Kathryn V. Johnston

Tracing Galaxy Formation with Stellar Halos. I. Methods

If the favored hierarchical cosmological model is correct, then the Milky Way system should have accreted � 100– 200 luminous satellite galaxies in the past � 12 Gyr. We model this process using a hybrid semianalytic plus N-body approach that distinguishes explicitly between the evolution of light and dark matter in accreted satellites. This distinction is essential to our ability to produce a realistic stellar halo, with mass and density profile much like that of our own Galaxy, and a surviving satellite population that matches the observed number counts and structural parameter distributions of the satellite galaxies of the Milky Way. Our accreted stellar halos have density profiles that typically drop off with radius faster than the dark matter and follow power laws at rk30 kpc with � / r � � , � ’ 3 4. They are well fit by Hernquist profiles over the full radial range. We find that stellar halos are assembled from the inside out, with the majority of mass (� 80%) coming from the � 15 most massive accretion events. The satellites that contributetothestellarhalohavemedianaccretiontimes of � 9Gyrinthepast,whilesurvivingsatellitesystems have median accretion times of � 5 Gyr in the past. This implies that stars associated with the inner halo should be quite differentchemicallyfromstarsinsurvivingsatellitesandalsofromstarsintheouterhaloorthoseliberatedinrecent disruptionevents.Webrieflydiscusstheexpectedspatialstructureandphase-spacestructureforhalosformedinthismanner. Searches for this type of structure offer a direct test of whether cosmology is indeed hierarchical on small scales. Subject headingg dark matter — galaxies: dwarf — galaxies: evolution — galaxies: formation — galaxies: halos — galaxies: kinematics and dynamics — Galaxy: evolution — Galaxy: formation — Galaxy: halo — Galaxy: kinematics and dynamics — Local Group

The Accretion Origin of the Milky Way's Stellar Halo

We have used data from the Sloan Digital Sky Survey (SDSS) Data Release 5 to explore the overall structure and substructure of the stellar halo of the Milky Way using ~4 million color-selected main-sequence turnoff stars with -->0.2 18.5 ? r 0.5 3.7 ? 1.2 ? 108 M?. The density profile of the stellar halo is approximately -->? r??, where ? -->2 > ? > ? 4. Yet, we found that all smooth and symmetric models were very poor fits to the distribution of stellar halo stars because the data exhibit a great deal of spatial substructure. We quantified deviations from a smooth oblate/triaxial model using the rms of the data around the model profile on scales 100 pc, after accounting for the (known) contribution of Poisson uncertainties. Within the DR5 area of the SDSS, the fractional rms deviation ?/total of the actual stellar distribution from any smooth, parameterized halo model is 40%: hence, the stellar halo is highly structured. We compared the observations with simulations of galactic stellar halos formed entirely from the accretion of satellites in a cosmological context by analyzing the simulations in the same way as the SDSS data. While the masses, overall profiles, and degree of substructure in the simulated stellar halos show considerable scatter, the properties and degree of substructure in the Milky Ways halo match well the properties of a typical stellar halo built exclusively out of the debris from disrupted satellite galaxies. Our results therefore point toward a picture in which an important fraction of the stellar halo of the Milky Way has been accreted from satellite galaxies.

A TWO MICRON ALL-SKY SURVEY VIEW OF THE SAGITTARIUS DWARF GALAXY. IV. MODELING THE SAGITTARIUS TIDAL TAILS

M giants recovered from the Two Micron All-Sky Survey have recently been used to map the position and velocity distributions of tidal debris from the Sagittarius (Sgr) dwarf spheroidal galaxy around the entire Galaxy. We compare this data set to both test-particle orbits and N-body simulations of satellite destruction run within a variety of rigid Milky Way potentials, and we find that the mass of the Milky Way within 50 kpc of its center should be × 1011 M☉ in order for any Sgr orbit to simultaneously fit the velocity gradient in the Sgr trailing debris and the apocenter of the Sgr leading debris. Orbital pole precession of young debris and leading debris velocities in regions corresponding to older debris provide contradictory evidence in favor of oblate/prolate Galactic halo potentials, respectively, leading us to conclude that the orbit of Sgr has evolved over the past few gigayears. In light of this discrepancy, we consider constraints from only the younger portions of the debris within three models of the flattening of the Galactic potential [q = 0.90 (O), 1.0 (S), and 1.25 (P), i.e., oblate, spherical, and prolate] in our further N-body simulations. On the basis of the velocity dispersion and width along the trailing tidal stream, we estimate the current bound mass of Sgr to be MSgr = × 108 M☉ independent of the form of the Galactic potential; this corresponds to a range of mass-to-light ratios (M/L)Sgr = 14-36(M/L)☉ for the Sgr core. Models with masses in this range best fit the apocenter of leading Sgr tidal debris when they orbit with a radial period of roughly 0.85 Gyr and have perigalactica and apogalactica of about 15 and 60 kpc, respectively. These distances scale with the assumed distance to the Sgr dwarf and the assumed depth of the Galactic potential. The density distribution of debris along the orbit in these models is consistent with the M giant observations, and debris at all orbital phases in which M giants are obviously present is younger (i.e., was lost more recently from the satellite) than the typical age of a Sgr M giant star.

TRACING GALAXY FORMATION WITH STELLAR HALOS II: RELATING SUBSTRUCTURE IN PHASE- AND ABUNDANCE-SPACE TO ACCRETION HISTORIES

This paper explores the mapping between the observable properties of a stellar halo in phase- and abundance-space and the parent galaxy’s accretion history in terms of the characteristic epoch of accretion and mass and orbits of progenitor objects. The study utilizes a suite of eleven stellar halo models constructed within the context of a standard CDM cosmology. The results demonstrate that coordinate-space studies are sensitive to the recent (0-8 Gyears ago) merger histories of galaxies (this timescale corresponds to the last few to tens of percent of mass accretion for a Milky-Way-type galaxy). Specically, the frequency, sky coverage and fraction of stars in substructures in the stellar halo as a function of surface brightness are indicators of the importance of recent merging and of the luminosity function of infalling dwarfs. The morphology of features serves as a guide to the orbital distribution of those dwarfs. Constraints on the earlier merger history (> 8 Gyears ago) can be gleaned from the abundance patterns in halo stars: within our models, dramatic dierences in the dominant epoch of accretion or luminosity function of progenitor objects leave clear signatures in the [ /Fe] and [Fe/H] distributions of the stellar halo | halos dominated by very early accretion have higher average [ /Fe], while those dominated by high luminosity satellites have higher [Fe/H]. This intuition can be applied to reconstruct much about the merger histories of nearby galaxies from current and future data sets. Subject headings: Galaxy: evolution | Galaxy: formation | Galaxy:halo | Galaxy: kinematics and dynamics | galaxies: dwarf | galaxies: evolution | galaxies: formation | galaxies: halos | galaxies: kinematics and dynamics | Local Group | dark matter

The disruption of the sagittarius dwarf galaxy.

Numerical simulations of dwarf spheroidal galaxies undergoing several close encounters with the Milky Way are described. By comparing our models to observed properties of the recently discovered dwarf galaxy in Sagittarius (Sgr), we discuss implications of our results for the formation and evolution of the Milky Way system. We find that existing observations are not sufficient to allow us to place precise limits on either the orbit or the initial state of the dwarf. Debris from the ongoing tidal stripping of the Sagittarius galaxy are expected to form moving groups in the halo of the Galaxy and the discovery of such stars would strongly constrain the history and dynamical state of the dwarf. Furthermore, if Sgr is presently being disrupted, we predict that its remains will be detectable as a moving group in the halo for more than 1 Gyr. Thus, if similar accretion events have occurred in the recent history of the Galaxy, their aftereffects may still be observable.

Fossil signatures of ancient accretion events in the halo

The role that minor mergers have played in the formation and structure of the Milky Way is still an open question, about which there is much debate. We use numerical simulations to explore the evolution of debris from a tidally disrupted satellite, with the aim of developing a method that can be used to identify and quantify signatures of accretion in a survey of halo stars. For a Milky Way with a spherical halo, we find that debris from minor mergers can remain aligned along great circles throughout the lifetime of the Galaxy. We exploit this result to develop the method of Great Circle Cell Counts (GC3), which we test by applying it to artificially constructed halo distributions. Our results suggest that if as few as 1\% of the stars in a halo survey are accreted from the disruption of a single subsystem smaller than the Large Magellanic Cloud, GC3 can recover the great circle associated with this debris. The dispersion in GC3 can also be used to detect the presence of structure characteristic of accretion in distributions containing a much smaller percentage of material accreted from any single satellite.

A COLD DARK MATTER, STELLAR FEEDBACK, AND THE GALACTIC HALO ABUNDANCE PATTERN

The hierarchical formation scenario for the stellar halo requires the accretion and disruption of dwarf galaxies, yet low-metallicity halo stars are enriched in � -elements compared to similar, low-metallicity stars in dwarf spheroidal (dSph) galaxies. We address this primary challenge for the hierarchical formation scenario for the stellar halo by combining chemical evolution modeling with cosmologically motivated mass accretion histories for the MilkyWaydarkhaloanditssatellites.Wedemonstratethatstellarhaloanddwarfgalaxyabundancepatternscanbe explained naturally within the CDM framework. Our solution relies fundamentally on the CDM model prediction that the majority of the stars in the stellar halo were formed within a few relatively massive, � 5 ;10 10 M� , dwarf irregular (dIrr) sized dark matter halos, which were accreted and destroyed � 10 Gyr in the past. These systems necessarily have short-lived, rapid star formation histories, are enriched primarily by Type II supernovae, and host stars with enhanced [� /Fe] abundances. In contrast, dwarf dSph galaxies exist within low-mass dark matter hostsof � 10 9 M� , wheresupernovaewindsareimportantin settingtheintermediate[� /Fe]ratios observed. Our model includes enrichment from Type Ia and Type II supernovae, as well as stellar winds, and includes a physicallymotivatedsupernovaefeedbackprescriptioncalibratedtoreproducethelocaldwarfgalaxystellarmass– metallicity relation. We use representative examples of the type of dark matter halos that we expect to host a destroyed ‘‘stellar halo progenitor’’ dwarf, a surviving dIrr, and a surviving dSph galaxy, and show that their derived abundance patterns, stellar masses, and gas masses are consistent with those observed for each type of system. Our model also self-consistently reproduces the observed stellar mass–vcirc relation for local group satellites and produces the correct cumulative mass for the Milky Way stellar halo. We predict that the lowest metallicity stars in intermediate-mass dIrr galaxies such as the SMC and LMC should follow abundance patterns similar to that observed in the stellar halo. Searches for accreted, disrupted, low-mass dwarfs may be enhanced by searching for unbound stars with dSph-like chemical abundance patterns.

Chemical Abundance Distributions of Galactic Halos and Their Satellite Systems in a ΛCDM Universe

We present a cosmologically motivated model for the hierarchical formation of the stellar halo that includes a semianalytic treatment of galactic chemical enrichment coupled to numerical simulations that track the orbital evolution and tidal disruption of satellites. A major motivating factor in this investigation is the observed systematic difference between the chemical abundances of stars in satellite galaxies and those in the Milky Way halo. Specifically, for the same [Fe/H] values, stars in neighboring satellite galaxies display significantly lower [α/Fe] ratios than stars in the halo. We find that the observed chemical abundance patterns are a natural outcome of the process of hierarchical assembly of the Galaxy. This result follows because the stellar halo in this context is built up from satellite galaxies accreted early on (more than 8-9 Gyr ago) and enriched in α-elements produced in Type II supernovae. In contrast, satellites that still survive today are typically accreted late (within the last 4-5 Gyr) with nearly solar [α/Fe] values as a result of contributions from both Type II and Type Ia supernovae. We use our model to investigate the abundance distribution functions (using both [Fe/H] and [α/Fe] ratios) for stars in the halo and in surviving satellites. Our results suggest that the shapes and peaks in the abundance distribution functions provide a direct probe of the accretion histories of galaxies.

A Two Micron All Sky Survey View of the Sagittarius Dwarf Galaxy. III. Constraints on the Flattening of the Galactic Halo

M giants selected from the Two Micron All Sky Survey (2MASS) have been used to trace streams of tidal debris apparently associated with the Sagittarius dwarf spheroidal galaxy (Sgr) that entirely encircle the Galaxy. While the Sgr M giants are generally aligned with a single great circle on the sky, we measure a difference of 104 ± 26 between the mean orbital poles of the great circles that best fit debris leading and trailing Sgr, which can be attributed to the precession of Sgrs orbit over the range of phases explored by the data set. Simulations of the destruction of Sgr in potentials containing bulge, disk, and halo components best reproduce this level of precession along the same range of orbital phases if the potential contours of the halo are only slightly flattened, with the ratio of the axis length perpendicular to and in the disk in the range q = 0.90-0.95 (corresponding to isodensity contours with qρ ~ 0.83-0.92). Oblate halos are strongly preferred over prolate (qρ > 1) halos, and flattenings in the potential of q ≤ 0.85 (qρ ≤ 0.75) and q ≥ 1.05 (qρ ≥ 1.1) are ruled out at the 3 σ level. More extreme values of q ≤ 0.80 (qρ ≤ 0.6) and q ≥ 1.25 (qρ ≥ 1.6) are ruled out at the 7 and 5 σ levels, respectively. These constraints will improve as debris with larger separation in orbital phases is found.

GALAXIA: A CODE TO GENERATE A SYNTHETIC SURVEY OF THE MILKY WAY

We present here a fast code for creating a synthetic survey of the Milky Way. Given one or more color-magnitude bounds, a survey size, and geometry, the code returns a catalog of stars in accordance with a given model of the Milky Way. The model can be specified by a set of density distributions or as an N-body realization. We provide fast and efficient algorithms for sampling both types of models. As compared to earlier sampling schemes which generate stars at specified locations along a line of sight, our scheme can generate a continuous and smooth distribution of stars over any given volume. The code is quite general and flexible and can accept input in the form of a star formation rate, age-metallicity relation, age-velocity-dispersion relation, and analytic density distribution functions. Theoretical isochrones are then used to generate a catalog of stars, and support is available for a wide range of photometric bands. As a concrete example, we implement the Besancon Milky Way model for the disk. For the stellar halo we employ the simulated stellar halo N-body models of Bullock & Johnston. In order to sample N-body models, we present a scheme that disperses the stars spawned by an N-body particle, in such a way that the phase-space density of the spawned stars is consistent with that of the N-body particles. The code is ideally suited to generating synthetic data sets that mimic near future wide area surveys such as GAIA, LSST, and HERMES. As an application we study the prospect of identifying structures in the stellar halo with a simulated GAIA survey. We plan to make the code publicly available.