Keywords

We obtained deep Gemini GMOS-S g, r photometry of the ultra-faint dwarf galaxy candidate Cetus II with the aim of providing stronger constraints on its size, luminosity, and stellar population. Cetus II is an important object in the size–luminosity plane, as it occupies the transition zone between dwarf galaxies and star clusters. All known objects smaller than Cetus II (r_h ∼ 20 pc) are reported to be star clusters, while most larger objects are likely dwarf galaxies. We found a prominent excess of main-sequence stars in the color–magnitude diagram of Cetus II, best described by a single stellar population with an age of 11.2 Gyr, metallicity of [Fe/H] = −1.28 dex, an [α/Fe] = 0.0 dex at a heliocentric distance of 26.3 ± 1.2 kpc. As well as being spatially located within the Sagittarius dwarf tidal stream, these properties are well matched to the Sagittarius galaxy's Population B stars. Interestingly, like our recent findings on the ultra-faint dwarf galaxy candidate Tucana V, the stellar field in the direction of Cetus II shows no evidence of a concentrated overdensity despite tracing the main sequence for over six magnitudes. These results strongly support the picture that Cetus II is not an ultra-faint stellar system in the Milky Way halo, but made up of stars from the Sagittarius tidal stream.

Using a spectroscopically confirmed sample of M giants, M dwarfs, and quasars from the LAMOST survey, we assess how well Wide-field Infrared Survey Explorer (WISE) and Two Micron All Sky Survey color cuts can be used to select M giant stars. The WISE bands are very efficient at separating M giants from M dwarfs, and we present a simple classification that can produce a clean and relatively complete sample of M giants. We derive a new photometric relation to estimate the metallicity for M giants, calibrated using data from the APOGEE survey. We find a strong correlation between the $(W1-W2)$ color and $[{\rm{M}}/{\rm{H}}]$, where almost all of the scatter is due to photometric uncertainties. We show that previous photometric distance relations, which are mostly based on stellar models, may be biased and devise a new empirical distance relation, investigating trends with metallicity and star formation history. Given these relations, we investigate the properties of M giants in the Sagittarius stream. The offset in the orbital plane between the leading and trailing tails is reproduced, and by identifying distant M giants in the direction of the Galactic anticenter, we confirm that the previously detected debris in the outer halo is the apocenter of the trailing tail. We also find tentative evidence supporting an existing overdensity near the leading tail in the northern Galactic hemisphere, possibly an extension to the trailing tail (so-called Branch C). We have measured the metallicity distribution along the stream, finding a clear metallicity offset between the leading and trailing tails, in agreement with models for the stream formation. We include an online table of M giants to facilitate further studies.

We present deep color–magnitude diagrams (CMDs) for two Subaru Suprime-Cam fields in the Virgo Stellar Stream (VSS)/Virgo Overdensity (VOD) and compare them to a field centered on the highest concentration of Sagittarius (Sgr) Tidal Stream stars in the leading arm, Branch A of the bifurcation. A prominent population of main-sequence stars is detected in all three fields and can be traced as faint as g ≈ 24 mag. Using theoretical isochrone fitting, we derive an age of $9.1^{+1.0}_{-1.1}$ Gyr, a median abundance of [Fe/H] = $-0.70^{+0.15}_{-0.20}$ dex, and a heliocentric distance of 30.9 ± 3.0 kpc for the main sequence of the Sgr Stream Branch A. The dominant main-sequence populations in the two VSS/VOD fields (Λ_☉ ≈ 265°, B_☉ ≈ 13°) are located at a mean distance of 23.3 ± 1.6 kpc and have an age of ∼8.2 Gyr, and an abundance of [Fe/H] = $-0.67^{+0.16}_{-0.12}$ dex, similar to the Sgr Stream stars. These statistically robust parameters, derived from the photometry of 260 main-sequence stars, are also in good agreement with the age of the main population in the Sgr dwarf galaxy (8.0 ± 1.5 Gyr). They also agree with the peak in the metallicity distribution of 2–3 Gyr old M giants, [Fe/H] ≈−0.6 dex, in the Sgr north leading arm. We then compare the results from the VSS/VOD fields with the Sgr Tidal Stream model by Law & Majewski based on a triaxial Galactic halo shape that is empirically calibrated with Sloan Digital Sky Survey Sgr A-branch and Two Micron All Sky Survey M-giant stars. We find that the most prominent feature in the CMDs, the main-sequence population at 23 kpc, is not explained by the model. Instead the model predicts in these directions a low-density filamentary structure of Sgr debris stars at ∼9 kpc and a slightly higher concentration of Sgr stars spread over a heliocentric distance range of 42–53 kpc. At best there is only marginal evidence for the presence of these populations in our data. Our findings then suggest that while there are probably some Sgr debris stars present, the dominant stellar population in the VOD originates from a different halo structure that has an almost identical age and metallicity as some sections of the Sgr tidal stream.

Using multi-epoch observations of the Stripe 82 region from the Sloan Digital Sky Survey (SDSS), we measure precise statistical proper motions of the stars in the Sagittarius (Sgr) stellar stream. The multi-band photometry and SDSS radial velocities allow us to efficiently select Sgr members and thus enhance the proper-motion precision to ∼0.1 mas yr⁻¹. We measure separately the proper motion of a photometrically selected sample of the main-sequence turn-off stars, as well as spectroscopically selected Sgr giants. The data allow us to determine the proper motion separately for the two Sgr streams in the south found in Koposov et al. Together with the precise velocities from SDSS, our proper motions provide exquisite constraints of the three-dimensional motions of the stars in the Sgr streams.

The structure of the Sagittarius stream in the southern Galactic hemisphere is analyzed with the Sloan Digital Sky Survey Data Release 8. Parallel to the Sagittarius tidal track, but ∼10° away, there is another fainter and more metal-poor stream. We provide evidence that the two streams follow similar distance gradients but have distinct morphological properties and stellar populations. The brighter stream is broader, contains more metal-rich stars, and has a richer color–magnitude diagram with multiple turnoffs and a prominent red clump as compared to the fainter stream. Based on the structural properties and the stellar population mix, the stream configuration is similar to the Northern "bifurcation." In the region of the South Galactic Cap, there is overlapping tidal debris from the Cetus stream, which crosses the Sagittarius stream. Using both photometric and spectroscopic data, we show that the blue straggler population belongs mainly to Sagittarius and the blue horizontal branch stars belong mainly to the Cetus stream in this confused location in the halo.

The tidal stirring model envisions the formation of dwarf spheroidal (dSph) galaxies in the Local Group and similar environments via the tidal interaction of disky dwarf systems with a larger host galaxy like the Milky Way. These progenitor disks are embedded in extended dark halos and during the evolution both components suffer strong mass loss. In addition, the disks undergo the morphological transformation into spheroids and the transition from ordered to random motion of their stars. Using collisionless N-body simulations, we construct a model for the nearby and highly elongated Sagittarius (Sgr) dSph galaxy within the framework of the tidal stirring scenario. Constrained by the present orbit of the dwarf, which is fairly well known, the model suggests that in order to produce the majority of tidal debris observed as the Sgr stream, but not yet transform the core of the dwarf into a spherical shape, Sgr must have just passed the second pericenter of its current orbit around the Milky Way. In the model, the stellar component of Sgr is still very elongated after the second pericenter and morphologically intermediate between the strong bar formed at the first pericenter and the almost spherical shape existing after the third pericenter. This is thus the first model of the evolution of the Sgr dwarf that accounts for its observed very elliptical shape. At the present time, there is very little intrinsic rotation left and the velocity gradient detected along the major axis is almost entirely of tidal origin. We model the recently measured velocity dispersion profile for Sgr assuming that mass traces light and estimate its current total mass within 5 kpc to be 5.2 × 10⁸ M_☉. To have this mass at present, the model requires that the initial virial mass of Sgr must have been as high as 1.6 × 10¹⁰ M_☉, comparable to that of the Large Magellanic Cloud, which may serve as a suitable analog for the pre-interaction, Sgr progenitor.

We present multi-slit radial velocity measurements for 111 stars in the direction of the Virgo Stellar Stream (VSS). The stars were photometrically selected to be probable main-sequence stars in the Galactic halo. When compared with the radial velocity distribution expected for the halo of the Milky Way, as well as the distribution seen in a control field, we observe a significant excess of negative velocity stars in the field, which can likely be attributed to the presence of a stellar stream. This kinematic excess peaks at a Galactic standard of rest radial velocity of −75 km s⁻¹. A rough distance estimate suggests that this feature extends from ∼15 kpc out to, and possibly beyond, the ∼30 kpc limit of the study. The mean velocity of these stars is incompatible with those of the VSS itself (V_gsr ∼ 130 km s⁻¹), which we weakly detect, but it is consistent with radial velocity measurements of nearby 2MASS M-giants and SDSS+SEGUE K/M-giants and blue horizontal branch stars that constitute the leading tidal tail of the Sagittarius dwarf spheroidal galaxy. Some oblate models for the shape of the Milky Way's dark matter halo predict that the leading arm of the Sagittarius Stream should pass through this volume, and have highly negative (V_gsr ≲ −200 km s⁻¹) radial velocities, as it descends down from the northern Galactic hemisphere toward the Galactic plane. The kinematic feature observed in this study, if it is in fact Sagittarius debris, is not consistent with these predictions, and instead, like other leading stream radial velocity measurements, is consistent with a recently published triaxial halo model, or, if axisymmetry is imposed, favors a prolate shape for the Galactic halo potential. However, a rough distance estimate to the observed kinematic feature places it somewhat closer (D ∼ 15–30 kpc) than the Sagittarius models predict (D ∼ 35–45 kpc).

Numerical models of the tidal disruption of the Sagittarius (Sgr) dwarf galaxy have recently been developed that for the first time simultaneously satisfy most observational constraints on the angular position, distance, and radial velocity trends of both leading and trailing tidal streams emanating from the dwarf. We use these dynamical models in combination with extant three-dimensional position and velocity data for Galactic globular clusters and dSph galaxies to identify those Milky Way satellites that are likely to have originally formed in the gravitational potential well of the Sgr dwarf, and have been stripped from Sgr during its extended interaction with the Milky Way. We conclude that the globular clusters Arp 2, M 54, NGC 5634, Terzan 8, and Whiting 1 are almost certainly associated with the Sgr dwarf, and that Berkeley 29, NGC 5053, Pal 12, and Terzan 7 are likely to be as well (albeit at lower confidence). The initial Sgr system therefore may have contained five to nine globular clusters, corresponding to a specific frequency S_N = 5–9 for an initial Sgr luminosity M_V = −15.0. Our result is consistent with the 8 ± 2 genuine Sgr globular clusters expected on the basis of statistical modeling of the Galactic globular cluster distribution and the corresponding false-association rate due to chance alignments with the Sgr streams. The globular clusters identified as most likely to be associated with Sgr are consistent with previous reconstructions of the Sgr age–metallicity relation, and show no evidence for a second-parameter effect shaping their horizontal branch morphologies. We find no statistically significant evidence to suggest that any of the recently discovered population of ultrafaint dwarf galaxies are associated with the Sgr tidal streams, but are unable to rule out this possibility conclusively for all systems.

We present a new N-body model for the tidal disruption of the Sagittarius (Sgr) dwarf that is capable of simultaneously satisfying the majority of angular position, distance, and radial velocity constraints imposed by current wide-field surveys of its dynamically young (≲3 Gyr) tidal debris streams. In particular, this model resolves the conflicting angular position and radial velocity constraints on the Sgr leading tidal stream that have been highlighted in recent years. While the model does not reproduce the apparent bifurcation observed in the leading debris stream, recent observational data suggest that this bifurcation may represent a constraint on the internal properties of the Sgr dwarf rather than the details of its orbit. The key element in the success of this model is the introduction of a non-axisymmetric component to the Galactic gravitational potential that can be described in terms of a triaxial dark matter halo whose minor/major axis ratio (c/a)_Φ = 0.72 and intermediate/major axis ratio (b/a)_Φ = 0.99 at radii 20 kpc < r < 60 kpc. The minor/intermediate/major axes of this halo lie along the directions (l, b) = (7°, 0°), (0°, 90°), and (97°, 0°) respectively, corresponding to a nearly oblate ellipsoid whose minor axis is contained within the Galactic disk plane. This particular disk/halo orientation is difficult to reconcile within the general context of galactic dynamics (and cold dark matter models in particular), suggesting either that the orientation may have evolved significantly with time or that inclusion of other non-axisymmetric components (such as the gravitational influence of the Magellanic Clouds) in the model may obviate the need for triaxiality in the dark matter halo. The apparent proper motion of Sgr in this model is estimated to be (μ_lcos b, μ_b) = (−2.16, 1.73) mas yr⁻¹, corresponding to a Galactocentric space velocity (U, V, W) = (230, −35, 195) km s⁻¹ . Based on the velocity dispersion in the stellar tidal streams, we estimate that Sgr has a current bound mass M_Sgr = 2.5^+1.3_−1.0 × 10⁸ M_☉. We demonstrate that with simple assumptions about the star formation history of Sgr, tidal stripping models naturally give rise to gradients in the metallicity distribution function (MDF) along the stellar debris streams similar to those observed in recent studies. These models predict a strong evolution in the MDF of the model Sgr dwarf with time, indicating that the chemical abundances of stars in Sgr at the present day may be significantly different than the abundances of those already contributed to the Galactic stellar halo. We conclude by using the new N-body model to re-evaluate previous claims of the association of miscellaneous halo substructure with the Sgr dwarf.

What was the mass of the progenitor of the Sagittarius (Sgr) dwarf galaxy? Here, we reassemble the stellar debris using Sloan Digital Sky Survey and Two Micron All-Sky Survey data to find the total luminosity and likely mass. We find that the luminosity is in the range (9.6–13.2) × 10⁷L_☉ or M_V ∼ −15.1 to −15.5, with 70% of the light residing in the debris streams. The progenitor is somewhat fainter than the present-day Small Magellanic Cloud, and comparable in brightness to the M31 dwarf spheroidals NGC 147 and NGC 185. Using cosmologically motivated models, we estimate that the mass of Sgr's dark matter halo prior to tidal disruption was ∼10¹⁰M_☉.