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Register a new userEvidence for an Intermediate-Mass Milky Way from Gaia DR2 Halo Globular Cluster Motions
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Laura L. Watkins
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We estimate the mass of the Milky Way (MW) within 21.1 kpc using the kinematics of halo globular clusters (GCs) determined by Gaia. The second Gaia data release (DR2) contained a catalogue of absolute proper motions (PMs) for a set of Galactic GCs and satellite galaxies measured using Gaia DR2 data. We select from the catalogue only halo GCs, identifying a total of 34 GCs spanning $2.0 < r < 21.1$ kpc, and use their 3D kinematics to estimate the anisotropy over this range to be $beta = 0.46^{+0.15}_{-0.19}$, in good agreement, though slightly lower than, a recent estimate for a sample of halo GCs using HST PM measurements further out in the halo. We then use the Gaia kinematics to estimate the mass of the MW inside the outermost GC to be $M(< 21.1 mathrm{kpc}) = 0.21^{+0.04}_{-0.03} 10^{12} mathrm{M_odot}$, which corresponds to a circular velocity of $v_mathrm{circ}(21.1 mathrm{kpc}) = 206^{+19}_{-16}$ km/s. The implied virial mass is $M_mathrm{virial} = 1.28^{+0.97}_{-0.48} 10^{12} mathrm{M_odot}$. The error bars encompass the uncertainties on the anisotropy and on the density profile of the MW dark halo, and the scatter inherent in the mass estimator we use. We get improved estimates when we combine the Gaia and HST samples to provide kinematics for 46 GCs out to 39.5 kpc: $beta = 0.52^{+0.11}_{-0.14}$, $M(< 39.5 mathrm{kpc}) = 0.42^{+0.07}_{-0.06} 10^{12} mathrm{M_odot}$, and $M_mathrm{virial} = 1.54^{+0.75}_{-0.44} 10^{12} mathrm{M_odot}$. We show that these results are robust to potential substructure in the halo GC distribution. While a wide range of MW virial masses have been advocated in the literature, from below $10^{12} mathrm{M_odot}$ to above $2 times 10^{12}mathrm{M_odot}$, these new data imply that an intermediate mass is most likely.
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We present new mass estimates and cumulative mass profiles (CMPs) with Bayesian credible regions for the Milky Way (MW) Galaxy, given the kinematic data of globular clusters as provided by (1) the $textit{Gaia}$ DR2 collaboration and the HSTPROMO team, and (2) the new catalog in Vasiliev (2019). We use globular clusters beyond 15kpc to estimate the CMP of the MW, assuming a total gravitational potential model $Phi(r) = Phi_{circ}r^{-gamma}$, which approximates an NFW-type potential at large distances when $gamma=0.5$. We compare the resulting CMPs given data sets (1) and (2), and find the results to be nearly identical. The median estimate for the total mass is $M_{200}= 0.70 times 10^{12} M_{odot}$ and the $50%$ Bayesian credible interval is $(0.62, 0.81)times10^{12}M_{odot}$. However, because the Vasiliev catalog contains more complete data at large $r$, the MW total mass is slightly more constrained by these data. In this work, we also supply instructions for how to create a CMP for the MW with Bayesian credible regions, given a model for $M(<r)$ and samples drawn from a posterior distribution. With the CMP, we can report median estimates and $50%$ Bayesian credible regions for the MW mass within any distance (e.g., $M(r=25text{kpc})= 0.26~(0.20, 0.36)times10^{12}M_{odot}$, $M(r=50text{kpc})= 0.37~(0.29, 0.51) times10^{12}M_{odot}$, $M(r=100text{kpc}) = 0.53~(0.41, 0.74) times10^{12}M_{odot}$, etc.), making it easy to compare our results directly to other studies.
We employ Gaia DR2 proper motions for 151 Milky Way globular clusters from Vasiliev (2019) in tandem with distances and line-of-sight velocities to derive their kinematical properties. To assign clusters to the Milky Way thick disk, bulge, and halo we follow the approach of Posti et al. (2018) who distinguished among different Galactic stellar components using starss orbits. In particular, we use the ratio $L_{z}/e$, the $Z$ projection of the angular momentum to the eccentricity, as population tracer, which we complement with chemical abundances extracted from the literature and Monte-Carlo simulations. We find that 20 globular clusters belong to the bar/bulge of the Milky Way, 35 exhibit disk properties, and 96 are members of the halo. Moreover, we find that halo globular clusters have close to zero rotational velocity with average value $<Theta>$ =1$pm$ 4 km s$^{-1}$. On the other hand, the sample of clusters that belong to the thick disk possesses a significant rotation with average rotational velocity 179 $pm$ 6 km s$^{-1}$. The twenty globular clusters orbiting within the bar/bulge region of the Milky Way galaxy have average rotational velocity of 49 $pm$ 11 km s$^{-1}$.
Using data from Gaia DR2, we study the radial number density profiles of the Galactic globular cluster sample. Proper motions are used for accurate membership selection, especially crucial in the cluster outskirts. Due to the severe crowding in the centres, the Gaia data is supplemented by literature data from HST and surface brightness measurements, where available. This results in 81 clusters with a complete density profile covering the full tidal radius (and beyond) for each cluster. We model the density profiles using a set of single-mass models ranging from King and Wilson models to generalised lowered isothermal limepy models and the recently introduced spes models, which allow for the inclusion of potential escapers. We find that both King and Wilson models are too simple to fully reproduce the density profiles, with King (Wilson) models on average underestimating(overestimating) the radial extent of the clusters. The truncation radii derived from the limepy models are similar to estimates for the Jacobi radii based on the cluster masses and their orbits. We show clear correlations between structural and environmental parameters, as a function of Galactocentric radius and integrated luminosity. Notably, the recovered fraction of potential escapers correlates with cluster pericentre radius, luminosity and cluster concentration. The ratio of half mass over Jacobi radius also correlates with both truncation parameter and PE fraction, showing the effect of Roche lobe filling.
We use data from the Radial Velocity Experiment (RAVE) and the Tycho-Gaia astrometric solution catalogue (TGAS) to compute the velocity fields yielded by the radial (VR), azimuthal (Vphi) and vertical (Vz) components of associated Galactocentric velocity. We search in particular for variation in all three velocity components with distance above and below the disc midplane, as well as how each component of Vz (line-of-sight and tangential velocity projections) modifies the obtained vertical structure. To study the dependence of velocity on proper motion and distance we use two main samples: a RAVE sample including proper motions from the Tycho-2, PPMXL and UCAC4 catalogues, and a RAVE-TGAS sample with inferred distances and proper motions from the TGAS and UCAC5 catalogues. In both samples, we identify asymmetries in VR and Vz. Below the plane we find the largest radial gradient to be dVR / dR = -7.01+- 0.61 kms kpc, in agreement with recent studies. Above the plane we find a similar gradient with dVR / dR= -9.42+- 1.77 kms kpc. By comparing our results with previous studies, we find that the structure in Vz is strongly dependent on the adopted proper motions. Using the Galaxia Milky Way model, we demonstrate that distance uncertainties can create artificial wave-like patterns. In contrast to previous suggestions of a breathing mode seen in RAVE data, our results support a combination of bending and breathing modes, likely generated by a combination of external or internal and external mechanisms.
Based on Gaia Early Data Release 3 (EDR3), we estimate the proper motions for 46 dwarf spheroidal galaxies (dSphs) of the Milky Way. The uncertainties in proper motions, determined by combining both statistical and systematic errors, are smaller by a factor 2.5, when compared with Gaia Data Release 2. We have derived orbits in four Milky Way potential models that are consistent with the MW rotation curve, with total mass ranging from $2.8times10^{11}$ $M_{odot}$ to $15times10^{11}$ $M_{odot}$. Although the type of orbit (ellipse or hyperbola) are very dependent on the potential model, the pericenter values are firmly determined, largely independent of the adopted MW mass model. By analyzing the orbital phases, we found that the dSphs are highly concentrated close to their pericenter, rather than to their apocenter as expected from Keplers law. This may challenge the fact that most dSphs are Milky Way satellites, or alternatively indicates an unexpected large number of undiscovered dSphs lying very close to their apocenters. Between half and two thirds of the satellites have orbital poles that indicate them to orbit along the Vast Polar Structure (VPOS), with the vast majority of these co-orbiting in a common direction also shared by the Magellanic Clouds, which is indicative of a real structure of dSphs.
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