We present the analysis of the radial distributions and kinematic properties of the multiple stellar populations (mPOPs) hosted in the globular cluster (GC) NGC 6352 as part of the Hubble Space ...Telescope "UV Legacy Survey of Galactic Globular Clusters" program. NGC 6352 is one of the few GCs for which the mPOP tagging in appropriate color-magnitude diagrams is clear in all evolutionary sequences. We computed high-precision stellar proper motions for the stars from the cluster's core out to 75 arcsec (∼1.5 core radii, or ∼0.6 half-light radii). We find that, in the region explored, first- and second-generation stars share the same radial distribution and kinematic properties. Velocity dispersions, anisotropy radial profiles, differential rotation, and level of energy equipartition, all suggest that NGC 6352 is probably in an advanced evolutionary stage, and any possible difference in the structural and kinematic properties of its mPOPs have been erased by dynamical processes in the core of the cluster. We also provide an estimate of the mass of blue stragglers and of main-sequence binaries through kinematics alone. In general, in order to build a complete dynamical picture of this and other GCs, it will be essential to extend the analyses presented in this paper to the GCs' outer regions where some memories of the initial differences in the mPOP properties, and those imprinted by dynamical processes, might still be present.
We present kinematical analyses of 22 Galactic globular clusters using the Hubble Space Telescope proper motion catalogs recently presented in Bellini et al. For most clusters, this is the first ...proper-motion study ever performed, and, for many, this is the most detailed kinematic study of any kind. We use cleaned samples of bright stars to determine binned velocity-dispersion and velocity-anisotropy radial profiles and two-dimensional velocity-dispersion spatial maps. Using these profiles, we search for correlations between cluster kinematics and structural properties. We find the following: (1) more centrally concentrated clusters have steeper radial velocity-dispersion profiles; (2) on average, at 1sigma confidence in two dimensions, the photometric and kinematic centers of globular clusters agree to within ~1", with a cluster-to-cluster rms of 4"(including observational uncertainties); (3) on average, the cores of globular clusters have isotropic velocity distributions to within 1% (sigmat/sigmar = 0.992 + or - 0.005), with a cluster-to-cluster rms of 2% (including observational uncertainties); (4) clusters generally have mildly radially anisotropic velocity distributions ((sigmat/sigmar approx = 0.8-1.0) near the half-mass-radius, with bigger deviations from isotropy for clusters with longer relaxation times; and (5) there is a relation between sigma sub(minor)/sigma sub(major) and ellipticity, such that the more flattened clusters in the sample tend to be more anisotropic, with sigma sub(minor)/sigma sub(major) approx = 0.9-1.0. Aside from these general results and correlations, the profiles and maps presented here can provide a basis for detailed dynamical modeling of individual globular clusters. Given the quality of the data, this is likely to provide new insights into a range of topics concerning globular cluster mass profiles, structure, and dynamics.
Abstract
In this second installment of the series, we look at the internal kinematics of the multiple stellar populations of the globular cluster
ω
Centauri in one of the parallel
Hubble Space ...Telescope
(
HST
) fields, located at about 3.5 half-light radii from the center of the cluster. Thanks to the over 15 yr long baseline and the exquisite astrometric precision of the
HST
cameras, well-measured stars in our proper-motion catalog have errors as low as ∼10
μ
as yr
−1
, and the catalog itself extends to near the hydrogen-burning limit of the cluster. We show that second-generation (2G) stars are significantly more radially anisotropic than first-generation (1G) stars. The latter are instead consistent with an isotropic velocity distribution. In addition, 1G stars have excess systemic rotation in the plane of the sky with respect to 2G stars. We show that the six populations below the main-sequence (MS) knee identified in our first paper are associated with the five main population groups recently isolated on the upper MS in the core of cluster. Furthermore, we find both 1G and 2G stars in the field to be far from being in energy equipartition, with
for the former and
for the latter, where
η
is defined so that the velocity dispersion
scales with stellar mass as
. The kinematical differences reported here can help constrain the formation mechanisms for the multiple stellar populations in
ω
Centauri and other globular clusters. We make our astro-photometric catalog publicly available.
We present the first detailed assessment of the large-scale rotation of any galaxy based on full three-dimensional velocity measurements. We do this for the LMC by combining our Hubble Space ...Telescope average proper motion (PM) measurements for stars in 22 fields, with existing line-of-sight (LOS) velocity measurements for 6790 individual stars. We interpret these data with a model of circular rotation in a flat disk. The PM and LOS data paint a consistent picture of the LMC rotation, and their combination yields several new insights. The PM data imply a stellar dynamical center that coincides with the HI dynamical center, and a rotation curve amplitude consistent with that inferred from LOS velocity studies. The implied disk viewing angles agree with the range of values found in the literature, but continue to indicate variations with stellar population and/or radius. Young (red supergiant) stars rotate faster than old (red and asymptotic giant branch) stars due to asymmetric drift. Outside the central region, the circular velocity is approximately flat at V sub(circ) = 91.7 + or - 18.8 km s super(-1). This is consistent with the baryonic Tully-Fisher relation and implies an enclosed mass M(8.7 kpc) = (1.7 + or - 0.7) x 10 super(10) M sub(middot in circle). The virial mass is larger, depending on the full extent of the LMC's dark halo. The tidal radius is 22.3 + or - 5.2 kpc (24degrees.0 + or - 5degrees.6). Combination of the PM and LOS data yields kinematic distance estimates for the LMC, but these are not yet competitive with other methods.
With the release of Gaia DR2, it is now possible to measure the proper motions (PMs) of the lowest-mass, ultrafaint satellite galaxies in the Milky Way's (MW) halo for the first time. Many of these ...faint satellites are posited to have been accreted as satellites of the Magellanic Clouds (MCs). Using their six-dimensional phase-space information, we calculate the orbital histories of 13 ultrafaint satellites and five classical dwarf spheroidals in a combined MW+LMC+SMC potential to determine which galaxies are dynamically associated with the MCs. These 18 galaxies are separated into four classes: (i) long-term Magellanic satellites that have been bound to the MCs for at least the last two consecutive orbits around the MCs (Carina 2, Carina 3, Horologium 1, Hydrus 1); (ii) Magellanic satellites that were recently captured by the MCs < 1 Gyr ago (Reticulum 2, Phoenix 2); (iii) MW satellites that have interacted with the MCs (Sculptor 1, Tucana 3, Segue 1); and (iv) MW satellites (Aquarius 2, Canes Venatici 2, Crater 2, Draco 1, Draco 2, Hydra 2, Carina, Fornax, Ursa Minor). Results are reported for a range of MW and LMC masses. Contrary to previous work, we find no dynamical association between Carina, Fornax, and the MCs. Finally, we determine that the addition of the SMC's gravitational potential affects the longevity of satellites as members of the Magellanic system (long-term versus recently captured), but it does not change the total number of Magellanic satellites.
According to LCDM theory, hierarchical evolution occurs on all mass scales, implying that satellites of the Milky Way should also have companions. The recent discovery of ultra-faint dwarf galaxy ...candidates in close proximity to the Magellanic Clouds provides an opportunity to test this theory. We present proper motion (PM) measurements for 13 of the 32 new dwarf galaxy candidates using Gaia data release 2. All 13 also have radial velocity measurements. We compare the measured 3D velocities of these dwarfs to those expected at the corresponding distance and location for the debris of a Large Magellanic Cloud (LMC) analog in a cosmological numerical simulation. We conclude that four of these galaxies (Hor1, Car2, Car3, and Hyi1) have come in with the Magellanic Clouds, constituting the first confirmation of the type of satellite infall predicted by LCDM. Ret2, Tuc2, and Gru1 have velocity components that are not consistent within 3 of our predictions and are therefore less favorable. Hya2 and Dra2 could be associated with the LMC and merit further attention. We rule out Tuc3, Cra2, Tri2, and Aqu2 as potential members. Of the dwarfs without measured PMs, five of them are deemed unlikely on the basis of their positions and distances alone being too far from the orbital plane expected for LMC debris (Eri2, Ind2, Cet2, Cet3, and Vir1). For the remaining sample, we use the simulation to predict PMs and radial velocities, finding that Phx2 has an overdensity of stars in DR2 consistent with this PM prediction.
We present proper motions for the Large and Small Magellanic Clouds (LMC and SMC) based on three epochs of Hubble Space Telescope data, spanning a ~7 yr baseline, and centered on fields with ...background QSOs. The first two epochs, the subject of past analyses, were obtained with ACS/HRC, and have been reanalyzed here. The new third epoch with WFC3/UVIS increases the time baseline and provides better control of systematics. The three-epoch data yield proper-motion random errors of only 1%-2% per field. For the LMC this is sufficient to constrain the internal proper-motion dynamics, as will be discussed in a separate paper.Herewe focus on the implied center-of-mass proper motions: mu sub(W) sub(,LMC) = -1.910 + or - 0.020 mas yr-1, mu sub(N) sub(,LMC) = 0.229 + or - 0.047 mas yr super(-1), and mu sub(W) sub(,SMC) = -0.772 + or - 0.063 mas yr-1, mu sub(N) sub(,SMC) = -1.117+ or -0.061 mas yr super(-1).We combine the results with a revised understanding of the solar motion in the MilkyWay to derive Galactocentric velocities: v sub(tot,LMC) = 321+ or -24 km s-1 and v sub(tot,SMC) = 217 + or - 26 km s super(-1). Our proper-motion uncertainties are now dominated by limitations in our understanding of the internal kinematics and geometry of theClouds, and our velocity uncertainties are dominated by distance errors. Orbit calculations for the Clouds around the Milky Way allow a range of orbital periods, depending on the uncertain masses of the Milky Way and LMC. Periods lap4 Gyr are ruled out, which poses a challenge for traditional Magellanic Stream models. First-infall orbits are preferred (as supported by other arguments as well) if one imposes the requirement that the LMC and SMC must have been a bound pair for at least several Gyr.
ABSTRACT We report the discovery of one RR Lyrae star in the ultra-faint satellite galaxy Hydra II based on time series photometry in the g, r and i bands obtained with the Dark Energy Camera at ...Cerro Tololo Inter-American Observatory, Chile. The association of the RR Lyrae star discovered here with Hydra II is clear because is located at 42 ″ from the center of the dwarf, well within its half-light radius of 102 ″ . The RR Lyrae star has a mean magnitude of i = 21.30 0.04 which is too faint to be a field halo star. This magnitude translates to a heliocentric distance of 151 8 kpc for Hydra II; this value is ∼ 13 % larger than the estimate from the discovery paper based on the average magnitude of several blue horizontal branch star candidates. The new distance implies a slightly larger half-light radius of 76 − 10 + 12 pc and a brighter absolute magnitude of M V = − 5.1 0.3 , which keeps this object within the realm of the dwarf galaxies. A comparison with other RR Lyrae stars in ultra-faint systems indicates similar pulsational properties among them, which are different to those found among halo field stars and those in the largest of the Milky Way satellites. We also report the discovery of 31 additional short period variables in the field of view (RR Lyrae, SX Phe, eclipsing binaries, and a likely anomalous cepheid) which are likely not related with Hydra II.
We present Hubble Space Telescope (HST) absolute proper motion (PM) measurements for 20 globular clusters (GCs) in the Milky Way (MW) halo at Galactocentric distances R GC 10 - 100 kpc, with a median ...per-coordinate PM uncertainty of 0.06 mas yr − 1 . Young and old halo GCs do not show systematic differences in their 3D Galactocentric velocities, derived from combining existing line-of-sight velocities. We confirm the association of Arp 2, Pal 12, Terzan 7, and Terzan 8 with Sgr. These clusters and NGC 6101 have tangential velocity v tan > 290 km s−1, whereas all other clusters have v tan < 200 km s−1. NGC 2419, the most distant GC in our sample, is also likely associated with the Sgr stream, whereas NGC 4147, NGC 5024, and NGC 5053 definitely are not. We use the distribution of orbital parameters derived using the 3D velocities to separate halo GCs that either formed within the MW or were accreted. We also assess the specific formation history of, e.g., Pyxis and Terzan 8. We constrain the MW mass via an estimator that considers the full 6D phase-space information for 16 of the GCs from R GC = 10 to 40 kpc. The velocity dispersion anisotropy parameter β = 0.609 − 0.229 + 0.130 . The enclosed mass M ( < 39.5 kpc ) = 0.61 − 0.12 + 0.18 × 10 12 M , and the virial mass M vir = 2.05 − 0.79 + 0.97 × 10 12 M . These are consistent with, but on the high side among, recent mass estimates in the literature.
We present an improved data-reduction technique to obtain high-precision proper motions (PMs) of globular clusters (GCs) using Hubble Space Telescope data. The new reduction is superior to the one ...presented in the first paper of this series for the faintest sources in very crowded fields. We choose the GC NGC 362 as a benchmark to test our new procedures. We measure PMs of 117,450 sources in the field, showing that we are able to obtain a PM precision better than 10 as yr−1 for bright stars. We make use of this new PM catalog of NGC 362 to study the cluster's internal kinematics. We investigate the velocity dispersion profiles of the multiple stellar populations hosted by NGC 362 and find new pieces of information on the kinematics of first- and second-generation stars. We analyze the level of energy equipartition of the cluster and find direct evidence for its post-core-collapsed state from kinematic arguments alone. We refine the dynamical mass of the blue stragglers (BSs) and study possible kinematic differences between BSs formed by collisions and mass transfer. We also measure no significant cluster rotation in the plane of the sky. Finally, we measure the absolute PM of NGC 362 and of the background stars belonging to the Small Magellanic Cloud, finding a good agreement with previous estimates in the literature. We make the PM catalog publicly available.
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