[en] This paper describes the discovery of seven dwarf objects of spectral type L (objects cooler than the latest M dwarfs) in commissioning imaging data taken by the Sloan Digital Sky Survey (SDSS). Low-resolution spectroscopy shows that they have spectral types from L0 to L8. Comparison of the SDSS and the Two Micron All Sky Survey (2MASS) photometry for several of these objects indicates the presence of significant opacity at optical wavelengths. This comparison also demonstrates the high astrometric accuracy (better than 1^'' for these faint sources) of both surveys. The L dwarfs are shown to occupy a distinctive region of color-color space as measured in the SDSS filters, which should enables their identification in a straightforward way. This should lead eventually to a complete sample of many hundreds of these low-mass objects, or about 1 per 15 deg2 to i^' ≅20, in the complete SDSS data set. (c) (c) 2000. The American Astronomical Society

[en] We identify 10-seven for the first time-elements of cold halo substructure (ECHOS) in the volume within 17.5 kpc of the Sun in the inner halo of the Milky Way. Our result is based on the observed spatial and radial velocity distribution of metal-poor main-sequence turnoff (MPMSTO) stars in 137 Sloan Extension for Galactic Understanding and Exploration lines of sight. We point out that the observed radial velocity distribution is consistent with a smooth stellar component of the Milky Way's inner halo overall, but disagrees significantly at the radial velocities that correspond to our detections. We show that all of our detections are statistically significant and that we expect no false positives. These ECHOS represent the observable stellar debris of ancient merger events in the stellar accretion history of the Milky Way, and we use our detections and completeness estimates to infer a formal upper limit of 0.34^+0.02_-0.02 on the fraction of the MPMSTO population in the inner halo that belong to ECHOS. Our detections and completeness calculations also suggest that there is a significant population of low fractional overdensity ECHOS in the inner halo, and we predict that 1/3 of the inner halo (by volume) harbors ECHOS with MPMSTO star number densities n ∼ 15 kpc^-3. In addition, we estimate that there are of order 10³ ECHOS in the entire inner halo. ECHOS are likely older than known surface brightness substructure, so our detections provide us with a direct measure of the accretion history of the Milky Way in a region and time interval that has yet to be fully explored. In concert with previous studies, our result suggests that the level of merger activity has been roughly constant over the past few Gyr and that there has been no accretion of single stellar systems more massive than a few percent of a Milky Way mass in that interval.

[en] We quantify the variability of faint unresolved optical sources using a catalog based on multiple SDSS imaging observations. The catalog covers SDSS Stripe 82, which lies along the celestial equator in the Southern Galactic Hemisphere (22h 24m < α_J2000 < 04h 08m, -1.27^o < (delta)_J2000 < +1.27^o, ∼ 290 deg²), and contains 58 million photometric observations in the SDSS ugriz system for 1.4 million unresolved sources that were observed at least 4 times in each of the gri bands (with a median of 10 observations obtained over ∼5 years). In each photometric bandpass we compute various low-order lightcurve statistics such as root-mean-square scatter (rms), χ² per degree of freedom, skewness, minimum and maximum magnitude, and use them to select and study variable sources. We find that 2% of unresolved optical sources brighter than g = 20.5 appear variable at the 0.05 mag level (rms) simultaneously in the g and r bands. The majority (2/3) of these variable sources are low-redshift (< 2) quasars, although they represent only 2% of all sources in the adopted flux-limited sample. We find that at least 90% of quasars are variable at the 0.03 mag level (rms) and confirm that variability is as good a method for finding low-redshift quasars as is the UV excess color selection (at high Galactic latitudes). We analyze the distribution of lightcurve skewness for quasars and find that is centered on zero. We find that about 1/4 of the variable stars are RR Lyrae stars, and that only 0.5% of stars from the main stellar locus are variable at the 0.05 mag level. The distribution of lightcurve skewness in the g-r vs. u-g color-color diagram on the main stellar locus is found to be bimodal (with one mode consistent with Algol-like behavior). Using over six hundred RR Lyrae stars, we demonstrate rich halo substructure out to distances of 100 kpc. We extrapolate these results to expected performance by the Large Synoptic Survey Telescope and estimate that it will obtain well-sampled 2% accurate, multi-color lightcurves for ∼2 million low-redshift quasars, and will discover at least 50 million variable stars

[en] We investigate in detail the probability distribution function (pdf) of the proper-motion measurement errors in the SDSS+USNO-B proper-motion catalog of Munn et al. using clean quasar samples. The pdf of the errors is well represented by a Gaussian core with extended wings, plus a very small fraction (<0.1%) of 'outliers'. We find that while formally the pdf could be well fit by a five-parameter fitting function, for many purposes it is also adequate to represent the pdf with a one-parameter approximation to this function. We apply this pdf to the calculation of the confidence intervals on the true proper motion for an SDSS+USNO-B proper-motion measurement, and discuss several scientific applications of the SDSS proper-motion catalog. Our results have various applications in studies of the galactic structure and stellar kinematics. Specifically, they are crucial for searching hyper-velocity stars in the Galaxy.

[en] We discuss a novel approach to ''weighing'' the Milky Way (MW) dark matter halo, one that combines the latest samples of halo stars selected from the Sloan Digital Sky Survey (SDSS) with state of the art numerical simulations of MW analogs. The fully cosmological runs employed in the present study include ''Eris'', one of the highest resolution hydrodynamical simulations of the formation of a M_vir = 8 × 10¹¹ M_☉ late-type spiral, and the dark-matter-only M_vir = 1.7 × 10¹² M_☉ ''Via Lactea II'' (VLII) simulation. Eris provides an excellent laboratory for creating mock SDSS samples of tracer halo stars, and we successfully compare their density, velocity anisotropy, and radial velocity dispersion profiles with the observational data. Most mock SDSS realizations show the same ''cold veil'' recently observed in the distant stellar halo of the MW, with tracers as cold as σ_los ≈ 50 km s^–1 between 100 and 150 kpc. Controlled experiments based on the integration of the spherical Jeans equation as well as a particle tagging technique applied to VLII show that a ''heavy'' M_vir ≈ 2 × 10¹² M_☉ realistic host produces a poor fit to the kinematic SDSS data. We argue that these results offer added evidence for a ''light'', centrally concentrated MW halo

[en] We present the line-of-sight (LOS) velocities for 13 distant main sequence Milky Way halo stars with published proper motions (PMs). The PMs were measured using long baseline (5–7 years) multi-epoch Hubble Space Telescope/Advanced Camera for Surveys photometry, and the LOS velocities were extracted from deep (5–6 hr integrations) Keck II/DEIMOS spectra. We estimate the parameters of the velocity ellipsoid of the stellar halo using a Markov chain Monte Carlo ensembler sampler method. The velocity second moments in the directions of the Galactic (l, b, LOS) coordinate system are ${⟨<!--\; ???\; -->v_l^2⟩<!--\; ???\; -->}^{1/2}={138}_{-<!--\; ???\; -->26}^{+43}$ km s⁻¹, ${⟨<!--\; ???\; -->v_b^2⟩<!--\; ???\; -->}^{1/2}={88}_{-<!--\; ???\; -->17}^{+28}{\text{km s}}^{-<!--\; ???\; -->1}$, and ${⟨<!--\; ???\; -->v_{\mathrm{L}\mathrm{O}\mathrm{S}}^2⟩<!--\; ???\; -->}^{1/2}={91}_{-<!--\; ???\; -->14}^{+27}{\text{km s}}^{-<!--\; ???\; -->1}$. We use these ellipsoid parameters to constrain the velocity anisotropy of the stellar halo. Ours is the first measurement of the anisotropy parameter β using 3D kinematics outside of the solar neighborhood. We find $\mathrm{\beta <!--\; ??\; -->}=-<!--\; ???\; -->{0.3}_{-<!--\; ???\; -->0.9}^{+0.4}$, consistent with isotropy and lower than solar neighborhood β measurements by 2σ (β_SN ∼ 0.5–0.7). We identify two stars in our sample that are likely members of the known TriAnd substructure, and excluding these objects from our sample increases our estimate of the anisotropy to $\mathrm{\beta <!--\; ??\; -->}={0.1}_{-<!--\; ???\; -->1.0}^{+0.4}$, which is still lower than solar neighborhood measurements by 1σ. The potential decrease in β with Galactocentric radius is inconsistent with theoretical predictions, though consistent with recent observational studies, and may indicate the presence of large, shell-type structure (or structures) at r ∼ 25 kpc. The methods described in this paper will be applied to a much larger sample of stars with 3D kinematics observed through the ongoing HALO7D program.

[en] We determine the average metallicities of the elements of cold halo substructure (ECHOS) that we previously identified in the inner halo of the Milky Way within 17.5 kpc of the Sun. As a population, we find that stars kinematically associated with ECHOS are chemically distinct from the background kinematically smooth inner halo stellar population along the same Sloan Extension for Galactic Understanding and Exploration (SEGUE) line of sight. ECHOS are systematically more iron-rich, but less α-enhanced than the kinematically smooth component of the inner halo. ECHOS are also chemically distinct from other Milky Way components: more iron-poor than typical thick-disk stars and both more iron-poor and α-enhanced than typical thin-disk stars. In addition, the radial velocity dispersion distribution of ECHOS extends beyond σ ∼ 20 km s^-1. Globular clusters are unlikely ECHOS progenitors, as ECHOS have large velocity dispersions and are found in a region of the Galaxy in which iron-rich globular clusters are very rare. Likewise, the chemical composition of stars in ECHOS does not match predictions for stars formed in the Milky Way and subsequently scattered into the inner halo. Dwarf spheroidal (dSph) galaxies are possible ECHOS progenitors, and if ECHOS are formed through the tidal disruption of one or more dSph galaxies, the typical ECHOS [Fe/H] ∼ - 1.0 and radial velocity dispersion σ ∼ 20 km s^-1 implies a dSph with M_tot ∼> 10⁹ M_sun. Our observations confirm the predictions of theoretical models of Milky Way halo formation that suggest that prominent substructures are likely to be metal-rich, and our result implies that the most likely metallicity for a recently accreted star currently in the inner halo is [Fe/H] ∼ - 1.0.

[en] We examine the effects that unresolved binaries have on the determination of various stellar atmospheric parameters for targets from the Sloan Extension for Galactic Understanding and Exploration (SEGUE) using numerical modeling, a grid of synthetic spectra, and the SEGUE Stellar Parameter Pipeline (SSPP). The SEGUE survey, a component of the Sloan Digital Sky Survey-II (SDSS-II) project focusing on Galactic structure, provides medium resolution spectroscopy for over 200,000 stars of various spectral types over a large area on the sky. To model undetected binaries that may be in this sample, we use a variety of mass distributions for the primary and secondary stars in conjunction with empirically determined relationships for orbital parameters to determine the fraction of G - K dwarf stars, defined by SDSS color cuts as having 0.48 ≤ (g - r)₀ ≤ 0.75, that will be blended with a secondary companion. We focus on the G-K dwarf sample in SEGUE as it records the history of chemical enrichment in our galaxy. To determine the effect of the secondary on the spectroscopic parameters, specifically effective temperature, surface gravity, metallicity, and [α/Fe], we synthesize a grid of model spectra from 3275 to 7850 K and [Fe/H] = -0.5 to -2.5 from MARCS model atmospheres using TurboSpectrum. These temperature and metallicity ranges roughly correspond to a stellar mass range of 0.1-1.0 M_sun. We assume that both stars in the pair have the same metallicity. We analyze both 'infinite' signal-to-noise ratio (S/N) models and degraded versions of the spectra, at median S/N of 50, 25, and 10. By running individual and combined spectra (representing the binaries) through the SSPP, we determine that ∼10% of the blended G - K dwarf pairs with S/N ≥ 25 will have their atmospheric parameter determinations, in particular temperature and metallicity, noticeably affected by the presence of an undetected secondary; namely, they will be shifted beyond the expected SSPP uncertainties. Shifts in [Fe/H] largely result from the shifts in temperature caused by a secondary. The additional uncertainty from binarity in targets with S/N ≥ 25 is ∼80 K in temperature and ∼0.1 dex in [Fe/H]. The effect on surface gravity and [α/Fe] is even smaller. As the S/N of targets decreases, the uncertainties from undetected secondaries increase. For S/N = 10, 40% of the G-K dwarf sample is shifted beyond expected uncertainties for this S/N in effective temperature and/or metallicity. To account for the additional uncertainty from binary contamination at an S/N ∼ 10, the most extreme scenario, uncertainties of ∼140 K and ∼0.17 dex in [Fe/H] must be added in quadrature to the published uncertainties of the SSPP.

[en] We utilize color information for an H I-selected sample of 195 galaxies to explore the star formation histories and physical conditions that produce the observed colors. We show that the H I selection creates a significant offset toward bluer colors that can be explained by enhanced recent bursts of star formation. There is also no obvious color bimodality, because the H I selection restricts the sample to bluer, actively star-forming systems, diminishing the importance of the red sequence. Rising star formation rates are still required to explain the colors of galaxies bluer than g - r< 0.3. We also demonstrate that the colors of the bluest galaxies in our sample are dominated by emission lines and that stellar population synthesis models alone (without emission lines) are not adequate for reproducing many of the galaxy colors. These emission lines produce large changes in the r - i colors but leave the g - r color largely unchanged. In addition, we find an increase in the dispersion of galaxy colors at low masses that may be the result of a change in the star formation process in low-mass galaxies.

[en] We find that the relative contribution of satellite galaxies accreted at high redshift to the stellar population of the Milky Way's smooth halo increases with distance, becoming observable relative to the classical smooth halo about 15 kpc from the Galactic center. In particular, we determine line-of-sight-averaged [Fe/H] and [α/Fe] in the metal-poor main-sequence turnoff (MPMSTO) population along every Sloan Extension for Galactic Understanding and Exploration (SEGUE) spectroscopic line of sight. Restricting our sample to those lines of sight along which we do not detect elements of cold halo substructure (ECHOS), we compile the largest spectroscopic sample of stars in the smooth component of the halo ever observed in situ beyond 10 kpc. We find significant spatial autocorrelation in [Fe/H] in the MPMSTO population in the distant half of our sample beyond about 15 kpc from the Galactic center. Inside of 15 kpc however, we find no significant spatial autocorrelation in [Fe/H]. At the same time, we perform SEGUE-like observations of N-body simulations of Milky Way analog formation. While we find that halos formed entirely by accreted satellite galaxies provide a poor match to our observations of the halo within 15 kpc of the Galactic center, we do observe spatial autocorrelation in [Fe/H] in the simulations at larger distances. This observation is an example of statistical chemical tagging and indicates that spatial autocorrelation in metallicity is a generic feature of stellar halos formed from accreted satellite galaxies.