Recent high-precision proper motions from the Hubble Space Telescope suggest that the Large and Small Magellanic Clouds (LMC and SMC, respectively) are either on their first passage or on an ...eccentric long period (>6 Gyr) orbit about the Milky Way (MW). This differs markedly from the canonical picture in which the Clouds travel on a quasi-periodic orbit about the MW (period of {approx}2 Gyr). Without a short-period orbit about the MW, the origin of the Magellanic Stream, a young (1-2 Gyr old) coherent stream of H I gas that trails the Clouds {approx}150{sup 0} across the sky, can no longer be attributed to stripping by MW tides and/or ram pressure stripping by MW halo gas. We propose an alternative formation mechanism in which material is removed by LMC tides acting on the SMC before the system is accreted by the MW. We demonstrate the feasibility and generality of this scenario using an N-body/smoothed particle hydrodynamics simulation with cosmologically motivated initial conditions constrained by the observations. Under these conditions, we demonstrate that it is possible to explain the origin of the Magellanic Stream in a first infall scenario. This picture is generically applicable to any gas-rich dwarf galaxy pair infalling toward a massive host or interacting in isolation.
Abstract
High-precision proper motions of the globular cluster 47 Tuc have allowed us to measure for the first time the cluster rotation in the plane of the sky and the velocity anisotropy profile ...from the cluster core out to about 13′. These profiles are coupled with prior measurements along the line of sight (LOS) and the surface brightness profile and fit all together with self-consistent models specifically constructed to describe quasi-relaxed stellar systems with realistic differential rotation, axisymmetry, and pressure anisotropy. The best-fit model provides an inclination angle
i
between the rotation axis and the LOS direction of 30° and is able to simultaneously reproduce the full three-dimensional kinematics and structure of the cluster, while preserving a good agreement with the projected morphology. Literature models based solely on LOS measurements imply a significantly different inclination angle (
i
= 45°), demonstrating that proper motions play a key role in constraining the intrinsic structure of 47 Tuc. Our best-fit global dynamical model implies an internal rotation higher than previous studies have shown and suggests a peak of the intrinsic
V
/
σ
ratio of ∼0.9 at around two half-light radii, with a nonmonotonic intrinsic ellipticity profile reaching values up to 0.45. Our study unveils a new degree of dynamical complexity in 47 Tuc, which may be leveraged to provide new insights into the formation and evolution of globular clusters.
We present high-precision multiband photometry for the globular cluster (GC) M2. We combine the analysis of the photometric data obtained from the Hubble Space Telescope UV Legacy Survey of Galactic ...GCs GO-13297, with chemical abundances by Yong et al., and compare the photometry with models in order to analyse the multiple stellar sequences we identified in the colour–magnitude diagram. We find three main stellar components, composed of metal-poor, metal-intermediate, and metal-rich stars (hereafter referred to as population A, B, and C, respectively). The components A and B include stars with different s-process element abundances. They host six sub-populations with different light-element abundances, and exhibit an internal variation in helium up to ΔY ∼ 0.07 dex. In contrast with M22, another cluster characterized by the presence of populations with different metallicities, M2 contains a third stellar component, C, which shows neither evidence for sub-populations nor an internal spread in light-elements. Population C does not exhibit the typical photometric signatures that are associated with abundance variations of light elements produced by hydrogen burning at hot temperatures. We compare M2 with other GCs with intrinsic heavy-element variations and conclude that M2 resembles M22, but it includes an additional stellar component that makes it more similar to the central region of the Sagittarius galaxy, which hosts a GC (M54) and the nucleus of the Sagittarius galaxy itself.
Abstract
The DECam Local Volume Exploration survey (DELVE) is a 126-night survey program on the 4 m Blanco Telescope at the Cerro Tololo Inter-American Observatory in Chile. DELVE seeks to understand ...the characteristics of faint satellite galaxies and other resolved stellar substructures over a range of environments in the Local Volume. DELVE will combine new DECam observations with archival DECam data to cover ∼15,000 deg
2
of high Galactic latitude (∣
b
∣ > 10°) southern sky to a 5
σ
depth of
g
,
r
,
i
,
z
∼ 23.5 mag. In addition, DELVE will cover a region of ∼2200 deg
2
around the Magellanic Clouds to a depth of
g
,
r
,
i
∼ 24.5 mag and an area of ∼135 deg
2
around four Magellanic analogs to a depth of
g
,
i
∼ 25.5 mag. Here, we present an overview of the DELVE program and progress to date. We also summarize the first DELVE public data release (DELVE DR1), which provides point-source and automatic aperture photometry for ∼520 million astronomical sources covering ∼5000 deg
2
of the southern sky to a 5
σ
point-source depth of
g
= 24.3 mag,
r
= 23.9 mag,
i
= 23.3 mag, and
z
= 22.8 mag. DELVE DR1 is publicly available via the NOIRLab Astro Data Lab science platform.
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.
Abstract
We present Magellan/IMACS, Anglo-Australian Telescope/AAOmega+2dF, and Very Large Telescope/GIRAFFE+FLAMES spectroscopy of the Carina II (Car II) and Carina III (Car III) dwarf galaxy ...candidates, recently discovered in the Magellanic Satellites Survey (MagLiteS). We identify 18 member stars in Car II, including two binaries with variable radial velocities and two RR Lyrae stars. The other 14 members have a mean heliocentric velocity
and a velocity dispersion of
. Assuming Car II is in dynamical equilibrium, we derive a total mass within the half-light radius of
, indicating a mass-to-light ratio of
/
. From equivalent width measurements of the calcium triplet lines of nine red giant branch (RGB) stars, we derive a mean metallicity of
with dispersion
. Considering both the kinematic and chemical properties, we conclude that Car II is a dark-matter-dominated dwarf galaxy. For Car III, we identify four member stars, from which we calculate a systemic velocity of
. The brightest RGB member of Car III has a metallicity of
. Due to the small size of the Car III spectroscopic sample, we cannot conclusively determine its nature. Although these two systems have the smallest known physical separation (
) among Local Group satellites, the large difference in their systemic velocities,
, indicates that they are unlikely to be a bound pair. One or both systems are likely associated with the Large Magellanic Cloud (LMC), and may remain LMC satellites today. No statistically significant excess of
γ
-ray emission is found at the locations of Car II and Car III in eight years of
Fermi
-LAT data.
ABSTRACT
We present the star formation history (SFH) of the extremely metal-poor dwarf galaxy DDO 68, based on our photometry with the Advanced Camera for Surveys. With a metallicity of only
and a ...very isolated location, DDO 68 is one of the most metal-poor galaxies known. It has been argued that DDO 68 is a young system that started forming stars only ∼0.15 Gyr ago. Our data provide a deep and uncontaminated optical color–magnitude diagram (CMD) that allows us to disprove this hypothesis since we find a population of at least ∼1 Gyr old stars. The star formation activity has been fairly continuous over all the look-back time. The current rate is quite low, and the highest activity occurred between 10 and 100 Myr ago. The average star formation rate over the whole Hubble time is ≃0.01
M
⊙
yr
−1
, corresponding to a total astrated mass of ≃1.3 × 10
8
M
⊙
. Our photometry allows us to infer the distance from the tip of the red giant branch,
D
= 12.08 ± 0.67 Mpc; however, to let our synthetic CMD reproduce the observed ones, we need a slightly higher distance,
D
= 12.65 Mpc, or (
m
−
M
)
0
= 30.51, still inside the errors of the previous determination, and we adopt the latter. DDO 68 shows a very interesting and complex history, with its quite disturbed shape and a long tail, probably due to tidal interactions. The SFH of the tail differs from that of the main body mainly for enhanced activity at recent epochs likely triggered by the interaction.
We present and describe the astro-photometric catalog of more than 800,000 sources found in the Hubble Tarantula Treasury Project (HTTP). HTTP is a Hubble Space Telescope Treasury program designed to ...image the entire 30 Doradus region down to the sub-solar (~0.5 M sub(middot in circle)) mass regime using the Wide Field Camera 3 and the Advanced Camera for Surveys. We observed 30 Doradus in the near-ultraviolet (F275W, F336W), optical (F555W, F658N, F775W), and near-infrared (F110W, F160W) wavelengths. The stellar photometry was measured using point-spread function fitting across all bands simultaneously. The relative astrometric accuracy of the catalog is 0.4 mas. The astro-photometric catalog, results from artificial star experiments, and the mosaics for all the filters are available for download. Color-magnitude diagrams are presented showing the spatial distributions and ages of stars within 30 Dor as well as in the surrounding fields. HTTP provides the first rich and statistically significant sample of intermediate- and low-mass pre-main sequence candidates and allows us to trace how star formation has been developing through the region. The depth and high spatial resolution of our analysis highlight the dual role of stellar feedback in quenching and triggering star formation on the giant H II region scale. Our results are consistent with stellar sub-clustering in a partially filled gaseous nebula that is offset toward our side of the Large Magellanic Cloud.
In this paper, we combine Wide Field Camera3/UVIS F275W, F336W, and F438W data from the ‘UV Legacy Survey of Galactic Globular Clusters: Shedding Light on Their Populations and Formation’ (GO 13297) ...Hubble Space Telescope Treasury programme with F606W, F625W, F658N, and F814W Advanced Camera for Surveys archive data for a multiwavelength study of the globular cluster NGC 6352. In the colour–magnitude and two-colour diagrams obtained with appropriate combination of the photometry in the different bands, we separate two distinct stellar populations and trace them from the main sequence to the subgiant, red giant, horizontal and asymptotic giant branches. We infer that the two populations differ in He by ΔY = 0.029 ± 0.006. With a new method, we also estimate the age difference between the two sequences. Assuming no difference in Fe/H and α/Fe, and the uncertainties on ΔY, we found a difference in age between the two populations of 10 ± 120 Myr. If we assume Fe/H and α/Fe differences of 0.02 dex (well within the uncertainties of spectroscopic measurements), the total uncertainty in the relative age rises to ∼300 Myr.
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