Sagittarius Tidal Stream Research Articles

Stellar halo density with LAMOST K and M giants

Aims. We derive the morphology of the stellar component in the outer halo volume, and search for possible overdensities due to substructures therein. Methods. We made use of some of the data releases of the spectroscopic survey LAMOST DR8-DR9 in tandem with distance determinations for two subsamples, that is, of K-giants and M-giants, respectively, making up 60 000 stars. These distance are obtained through Bayesian techniques that derive absolute magnitudes as a function of measured spectroscopic parameters. Our calculation of the density from these catalogues requires: (1) derivation of the selection function; and (2) a correction for the convolution of the distance errors, which we carried out with Lucy’s inversion of the corresponding integral equation. Results. The stellar density distribution of the outer halo (distance to the Galactic centre, rG, of between 25 and 90 kpc) is a smooth monotonously decreasing function with a dependence of approximately ρ ∝ rG−n, with n = 4.6 ± 0.4 for K-giants and n = 4.5 ± 0.2 for M-giants, and with a insignificant oblateness. The value of n is independent of the angular distance to the Sagittarius tidal stream plane, which is what would be expected if such a stream did not exist in the anticenter positions or had a negligible imprint in the density distribution in the outer halo. Apart from random fluctuations or minor anomalies in some lines of sight, we do not see substructures superimposed in the outer halo volume within the resolution that we are using and limited by the error bars. This constrains the mass of over- and under-densities in the outer halo to be of ≲103 M⊙ deg−2, whereas the total mass of the stellar halo, including inner and outer parts, is ∼7 × 108 M⊙.

Water-maser survey towards off-plane O-rich AGBs around the orbital plane of the Sagittarius stellar stream

ABSTRACT A 22 GHz water-maser survey was conducted towards 178 O-rich asymptotic giant branch (AGB) stars with the aim of identifying maser emission associated with the Sagittarius stellar stream. In this survey, maser emissions were detected in 21 targets, 20 of which were new detections. We studied the Galactic distributions of H2O- and SiO-maser-traced AGBs towards the Sgr orbital plane, and found an elongated structure towards the (l, b) ∼ (340°, 40°) direction. In order to verify its association with the Sagittarius tidal stream, we further studied the 3D motion of these sources, but found that, kinematically, these maser-traced AGBs are still Galactic disc sources rather than stream debris. In addition, we found a remarkable outward motion, ∼50 km s−1 away from the Galactic Centre of these maser-traced AGBs, but with no systermatic lag of rotational speed as reported in 2000 for solar-neighbourhood Miras.

Globular Clusters Lost by the Sagittarius Dwarf Spheroidal Galaxy

In this work a search was carried out for globular clusters belonging to the Sagittarius (Sgr) tidal stream using the analysis of spatial positions, radial velocities relative to the Galactic Standard of Rest (V_{GSR}),proper motions and ratio of "age -- metallicity" ([Fe/H]) for globular clusters and for stars in the tidal stream. As a result, three categories of globular clusters were obtained: A -- most certainly in the stream: Terzan 8, Whiting 1, Arp 2, NGC 6715, Terzan 7, Pal 12; B -- kinematic outliers: Pal 5, NGC 5904, NGC 5024, NGC 5053, NGC 5272, NGC 288; C -- lowest rank candidates: NGC 6864, NGC 5466, NGC 5897, NGC 7492, NGC 4147.

Evidence for the accretion origin of halo stars with an extreme r-process enhancement

Small stellar systems like dwarf galaxies are suggested to be the main building blocks of our Galaxy by numerical simulations in Lambda CDM models. The existence of star streams like Sagittarius tidal stream indicates that dwarf galaxies play a role in the formation of the Milky Way. However, it is unclear how many and what kind of stars in our Galaxy are originated from satellite dwarf galaxies, which could be constrained by chemical abundances of metal-poor stars. Here we report on the discovery of a metal-poor star with an extreme r-process enhancement and alpha-element deficiency. In this star, the abundance ratio of the r-process element Eu with respect to Fe is more than one order of magnitude higher than the Sun and the metallicity is 1/20 of the solar one. Such kind of stars have been found in present-day dwarf galaxies, providing the clearest chemical signature of past accretion events. The long timescale of chemical evolution of the host dwarf galaxy expected from the abundance of alpha element with respect to Fe suggests that the accretion occurred in a relatively late phase compared to most of the accretions that formed the bulk of the Milky Way halo.

Effect of the Galactic Disk and Satellite Galaxies on the Spatial Distribution of Globular Clusters

The distribution anisotropy in systems of globular clusters (GCs) in satellite galaxies of the Milky Way is considered using the inertia tensor. The statistical significance of the results is estimated by repeating this analysis for random catalogs. This method reproduces a known planar structure in the distribution of satellite galaxies; however, the significance of this structure is slightly lower than that for other methods used in the literature. For the GCs at distances 2 < R < 10 kpc, the structure is coplanar with the galactic plane. At large distances, the structure orientation is close to the orientations of the satellite galaxies, i.e., is perpendicular to the Galaxy plane. The probability of stochastic realization of such distribution is 1.7%. The GC distribution anisotropy is also measured without the GCs belonging to the Sagittarius tidal stream. Measurements show that the effect of the stream on the GC perturbation is weak.

Fitting the Density Substructure of the Stellar Halo with MilkyWay@home

Abstract We propose and test a method for applying statistical photometric parallax to main-sequence turnoff stars in the Sloan Digital Sky Survey (SDSS). Using simulated data, we show that if our density model is similar to the actual density distribution of our data, we can reliably determine the density model parameters of three major substructures in the Milky Way halo using the computational resources available on MilkyWay@home (a 20-parameter fit). We fit the stellar density in SDSS stripe 19 with a smooth stellar spheroid component and three major streams. One of these streams is consistent with the Sagittarius tidal stream at 21.1 kpc away, one is consistent with the trailing tail of the Sagittarius tidal stream in the north Galactic cap at 48 kpc away, and one is possibly part of the Virgo Overdensity at 6 kpc away. We find the 1σ widths of these three streams to be 1.0 kpc, 17.6 kpc, and 6.1 kpc, respectively. The width of the trailing tail is extremely wide (41 kpc full width at half maximum). This large width could have implications for the shape of the Milky Way dark matter halo. The width of the Virgo Overdensity-like structure is consistent with what we might expect for a “cloudlike” structure; analysis of additional stripes of data are needed to outline the full extent of this structure and confirm its association with the Virgo Overdensity.

Chemical Abundances of Hydrostatic and Explosive Alpha-elements in Sagittarius Stream Stars

Abstract We analyze chemical abundances of stars in the Sagittarius (Sgr) tidal stream using high-resolution Gemini+GRACES spectra of 42 members of the highest surface-brightness portions of both the trailing and leading arms. Targets were chosen using a 2MASS+WISE color–color selection, combined with the Large Sky Area Multi-Object Fibre Spectroscopic Telescope (LAMOST) radial velocities. In this Letter, we analyze [Fe/H] and α-elements produced by both hydrostatic (O, Mg) and explosive (Si, Ca, Ti) nucleosynthetic processes. The average [Fe/H] for our Sgr stream stars is lower than that for stars in the Sgr core, and stars in the trailing and leading arms show systematic differences in [Fe/H]. Both hydrostatic and explosive elements are depleted relative to Milky Way (MW) disk and halo stars, with a larger gap between the MW trend and Sgr stars for the hydrostatic elements. Chemical abundances of Sgr stream stars show similar patterns to those measured in the core of the Sgr dSph. We explore the ratio of hydrostatic to explosive α-elements [α h/ex] (which we refer to as the “HEx ratio”). Our observed HEx ratio trends for Sgr debris are deficient relative to MW stars. Via simple chemical evolution modeling, we show that these HEx ratio patterns are consistent with a Sgr IMF that lacks the most massive stars. This study provides a link between the chemical properties in the intact Sgr core and the significant portion of the Sgr system’s luminosity that is estimated to currently reside in the streams.

On the Nature of Ultra-faint Dwarf Galaxy Candidates. II. The Case of Cetus II

Abstract 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.

The globular cluster NGC 7492 and the Sagittarius tidal stream: together but unmixed

We have derived from VIMOS spectroscopy the radial velocities for a sample of 71 stars selected from CFHT/Megacam photometry around the Galactic globular cluster NGC7492. In the resulting velocity distribution, it is possible to distinguish two relevant non-Galactic kinematic components along the same line of sight: a group of stars at $\langle{v_{\rm r}}\rangle \sim 125$km s$^{-1}$ which is compatible with the velocity of the old leading arm of the Sagittarius tidal stream, and a larger number of objects at $\langle{v_{\rm r}}\rangle \sim -110$km s$^{-1}$ that might be identified as members of the trailing wrap of the same stream. The systemic velocity of NGC7492 set at $v_{\rm r} \sim -177$km s$^{-1}$ differs significantly from that of both components, thus our results confirm that this cluster is not one of the globular clusters deposited by the Sagittarius dwarf spheroidal in the Galactic halo, even if it is immersed in the stream. A group of stars with $<v_{\rm r}> \sim -180$km s$^{-1}$ might be comprised of cluster members along one of the tidal tails of NGC7492.

The southern leading and trailing wraps of the Sagittarius tidal stream around the globular cluster Whiting 1

We present a study of the kinematics of 101 stars observed with VIsible MultiObject Spectrograph around Whiting 1, a globular cluster embedded in the Sagittarius tidal stream. The obtained velocity distribution shows the presence of two wraps of that halo substructure at the same heliocentric distance as that of the cluster and with well-differentiated mean radial velocities. The most prominent velocity component seems to be associated with the trailing arm of Sagittarius with 〈vr〉 ∼ −130 km s−1, which is consistent with the velocity of Whiting 1. This result supports that this globular cluster was formed in Sagittarius and recently accreted by the Milky Way. The second component with 〈vr〉 ∼ 120 km s−1 might correspond to the leading arm of Sagittarius, which has been predicted by numerical simulations but with no conclusive observational evidence of its existence presented so far. This detection of the old leading wrap of Sagittarius in the Southern hemisphere may be used to confirm and further constrain the models for its orbit and evolution.

Whiting 1: Confirmation of its accretion by the Milky Way

We investigate the association of Whiting 1 with the Sagittarius tidal stream by obtaining radial velocities for a sample of 101 stars observed with VIMOS. Our results reveal the presence of a component of the Sagittarius tidal stream with a radial velocity – and distance – compatible with that of the globular cluster. Therefore, we conclude that Whiting1 was formed in the interior of the Sagittarius dwarf spheroidal galaxy and later accreted by the Milky Way. In addition, our data also reveal the detection for the first time of an ancient wrap of the Sagittarius tidal stream along the same line-of-sight and at the same heliocentric distance.

New tidal debris nearby the Sagittarius leading tail from the LAMOST DR2 M giant stars

We report two new sets of tidal debris nearby the Sagittarius (Sgr) tidal stream in the north Galactic cap (NGC) identified from the M giant stars in LAMOST DR2. The M giant stars located in the sky area of 210° < Λ < 290°, and having a distance of 10-20 kpc and [Fe/H] < −0.75 show clear bimodality in their velocity distribution. We denote the two peaks as Vel−3+83 for the one within a mean velocity of −3 km s−1 with respect to that of the well observed Sgr leading tail at the same Λ and Vel+162+26 for the other one with a mean velocity of 162 km s−1 with respect to the Sgr leading tail. Although the projected Λ −Vgsr relation of Vel−3+83 is very similar to the Sgr leading tail, the opposite trend in the Λ—distance relation as compared to the Sgr leading tail suggests Vel−3+83 has a different 3D direction of motion with any branch of the simulated Sgr tidal stream from Law & Majewski. Therefore, we propose it is new tidal debris not related to the Sgr stream. Similarly, the other substructure Vel+162+26, which is the same one as the NGC group discovered by Chou et al., also moves toward a different direction with respect to the Sgr stream, implying that it may have a different origin than the Sgr tidal stream.

A search for stellar tidal debris of defunct dwarf galaxies around globular clusters in the inner Galactic halo

In the hierarchical formation scenario in which the outer halo of the Milky Way is the result of the continuous accretion of low-mass galaxies, a fraction of the Galactic globular cluster system might have originated in and been accreted with already extinct dwarf galaxies. In this context, we expect that the remnants of these progenitor galaxies might be still populating the surroundings of those accreted globulars. In this work, we present wide-field photometry of a sample of 23 globular clusters in the Galactocentric distance range 10 < Rg < 40kpc, which we use to search for remnants of their hypothetical progenitor systems. Our deep photometry reveals the presence of underlying stellar populations along the line-of-sight of about half of the globulars included in our sample. Among the detections lying in the footprint of the Sagittarius tidal stream, which we identify via the comparison with its orbit derived from numerical simulations, only Whiting1 and NGC7492 seem to be inmersed in that remnant at a compatible heliocentric distance. We also confirm the existence of a subjacent Main-Sequence feature in the surroundings of NGC1851. A tentative detection of the vast Hercules-Aquila cloud is unveiled in the background of NGC7006.

THE CATALINA SURVEYS PERIODIC VARIABLE STAR CATALOG

We present ~47,000 periodic variables found during the analysis of 5.4 million variable star candidates within a 20,000 square degree region covered by the Catalina Surveys Data Release-1 (CSDR1). Combining these variables with type-ab RR Lyrae from our previous work, we produce an on-line catalog containing periods, amplitudes, and classifications for ~61,000 periodic variables. By cross-matching these variables with those from prior surveys, we find that > 90% of the ~8,000 known periodic variables in the survey region are recovered. For these sources we find excellent agreement between our catalog and prior values of luminosity, period and amplitude, as well as classification. We investigate the rate of confusion between objects classified as contact binaries and type-c RR Lyrae (RRc's) based on periods, colours, amplitudes, metalicities, radial velocities and surface gravities. We find that no more than few percent of these variables in these classes are misidentified. By deriving distances for this clean sample of ~5,500 RRc's, we trace the path of the Sagittarius tidal streams within the Galactic halo. Selecting 146 outer-halo RRc's with SDSS radial velocities, we confirm the presence of a coherent halo structure that is inconsistent with current N-body simulations of the Sagittarius tidal stream. We also find numerous long-period variables that are very likely associated within the Sagittarius tidal streams system. Based on the examination of 31,000 contact binary light curves we find evidence for two subgroups exhibiting irregular lightcurves. One subgroup presents significant variations in mean brightness that are likely due to chromospheric activity. The other subgroup shows stable modulations over more than a thousand days and thereby provides evidence that the O'Connell effect is not due to stellar spots.

UPDATE ON THE CETUS POLAR STREAM AND ITS PROGENITOR

We trace the Cetus Polar Stream (CPS) with blue horizontal branch (BHB) and red giant stars (RGBs) from Data Release 8 of the Sloan Digital Sky Survey (SDSS DR8). Using a larger dataset than was available previously, we are able to refine the measured distance and velocity to this tidal debris star stream in the south Galactic cap. Assuming the tidal debris traces the progenitor's orbit, we fit an orbit to the CPS and find that the stream is confined between ~24-36 kpc on a rather polar orbit inclined 87 degrees to the Galactic plane. The eccentricity of the orbit is 0.20, and the period ~700 Myr. If we instead matched N-body simulations to the observed tidal debris, these orbital parameters would change by 10% or less. The CPS stars travel in the opposite direction to those from the Sagittarius tidal stream in the same region of the sky. Through N-body models of satellites on the best-fitting orbit, and assuming that mass follows light, we show that the stream width, line-of-sight depth, and velocity dispersion imply a progenitor of at least 10^8 solar masses. However, the density of stars along the stream requires either a disruption time on the order of one orbit, or a stellar population that is more centrally concentrated than the dark matter. We suggest that an ultra-faint dwarf galaxy progenitor could reproduce a large stream width and velocity dispersion without requiring a very recent deflection of the progenitor into its current orbit. We find that most Cetus stars have metallicities of -2.5 < [Fe/H] < -2.0, similar to the observed metallicities of the ultra-faint dwarfs. Our simulations suggest that the parameters of the dwarf galaxy progenitors, including their dark matter content, could be constrained by observations of their tidal tails through comparison of the debris with N-body simulations.

MAIN-SEQUENCE STAR POPULATIONS IN THE VIRGO OVERDENSITY REGION

We present deep CMDs for two Subaru Suprime-Cam fields in the Virgo Stellar Stream(VSS)/Virgo Overdensity(VOD) and compare them to a field centred on the highest concentration of Sagittarius (Sgr) Tidal Stream stars in the leading arm, Branch A of the bifurcation. A prominent population of MS 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 d_helio of 30.9+-3.0kpc for the MS of the Sgr Stream Branch A. The dominant main sequence populations in the two VSS/VOD fields (Lsun ~265 deg, Bsun ~13 deg) are located at a mean distance of 23.3+-1.6kpc and have an age ~8.2Gyr and an abundance [Fe/H]=-0.67(+0.16;-0.12)dex similar to the Sgr Stream stars. These parameters are also in good agreement with the age of the main population in the Sagittarius dwarf (8.0+-1.5Gyr). They also agree with the peak in the metallicity distribution of 2-3Gyr old M-giants, [Fe/H] ~ -0.6dex, in the Sgr leading arm north. 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 that is calibrated with SDSS Sgr A-branch and 2MASS M-giant stars. We find that the most prominent feature in the CMDs, the MS population at 23kpc, is not explained by the model. Instead the model predicts in these directions a low density filamentary structure of Sgr debris stars at ~9kpc and a slightly higher concentration of Sgr stars spread from 42-53kpc. 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 almost identical age and metallicity as some sections of the Sgr tidal stream.

PROBING THE OUTER GALACTIC HALO WITH RR LYRAE FROM THE CATALINA SURVEYS

We present the analysis of 12227 type-ab RR Lyrae found among the 200 million public lightcurves in the Catalina Surveys Data Release 1 (CSDR1). These stars span the largest volume of the Milky Way ever surveyed with RR Lyrae, covering ~20,000 square degrees of the sky (0 < RA < 360, -22 < Dec < 65 deg) to heliocentric distances of up to 60kpc. Each of the RR Lyrae are observed between 60 and 419 times over a six-year period. Using period finding and Fourier fitting techniques we determine periods and apparent magnitudes for each source. We find that the periods at generally accurate to sigma = 0.002% by comparison with 2842 previously known RR Lyrae and 100 RR Lyrae observed in overlapping survey fields. We photometrically calibrate the light curves using 445 Landolt standard stars and show that the resulting magnitudes are accurate to ~0.05 mags using SDSS data for ~1000 blue horizontal branch stars and 7788 of the RR Lyrae. By combining Catalina photometry with SDSS spectroscopy, we analyze the radial velocity and metallicity distributions for > 1500 of the RR Lyrae. Using the accurate distances derived for the RR Lyrae, we show the paths of the Sagittarius tidal streams crossing the sky at heliocentric distances from 20 to 60 kpc. By selecting samples of Galactic halo RR Lyrae, we compare their velocity, metallicity, and distance with predictions from a recent detailed N-body model of the Sagittarius system. We find that there are some significant differences between the distances and structures predicted and our observations.

A Pan-STARRS1 VIEW OF THE BIFURCATED SAGITTARIUS STREAM

We use data from the Pan-STARRS1 survey to present a panoramic view of the Sagittarius tidal stream in the southern Galactic hemisphere. As a result of the extensive sky coverage of Pan-STARRS1, the southern stream is visible along more than 60 degrees of its orbit, nearly double the length seen by the SDSS. The recently discovered southern bifurcation of the stream is also apparent, with the fainter branch of the stream visible over at least 30 degrees. Using a combination of fitting both the main sequence turn-off and the red clump, we measure the distance to both arms of the stream in the south. We find that the distances to the bright arm of the stream agree very well with the N-body models of Law & Majewski (2010). We also find that the faint arm lies ~5 kpc closer to the Sun than the bright arm, similar to the behavior seen in the northern hemisphere.

THE APACHE POINT OBSERVATORY GALACTIC EVOLUTION EXPERIMENT: FIRST DETECTION OF HIGH-VELOCITY MILKY WAY BAR STARS

Commissioning observations with the Apache Point Observatory Galactic Evolution Experiment (APOGEE), part of the Sloan Digital Sky Survey III, have produced radial velocities (RVs) for ~4700 K/M-giant stars in the Milky Way bulge. These high-resolution (R \sim 22,500), high-S/N (>100 per resolution element), near-infrared (1.51-1.70 um; NIR) spectra provide accurate RVs (epsilon_v~0.2 km/s) for the sample of stars in 18 Galactic bulge fields spanning -1<l<20 deg, |b|<20 deg, and dec>-32 deg. This represents the largest NIR high-resolution spectroscopic sample of giant stars ever assembled in this region of the Galaxy. A cold (sigma_v~30 km/s), high-velocity peak (V_GSR \sim +200 km/s) is found to comprise a significant fraction (~10%) of stars in many of these fields. These high RVs have not been detected in previous MW surveys and are not expected for a simple, circularly rotating disk. Preliminary distance estimates rule out an origin from the background Sagittarius tidal stream or a new stream in the MW disk. Comparison to various Galactic models suggests that these high RVs are best explained by stars in orbits of the Galactic bar potential, although some observational features remain unexplained.

THE METALLICITY OF THE MONOCEROS STREAM

We present low-resolution MMT Hectospec spectroscopy of 594 candidate Monoceros stream member stars. Based on strong color-magnitude diagram overdensities, we targeted three fields within the stream's footprint, with 178 deg < l < 203 deg and -25 deg < b < 25 deg. By comparing the measured iron abundances with those expected from smooth Galactic components alone, we measure, for the first time, the spectroscopic metallicity distribution function for Monoceros. We find the stream to be chemically distinct from both the thick disk and halo, with [Fe/H] = -1, and do not detect a trend in the stream's metallicity with Galactic longitude. Passing from b = +25 deg to b = -25 deg the median Monoceros metallicity trends upward by 0.1 dex, though uncertainties in modeling sample contamination by the disk and halo make this a marginal detection. In each field, we find Monoceros to have an intrinsic [Fe/H] dispersion of 0.10-0.22 dex. From the CaII K line, we measure [Ca/Fe] for a subsample of metal poor program stars with -1.1 < [Fe/H] < -0.5. In two of three fields, we find calcium deficiencies qualitatively similar to previously reported [Ti/Fe] underabundances in Monoceros and the Sagittarius tidal stream. Further, using 90 spectra of thick disk stars in the Monoceros pointings with b ~ +/-25 deg, we detect a 0.22 dex north/south metallicity asymmetry coincident with known stellar density asymmetry at R_GC ~ 12 kpc and |Z| ~ 1.7 kpc. Our median Monoceros [Fe/H] = -1.0 and its relatively low dispersion naturally fit the expectation for an appropriately luminous M_V ~ -13 dwarf galaxy progenitor.