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 turn-off stars with 0.2 < g − r < 0.4 and 18.5 ≤ r < 22.5. We fit oblate and triaxial broken power-law models to the data, and found a 'best-fit' oblateness of the stellar halo 0.5 < c/a < 0.8, and halo stellar masses between Galactocentric radii of 1 and 40 kpc of 3.7 ± 1.2 × 10 8 M ⊙ . The density profile of the stellar halo is approximately ρ ∝ r −α , where −2 > α > −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 σ/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 Way's halo match well the properties of a 'typical' stellar halo built exclusively out of the debris from disrupted satellite galaxies. Our results therefore point towards a picture in which an important fraction of the stellar halo of the Milky Way has been accreted from satellite galaxies.

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 11 stellar halo models constructed within the context of a standard ÃCDM cosmology. The results demonstrate that coordinate-space studies are sensitive to the recent (0Y8 Gyr ago) merger histories of galaxies (this timescale corresponds to the last few percent to tens of percent of mass accretion for a Milky WayYtype galaxy). Specifically, 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 Gyr ago) can be gleaned from the abundance patterns in halo stars: within our models, dramatic differences 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 insight can be applied to reconstruct much about the merger histories of nearby galaxies from current and future data sets.

We describe the catalogs assembled and the algorithms used to populate the revised TESS Input Catalog (TIC), based on the incorporation of the Gaia second data release. We also describe a revised ranking system for prioritizing stars for 2-minute cadence observations, and assemble a revised Candidate Target List (CTL) using that ranking. The TIC is available on the Mikulski Archive for Space Telescopes (MAST) server, and an enhanced CTL is available through the Filtergraph data visualization portal system at the URL http://filtergraph.vanderbilt.edu/tess_ctl.

We present here a fast code for creating a synthetic survey of the Milky Way. Given one or more colormagnitude 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 Besançon Milky Way model for the disc. For the stellar halo we employ the simulated stellar halo N-body models of . 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 at http://galaxia.sourceforge.net.

The Galactic Archaeology with HERMES (GALAH) survey is a large-scale stellar spectroscopic survey of the Milky Way and designed to deliver chemical information complementary to a large number of stars covered by the Gaia mission. We present the GALAH second public data release (GALAH DR2) containing 342,682 stars. For these stars, the GALAH collaboration provides stellar parameters and abundances for up to 23 elements to the community. Here we present the target selection, observation, data reduction and detailed explanation of how the spectra were analysed to estimate stellar parameters and element abundances. For the stellar analysis, we have used a multi-step approach. We use the physics-driven spectrum synthesis of Spectroscopy Made Easy (SME) to derive stellar labels (T eff , log g, [Fe/H], [X/Fe], v mic , v sin i, A K S ) for a representative training set of stars. This information is then propagated to the whole survey with the data-driven method of The Cannon. Special care has been exercised in the spectral synthesis to only consider spectral lines that have reliable atomic input data and are little affected by blending lines. Departures from local thermodynamic equilibrium (LTE) are considered for several key elements, including Li, O, Na, Mg, Al, Si, and Fe, using 1D stellar atmosphere models. Validation tests including repeat observations, Gaia benchmark stars, open and globular clusters, and K2 asteroseismic targets lend confidence to our methods and results. Combining the GALAH DR2 catalogue with the kinematic information from Gaia will enable a wide range of Galactic Archaeology studies, with unprecedented detail, dimensionality, and scope.