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We report chemical abundances for a sample of 66 M giants with high S/N high-resolution spectroscopy in the inner halo of the Milky Way. The program giant stars have radial velocities that vary significantly from those expected for stars moving on uniform circular orbits in the Galactic disk. Thus, based on kinematics, we expect a sample dominated by halo stars. Abundances are derived for α-elements and neutron capture elements. By analyzing the multi-dimensional abundance space, the formation site of the halo giants – in-situ or accreted – can be assessed. Of particular interest are a class of stars that form in-situ, deep in the Milky Way's gravitational potential well, but are “kicked out” of the disk into the halo due to a perturbation event. We find: (1) our sample is dominated by accreted stars and (2) tentative evidence of a small kicked-out population in our Milky Way halo sample.
We have obtained deep g, r, and i-band Subaru and ultra-deep 3.6 μm IRAC images of parts of the multiply-wrapped stellar stream around the nearby edge-on galaxy NGC 5907. We have fitted the surface brightness measurements of the stream with FSPS stellar population synthesis models to derive the metallicity and age of the brightest parts of the stream. The resulting relatively high metallicity ([Fe/H] = −0.3) is consistent with a major merger scenario but a satellite accretion event cannot be ruled out.
2.1 Somewhat historical: overview of the Local Group, dwarf galaxies, and their observed structures
Before taking on a discussion of the dynamics of Local Group (LG) galaxies and the contributing and competing effects of dark matter and tides, it is useful to have an understanding of the spatial distribution of these galaxies, the distribution of their types and masses, and their morphologies – all of which play critical roles in defining how dark matter and tides play out their dynamical tug-of-war. The most common types of galaxies – the dwarfs – which are the most dark matter dominated as well as those among LG galaxies to show the greatest evidence for tidal effects, are the primary focus of this chapter.
2.1.1 The Local Group in context
Large-scale galaxy redshift surveys over the past decades (e.g., Davis et al., 1982; Geller and Huchra, 1989; Shectman et al., 1996; York et al., 2000; Colless et al., 2001; Strauss et al., 2002; Abazajian et al., 2009; Jones et al., 2009) have revealed clearly the filamentary structure of the distribution of galaxies in the Universe. The nearest 100 Mpc shows vast voids but several large mass concentrations, such as the Perseus-Pisces, Pegasus, Pavo, Coma, Hydra-Centaurus, and Virgo Superclusters. The Milky Way (MW) and the LG of galaxies live on the outskirts of the Virgo Supercluster, whose center lies about 15 Mpc away.
Giant stars make particularly useful tracer stars for halo substructure because they are very bright and very common. I discuss several projects that use giant-star tracers to search the Galactic halo for tidal debris from known Galactic satellites, including that from the Sagittarius dwarf galaxy, and to search for tidal debris features from former, now destroyed satellites. Several cross-sections of the halo reveal it to be networked with extended, coherent substructures, indicating that it is likely to be predominantly made up of accreted satellites.
Rapid rotation in red giant stars may be one signature of the past engulfment of a planetary companion. Models of the future tidal interaction of known exoplanet host stars with their planets show that many of these stars will accrete one or more of their planets, and the orbital angular momentum of these accreted planets is sometimes sufficient to spin up the host stars to a level commonly accepted as “rapid rotation” for giant stars. Planets accreted during the red giant phase should leave behind a chemical signature in the form of unusual abundance patterns in the host red giant's atmosphere. Proposed signatures of planet accretion include the enhancement of Li and 12C; both species are generally depleted in giant star atmospheres by convection but could be replenished by planet accretion. Moreover, accreted planets may preferentially enhance the stellar abundance of refractory elements assuming that the refractory nature of these elements leads to their relative enhancements in the planets themselves. Here we present preliminary results of a search for these predicted chemical signatures through high resolution spectroscopic abundance analysis of both rapidly rotating giant stars (i.e., stars with a higher probability of having experienced planet accretion) and normally rotating giant stars. We find that the rapid rotators are enhanced in Li relative to the slow rotators — a result consistent with Li replenishment through planet absorption.
The Apache Point Observatory Galactic Evolution Experiment (APOGEE) is a large-scale, near-infrared (H-band), high-resolution (R ~ 30,000), high S/N (≳100) spectroscopic survey of Milky Way stellar populations. APOGEE will operate from 1.51–1.68μm, a region that includes useful absorption lines from at least fifteen chemical species including α, odd-Z, and iron peak elements. The APOGEE instrument has a novel design featuring 300 science fibers feeding light to a mosaiced VPH grating and a six-element camera encased in a liquid nitrogen-cooled cryostat. A three year bright-time observing campaign will enable APOGEE to observe approximately 100,000 red giants across the Galactic bulge, disk and halo.
The last decade has seen enormous progress in understanding the structure of the Milky Way and neighboring galaxies via the production of large-scale digital surveys of the sky like 2MASS and SDSS, as well as specialized, counterpart imaging surveys of other Local Group systems. Apart from providing snaphots of galaxy structure, these “cartographic” surveys lend insights into the formation and evolution of galaxies when supplemented with additional data (e.g., spectroscopy, astrometry) and when referenced to theoretical models and simulations of galaxy evolution. These increasingly sophisticated simulations are making ever more specific predictions about the detailed chemistry and dynamics of stellar populations in galaxies. To fully exploit, test and constrain these theoretical ventures demands similar commitments of observational effort as has been plied into the previous imaging surveys to fill out other dimensions of parameter space with statistically significant intensity. Fortunately the future of large-scale stellar population studies is bright with a number of grand projects on the horizon that collectively will contribute a breathtaking volume of information on individual stars in Local Group galaxies. These projects include: (1) additional imaging surveys, such as Pan-STARRS, SkyMapper and LSST, which, apart from providing deep, multicolor imaging, yield time series data useful for revealing variable stars (including critical standard candles, like RR Lyrae variables) and creating large-scale, deep proper motion catalogs; (2) higher accuracy, space-based astrometric missions, such as Gaia and SIM-Lite, which stand to provide critical, high precision dynamical data on stars in the Milky Way and its satellites; and (3) large-scale spectroscopic surveys provided by RAVE, APOGEE, HERMES, LAMOST, and the Gaia spectrometer, which will yield not only enormous numbers of stellar radial velocities, but extremely comprehensive views of the chemistry of stellar populations. Meanwhile, previously dust-obscured regions of the Milky Way will continue to be systematically exposed via large infrared surveys underway or on the way, such as the various GLIMPSE surveys from Spitzer's IRAC instrument, UKIDSS, APOGEE, JASMINE and WISE.
The Apache Point Observatory Galactic Evolution Experiment (APOGEE) is a large scale, high-resolution, near-infrared spectroscopic survey of Milky Way stellar populations and one of the four experiments in the Sloan Digital Sky Survey III (SDSS-III). APOGEE will be based on a new multi-fiber cryogenic spectrograph, currently under construction, expected to begin survey observations on the 2.5 m Sloan telescope in the Spring of 2011. APOGEE will measure high-precision radial velocities and elemental abundances for ~15 elements for ~ 105 stars, and is expected to shed new light on the processes that led to the formation of the Galaxy.
We present high-resolution spectroscopic measurements of the abundances of the α-like element titanium (Ti) and s-process elements yttrium (Y) and lanthanum (La) for M giant candidates of (a) the Sagittarius (Sgr) dwarf spheroidal + tidal tail system, (b) the Triangulum-Andromeda (TriAnd) Star Cloud, and (c) the Galactic Anticenter Stellar Structure (GASS, or Monoceros Stream). All three systems show abundance patterns unlike the Milky Way but typical of dwarf galaxies. The Sgr system abundance patterns resemble those of the Large Magellanic Cloud. GASS/Mon chemically resembles Sgr but is distinct from TriAnd, a result that does not support previous suggestions that TriAnd is a piece of the Monoceros Stream.
We describe an ongoing, large-scale, photometric and spectroscopic survey of the Large Magellanic Cloud (LMC) periphery. This survey uses Washington M, T2 + DDO51 photometry to identify distant LMC red giant branch (RGB) star candidates; multi-object spectroscopy is used to confirm the stellar surface gravities of these RGB stars and their association with the LMC (e.g., through radial velocities). The survey now encompasses hundreds of fields ranging from the LMC center with full azimuthal coverage around the LMC and out to 23° from the LMC center. We have confirmed the existence of RGB stars with (the unusual) Magellanic velocities out to the radial limit of this survey coverage. From data in a subsample of these fields, we show that this extended population of stars makes up a diffuse structure enveloping the LMC with a two-dimensional distribution resembling a classical halo with a shallow de Vaucouleurs profile and a broad metallicity spread around a typical mean value of [Fe/H] ~ −1.0.
A proper motion study of a field of 20′ × 20′ inside Plaut's low extinction window (l,b)=(0o, −8o), has been completed. Relative proper motions and photographic BV photometry have been derived for ~ 21,000 stars reaching to V ~ 20.5 mag, based on the astrometric reduction of 43 photographic plates, spanning over 21 years of epoch difference. Proper motion errors are typically 1 mas yr−1. Cross-referencing with the 2MASS catalog yielded a sample of ~ 8700 stars, from which predominantly disk and bulge subsamples were selected photometrically from the JH color-magnitude diagram. The two samples exhibited different proper-motion distributions, with the disk displaying the expected reflex solar motion. Galactic rotation was also detected for stars between ~2 and ~3 kpc from us. The bulge sample, represented by red giants, has an intrinsic proper motion dispersion of (σl, σb) = (3.39, 2.91)±(0.11, 0.09) mas yr−1, which is in good agreement with previous results. A mean distance of kpc has been estimated for the bulge sample, based on the observed K magnitude of the horizontal branch red clump. The metallicity [M/H] distribution was also obtained for a subsample of 60 bulge giants stars, based on calibrated photometric indices. The observed [M/H] shows a peak value at [M/H] ~ −0.1 with an extended metal poor tail and around 30% of the stars with supersolar metallicity. No change in proper motion dispersion was observed as a function of [M/H]. We are currently in the process of obtaining CCD UBV RI photometry for the entire proper-motion sample of ~ 21,000 stars.
2MASS has provided a three-dimensional map of the > 360°, wrapped tidal tails of the Sagittarius (Sgr) dwarf spheroidal galaxy, as traced by M giant stars. With the inclusion of radial velocity data for stars along these tails, strong constraints exist for dynamical models of the Milky Way-Sgr interaction. N-body simulations of Sgr disruption with model parameters spanning a range of initial conditions (e.g., Sgr mass and orbit, Galactic rotation curve, halo flattening) are used to find parameterizations that match almost every extant observational constraint of the Sgr system. We discuss the implications of the Sgr data and models for the orbit, mass and M/L of the Sgr bound core as well as the strength, flattening, and lumpiness of the Milky Way potential.
I would like to focus on one aspect regarding the evolution of Galactic stellar populations that is particularly relevant to discussions at this symposium: Where were the sites of early star formation in the Galaxy? The large scatter in abundance ratios for metal poor stars suggests multiple early settings of star formation in the Milky Way. In this and other ways, interpretation of detailed stellar chemical abundance analyses are converging with those of spatial-kinematical analyses of field stars, star clusters and satellite galaxies.
NASA’s Space Interferometry Mission (SIM), scheduled for launch in 2009, will determine the positions of thousands of stars as faint as V = 20 to a precision better than 4 microarcseconds (µas). A key part of the mission is the Astrometric Grid, which is a reference frame of several thousand stars with V ≤ 13 against which all relative measurements will be calibrated. To serve as a reliable inertial reference frame, the Grid must be astrometrically stable against photocenter jitter (from planets, binary companions, flaring or spotting) at the ~ 4µas level. Sub–solar metallicity giant stars, by virtue of their intrinsic luminosity, can probe the Galaxy to greater distances than almost any other stellar type at the same apparent magnitude. Thus, distant (> 3 kpc) giants with V < 13 will have proportionately smaller astrometric jitter compared to other potential Astrometric Grid star candidates. The Grid Giant Star Survey is a patchwork all-sky survey to find sub–solar metallicity K giants for the Grid, and to provide a unique database for studies of Galactic stellar populations. We describe here the survey characteristics and give examples of results to date.