A candidate diffuse stellar substructure was previously reported in the halo of the nearby dwarf starburst galaxy NGC 4449 by Karachentsev et al. We map and analyze this feature using a unique ...combination of deep integrated-light images from the BlackBird 0.5 m telescope, and high-resolution wide-field images from the 8 m Subaru Telescope, which resolve the nebulosity into a stream of red giant branch stars, and confirm its physical association with NGC 4449. The properties of the stream imply a massive dwarf spheroidal progenitor, which after complete disruption will deposit an amount of stellar mass that is comparable to the existing stellar halo of the main galaxy. The stellar mass ratio between the two galaxies is ~1:50, while the indirectly measured dynamical mass ratio, when including dark matter, may be ~1:10-1:5. This system may thus represent a "stealth" merger, where an infalling satellite galaxy is nearly undetectable by conventional means, yet has a substantial dynamical influence on its host galaxy. This singular discovery also suggests that satellite accretion can play a significant role in building up the stellar halos of low-mass galaxies, and possibly in triggering their starbursts.
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.
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 a database of structural and dynamical properties for 153 spatially resolved star clusters in the Milky Way, the Large and Small Magellanic Clouds, and the Fornax dwarf spheroidal. This ...database complements and extends others in the literature, such as those of Harris and Mackey & Gilmore. Our cluster sample comprises 50 "young massive clusters" in the LMC and SMC, and 103 old globular clusters between the four galaxies. The parameters we list include central and half-light-averaged surface brightnesses and mass densities; core and effective radii; central potentials, concentration parameters, and tidal radii; predicted central velocity dispersions and escape velocities; total luminosities, masses, and binding energies; central phase-space densities; half-mass relaxation times; and "-space" parameters. We use publicly available population-synthesis models to compute stellar-population properties (intrinsic B - V colors, reddenings, and V-band mass-to-light ratios) for the same 153 clusters plus another 63 globulars in the Milky Way. We also take velocity-dispersion measurements from the literature for a subset of 57 (mostly old) clusters to derive dynamical mass-to-light ratios for them, showing that these compare very well to the population-synthesis predictions. The combined data set is intended to serve as the basis for future investigations of structural correlations and the fundamental plane of massive star clusters, including especially comparisons between the systemic properties of young and old clusters. The structural and dynamical parameters are derived from fitting three different models - the modified isothermal sphere of King; an alternate modified isothermal sphere based on the ad hoc stellar distribution function of Wilson; and asymptotic power-law models with constant-density cores - to the surface-brightness profile of each cluster. Surface-brightness data for the LMC, SMC, and Fornax clusters are based in large part on the work of Mackey & Gilmore, but include significant supplementary data culled from the literature and important corrections to Mackey & Gilmore's V-band magnitude scale. The profiles of Galactic globular clusters are taken from Trager et al. We address the question of which model fits each cluster best, finding in the majority of cases that the Wilson models - which are spatially more extended than King models but still include a finite, "tidal" cutoff in density - fit clusters of any age, in any galaxy, as well as or better than King models. Untruncated, asymptotic power laws often fit about as well as Wilson models but can be significantly worse. We argue that the extended halos known to characterize many Magellanic Cloud clusters may be examples of the generic envelope structure of self-gravitating star clusters, not just transient features associated strictly with young age.
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.
Recent proper-motion measurements of the Large and Small Magellanic Clouds (LMC and SMC, respectively) by Kallivayalil and coworkers suggest that the 3D velocities of the Clouds are substantially ...higher ( similar to 100 km s super(-1)) than previously estimated and now approach the escape velocity of the Milky Way (MW). Previous studies have also assumed that the Milky Way can be adequately modeled as an isothermal sphere to large distances. Here we reexamine the orbital history of the Clouds using the new velocities and a ACDM-motivated MW model with virial mass M sub(vlr) = 10 super(12) M unk (e.g., Klypin and coworkers). We conclude that the LMC and SMC are either currently on their first passage about the MW or, if the MW can be accurately modeled by an isothermal sphere to distances unk200 kpc (i.e., M sub(vlr) > 2 x 10 super(12) M unk), that their orbital period and apogalacticon distance must be a factor of 2 larger than previously estimated, increasing to 3 Gyr and 200 kpc, respectively. A first passage scenario is consistent with the fact that the LMC and SMC appear to be outliers when compared to other satellite galaxies of the MW: they are irregular in appearance and are moving faster. We discuss the implications of this orbital analysis for our understanding of the star formation history, the nature of the warp in the MW disk and the origin of the Magellanic Stream (MS), a band of H I gas trailing the LMC and SMC that extends similar to 100 degree across the sky. Specifically, as a consequence of the new orbital history of the Clouds, the origin of the MS may not be explainable by current tidal and ram pressure stripping models.
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