A Two Micron All Sky Survey View of the Sagittarius Dwarf Galaxy. II. Swope Telescope Spectroscopy of M Giant Stars in the Dynamically Cold Sagittarius Tidal Stream (original) (raw)
Related papers
2003
We present the first all-sky view of the Sagittarius (Sgr) dwarf galaxy mapped by M giant star tracers detected in the complete Two Micron All-Sky Survey (2MASS). The main body is fit with a King profile of 30 deg limiting radius, but with a break in the density profile from stars in tidal tails. We argue that much of the observed structure beyond the 224' core radius may be unbound as the satellite undergoes catastrophic disruption. A striking, >150 deg trailing tidal tail extends from the Sgr center and arcs across the South Galactic Hemisphere. A prominent leading debris arm extends from the Sgr center northward of the Galactic plane to an ~40 kpc apoGalacticon, loops towards the North Galactic Cap (NGC) and descends back towards the Galactic plane, foreshortened and covering the NGC. The Sgr tails lie along a well-defined orbital plane that shows little precession, which supports the notion of a nearly spherical Galactic potential. The Sun lies near the path of leading Sgr debris; thus, former Sgr stars may be near or in the solar neighborhood. The number of M giants in the Sgr tails is >15% that within the King limiting radius of the Sgr center. That several gigayear old M giants are so widespread along the Sgr tidal arms not only places limits on the dynamical age of these arms but poses a timing problem that bears on the recent binding energy of the Sgr core and that is naturally explained by recent and catastrophic mass loss. Sgr appears to contribute >75% of the high latitude, halo M giants; no evidence for M giant tidal debris from the Magellanic Clouds is found. Generally good correspondence is found between the M giant, all-sky map of the Sgr system and all previously published detections of potential Sgr debris with the exception of Sgr carbon stars -- which must be subluminous to resolve the discrepancy.
The Astronomical …, 2004
We present high-resolution spectroscopic measurements of the abundances of the α element titanium (Ti) and s-process elements yttrium (Y) and lanthanum (La) for 59 candidate M giant members of the Sagittarius (Sgr) dwarf spheroidal (dSph) + tidal tail system pre-selected on the basis of position and radial velocity. As expected, the majority of these stars show peculiar abundance patterns compared to those of nominal Milky Way stars, but as a group the stars form a coherent picture of chemical enrichment of the Sgr dSph from [Fe/H] =-1.4 to solar abundance. This sample of spectra provides the largest number of Ti, La and Y abundances yet measured for a dSph, and spans metallicities not typically probed by studies of the other, generally more metal-poor Milky Way (MW) satellites. On the other hand, the overall [Ti/Fe], [Y/Fe], [La/Fe] and [La/Y] patterns with [Fe/H] of the Sgr stream plus Sgr core do, for the most part, resemble those seen in the Large Magellanic Cloud (LMC) and other dSphs, only shifted by ∆[Fe/H]∼+0.4 from the LMC and by ∼+1 dex from the other dSphs; these relative shifts reflect the faster and/or more efficient chemical evolution of Sgr compared to the other satellites, and show that Sgr has had an enrichment history more like the LMC than the other dSphs. By tracking the evolution of the abundance patterns along the Sgr stream we can follow the time variation of
The Astrophysical Journal, 2012
We present three-dimensional kinematics of Sagittarius (Sgr) trailing tidal debris in six fields located 70-130 • along the stream from the Sgr dwarf galaxy core. The data are from our proper-motion (PM) survey of Kapteyn's Selected Areas, in which we have measured accurate PMs to faint magnitudes in ∼ 40 ′ × 40 ′ fields evenly spaced across the sky. The radial velocity (RV) signature of Sgr has been identified among our followup spectroscopic data in four of the six fields and combined with mean PMs of spectroscopically-confirmed members to derive space motions of Sgr debris based on ∼15-64 confirmed stream members per field. These kinematics are compared to predictions of the Law & Majewski (2010a) model of Sgr disruption; we find reasonable agreement with model predictions in RVs and PMs along Galactic latitude. However, an upward adjustment of the Local Standard of Rest velocity (Θ LSR ) from its standard 220 km s −1 to at least 232 ± 14 km s −1 (and possibly as high as 264 ± 23 km s −1 ) is necessary to bring 3-D model debris kinematics and our measurements into agreement. Satisfactory model fits that simultaneously reproduce known position, distance, and radial velocity trends of the Sgr tidal streams, while significantly increasing Θ LSR , could only be achieved by increasing the Galactic bulge and disk mass while leaving the dark matter halo fixed to the best-fit values from Law & Majewski (2010a). We derive low-resolution spectroscopic abundances along this stretch of the Sgr stream and find a constant [Fe/H] ∼ -1.15 (with ∼ 0.5 dex scatter in each field -typical for dwarf galaxy populations) among the four fields with reliable measurements. A constant metallicity suggests that debris along the ∼ 60 • span of this study was all stripped from Sgr on the same orbital passage.
A catalogue of oxygen-rich pulsating giants in the Galactic halo and the Sagittarius stream
To construct a catalogue of oxygen-rich (M) asymptotic giant branch (AGB) stars in the halo, complementing the catalogues of carbon-rich (C) stars, previous lists of Miras and SRa semi-regulars located in the northern hemisphere are merged and cleaned of various defects. After putting aside known C stars, characteristics such as colours and periods indicate that most of the remaining objects are M stars. Distances are obtained through the period-luminosity relation. By considering their position in the sky, stars lying at |Z| > 5 kpc are confirmed to be in majority in the Sgr tidal arms. The M stars are more numerous than C ones. Our distance scale is supported by two cool variables located in the Pal 4 globular cluster. Along the Sgr arms, there is reasonable agreement on distances of our objects with recent RR Lyrae distances. A few stars may be as distant as 150 kpc, with possibly four at the trailing arm apocentre, and two in the A16 sub-structure, angularly close to two C stars. Ninety radial velocities are collected from Gaia and other sources. A catalogue with 417 M pulsating AGB stars is provided. This catalogue contains ∼260 stars in the halo with |Z| > 5 kpc. Their K s magnitudes range from 8 up to 13. For comparison, the catalogue also provides ∼150 stars in the disc having 5 < K s < 8.
2006
We present reliable measurements of the metallicity distribution function (MDF) at different points along the tidal stream of the Sagittarius (Sgr) dwarf spheroidal (dSph) galaxy, based on high resolution, echelle spectroscopy of candidate M giant members of the Sgr system. The Sgr MDF is found to evolve significantly from a median [Fe/H] ~-0.4 in the core to ~-1.1 dex over a Sgr leading arm length representing ~2.5-3.0 Gyr of dynamical (i.e. tidal stripping) age. This is direct evidence that there can be significant chemical differences between current dSph satellites and the bulk of the stars they have contributed to the halo. Our results suggest that Sgr experienced a significant change in binding energy over the past several Gyr, which has substantially decreased its tidal boundary across a radial range over which there must have been a significant metallicity gradient in the progenitor galaxy. By accounting for MDF variation along the debris arms, we approximate the MDF Sgr would have had several Gyr ago. We also analyze the MDF of a moving group of M giants we previously discovered towards the North Galactic Cap having opposite radial velocities to the infalling Sgr leading arm stars there and propose that most of these represent Sgr trailing arm stars overlapping the Sgr leading arm in this part of the sky. If so, these trailing arm stars further demonstrate the strong MDF evolution within the Sgr stream.
Discovery of new globular clusters in the Sagittarius dwarf galaxy
Astronomy & Astrophysics, 2021
Context. Globular clusters (GCs) are witnesses of the past accretion events onto the Milky Way. In particular, the GCs of the Sagittarius (Sgr) dwarf galaxy are important probes of an on-going merger. Aims. Our main goal is to search for new GC members of this dwarf galaxy using the VISTA Variables in the Via Lactea Extended Survey (VVVX) near-infrared database combined with the Gaia Early Data Release 3 (EDR3) optical database. Methods. We investigated all VVVX-enabled discoveries of GC candidates in a region covering about 180 sq. deg. toward the bulge and the Sgr dwarf galaxy. We used multiband point-spread function photometry to obtain deep color-magnitude diagrams (CMDs) and luminosity functions (LFs) for all GC candidates, complemented by accurate Gaia-EDR3 proper motions (PMs) to select Sgr members and variability information to select RR Lyrae which are potential GC members. Results. After applying a strict PM cut to discard foreground bulge and disk stars, the CMDs and LFs ...
The Astrophysical Journal, 2012
We present the latitude-normalized radial velocity (v b ) distribution of 3318 subsolar metallicity, V 13.5 stars from the Grid Giant Star Survey (GGSS) in Southern Hemisphere fields. The sample includes giants mostly within ∼5 kpc from the Galactic disks and halo. The nearby halo is found to (1) exhibit significant kinematical substructure, and (2) be prominently represented by several velocity coherent structures, including a very retrograde "cloud" of stars at l ∼ 285 • and extended, retrograde "streams" visible as relatively tight l-v b sequences. One sequence in the fourth Galactic quadrant lies within the l-v b space expected to contain tidal debris from the "star cluster" ωCentauri. Not only does ωCen lie precisely in this l-v b sequence, but the positions and v b of member stars match those of N-body simulations of tidally disrupting dwarf galaxies on orbits ending with ωCen's current position and space motion. But the ultimate proof that we have very likely found extended parts of the ωCen tidal stream comes from echelle spectroscopy of a subsample of the stars that reveals a very particular chemical abundance signature known to occur only in ωCen. The newly discovered ωCen debris accounts for almost all fourth Galactic quadrant retrograde stars in the southern GGSS, which suggests ωCen is a dominant contributor of retrograde giant stars in the inner Galaxy.
Astrophysical Journal, 2007
We have assembled a large-area spectroscopic survey of giant stars in the Sagittarius (Sgr) dwarf galaxy core. Using medium resolution (R ∼ 15,000), multifiber spectroscopy we have measured velocities of these stars, which extend up to 12 • from the galaxy's center (3.7 core radii or 0.4 times the King limiting radius). From these high-quality spectra we identify 1310 Sgr members out of 2296 stars surveyed, distributed across 24 different fields across the Sgr core. Additional slit spectra were obtained of stars bridging from the Sgr core to its trailing tail. Our systematic, large-area sample shows no evidence for significant rotation, a result at odds with the ∼20 km s −1 rotation required as an explanation for the bifurcation seen in the Sgr tidal stream; the observed small ( 4 km s −1 ) velocity trend primarily along the major axis is consistent with models of the projected motion of an extended body on the sky with no need for intrinsic rotation. The Sgr core is found to have a flat velocity dispersion (except for a kinematically colder center point) across its surveyed extent and into its tidal tails, a property that matches the velocity dispersion profiles measured for other Milky Way dwarf spheroidal (dSph) galaxies. We comment on the possible significance of this observed kinematical similarity for the dynamical state of the other classical Milky Way dSphs in light of the fact that Sgr is clearly a strongly tidally disrupted system.
Remnants of the Sagittarius Dwarf Spheroidal Galaxy around the Young Globular Cluster Palomar 12
Astrophysical Journal, 2002
Photometry of a large field around the young globular cluster Palomar 12 has revealed the main-sequence of a low surface-brightness stellar system. This main-sequence is indicative of a stellar population that varies significantly in metallicity and/or age, but in the mean is more metal poor than Pal 12. Under different assumptions for the properties of this population, we find distances from the Sun in the range 17-24 kpc, which encompasses the distance to Pal 12, 19.0pm0.919.0\pm0.919.0pm0.9 kpc. The stellar system is also detected in a field 2$\arcdeg$ North of Pal 12, which indicates it has a minimum diameter of sim0.9\sim0.9sim0.9 kpc. The orbit of Pal 12 (Dinescu et al. 2000), the color-magnitude diagram of the stellar system, their positions on the sky, and their distances suggest that they are debris from the tidal disruption of the Sgr dSph galaxy. We discuss briefly the implications for the evolution of Sgr and the Galactic halo.