tangential anisotropy) can improve the model fit to the observed kinematic profiles. The observed diversity in these properties illustrates that BCGs are not the homogeneous class of objects they are often assumed to be.
We have determined the masses and mass-to-light ratios of 50 Galactic globular clusters by comparing their velocity dispersion and surface brightness profiles against a large grid of 900 N-body simulations of star clusters of varying initial concentration, size and central black hole mass fraction. Our models follow the evolution of the clusters under the combined effects of stellar evolution and two-body relaxation allowing us to take the effects of mass segregation and energy equipartition between stars self-consistently into account. For a subset of 16 well observed clusters we also derive their kinematic distances. We find an average mass-to-light ratio of Galactic globular clusters of $<M/L_V>=1.98 pm 0.03$, which agrees very well with the expected M/L ratio if the initial mass function of the clusters was a standard Kroupa or Chabrier mass function. We do not find evidence for a decrease of the average mass-to-light ratio with metallicity. The surface brightness and velocity dispersion profiles of most globular clusters are incompatible with the presence of intermediate-mass black holes (IMBHs) with more than a few thousand $M_odot$ in them. The only clear exception is $omega$ Cen, where the velocity dispersion profile provides strong evidence for the presence of a $sim$40,000 $M_odot$ IMBH in the centre of the cluster.
Classical Cepheids in open clusters play an important role in benchmarking stellar evolution models, anchoring the cosmic distance scale, and invariably securing the Hubble constant. NGC 6649, NGC 6664 and Berkeley 55 are three pertinent clusters that host classical Cepheids and red (super)giants, and an analysis was consequently initiated to assess newly acquired spectra ($approx$50), archival photometry, and $Gaia$ DR2 data. Importantly, for the first time chemical abundances are determined for the evolved members of NGC 6649 and NGC 6664. We find that they are slightly metal-poor relative to the mean Galactic gradient, and an overabundance of Ba is observed. Those clusters likely belong to the thin disc, and the latter finding supports DOrazi et al. (2009) $s$-enhanced scenario. NGC 6664 and Berkeley 55 exhibit radial velocities consistent with Galactic rotation, while NGC 6649 displays a peculiar velocity. The resulting age estimates for the clusters ($approx$70 Ma) imply masses for the (super)giant demographic of $approx$6 M$_{sun}$. Lastly, the observed yellow-to-red (super)giant ratio is lower than expected, and the overall differences relative to models reflect outstanding theoretical uncertainties.
The mass segregation of stellar clusters could be primordial rather than dynamical. Despite the abundance of studies of mass segregation for stellar clusters, those for stellar progenitors are still scarce, so the question on the origin and evolution of mass segregation is still open. Our goal is to characterize the structure of the NGC 2264 molecular cloud and compare the populations of clumps and young stellar objects (YSOs) in this region whose rich YSO population has shown evidence of sequential star formation. We separated the Herschel column density map of NGC 2264 in three subregions and compared their cloud power spectra using a multiscale segmentation technique. We identified in the whole NGC 2264 cloud a population of 256 clumps with typical sizes of ~0.1 pc and masses ranging from 0.08 Msun to 53 Msun. Although clumps have been detected all over the cloud, the central subregion of NGC 2264 concentrates most of the massive, bound clumps. The local surface density and the mass segregation ratio indeed indicate a strong degree of mass segregation for the 15 most massive clumps, with a median $Sigma_6$ three time that of the whole clumps population and $Lambda_{MSR}$ about 8. We showed that this cluster of massive clumps is forming within a high-density cloud ridge, itself formed and probably still fed by the high concentration of gas observed on larger scales in the central subregion. The time sequence obtained from the combined study of the clump and YSO populations in NGC 2264 suggests that the star formation started in the northern subregion, that it is now actively developing at the center and will soon start in the southern subregion. Taken together, the cloud structure and the clump and YSO populations in NGC 2264 argue for a dynamical scenario of star formation.
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