Constraining the LRG Halo Occupation Distribution using Counts-in-Cylinders

The low number density of the Sloan Digital Sky Survey (SDSS) Luminous Red Galaxies (LRGs) suggests that LRGs occupying the same dark matter halo can be separated from pairs occupying distinct dark matter halos with high fidelity. We present a new technique, Counts-in-Cylinders (CiC), to constrain the parameters of the satellite contribution to the LRG Halo-Occupation Distribution (HOD). For a fiber collision-corrected SDSS spectroscopic LRG subsample at 0.16 < z < 0.36, we find the CiC multiplicity function is fit by a halo model where the average number of satellites in a halo of mass M is = ((M - Mcut)/M1)^alpha with Mcut = 5.0 +1.5/-1.3 (+2.9/-2.6) X 10^13 Msun, M1 = 4.95 +0.37/-0.26 (+0.79/-0.53) X 10^14 Msun, and alpha = 1.035 +0.10/-0.17 (+0.24/-0.31) at the 68% and 95% confidence levels using a WMAP3 cosmology and z=0.2 halo catalog. Our method tightly constrains the fraction of LRGs that are satellite galaxies, 6.36 +0.38/-0.39, and the combination Mcut/10^{14} Msun + alpha = 1.53 +0.08/-0.09 at the 95% confidence level. We also find that mocks based on a halo catalog produced by a spherical overdensity (SO) finder reproduce both the measured CiC multiplicity function and the projected correlation function, while mocks based on a Friends-of-Friends (FoF) halo catalog has a deficit of close pairs at ~1 Mpc/h separations. Because the CiC method relies on higher order statistics of close pairs, it is robust to the choice of halo finder. In a companion paper we will apply this technique to optimize Finger-of-God (FOG) compression to eliminate the 1-halo contribution to the LRG power spectrum.Comment: 40 pages, 9 figures, submitted to Astrophysical Journa

A Deeper Look at Leo IV: Star Formation History and Extended Structure

We present MMT/Megacam imaging of the Leo~IV dwarf galaxy in order to investigate its structure and star formation history, and to search for signs of association with the recently discovered Leo~V satellite. Based on parameterized fits, we find that Leo~IV is round, with $\epsilon < 0.23$ (at the 68\% confidence limit) and a half-light radius of $r_{h} \simeq 130$ pc. Additionally, we perform a thorough search for extended structures in the plane of the sky and along the line of sight. We derive our surface brightness detection limit by implanting fake structures into our catalog with stellar populations identical to that of Leo~IV. We show that we are sensitive to stream-like structures with surface brightness $\mu_{r}\lesssim29.6$ mag arcsec$^{-2}$, and at this limit, we find no stellar bridge between Leo IV (out to a radius of $\sim$0.5 kpc) and the recently discovered, nearby satellite Leo V. Using the color magnitude fitting package StarFISH, we determine that Leo~IV is consistent with a single age ($\sim$14 Gyr), single metallicity ($[Fe/H]\sim-2.3$) stellar population, although we can not rule out a significant spread in these value. We derive a luminosity of $M_{V}=-5.5\pm0.3$. Studying both the spatial distribution and frequency of Leo~IV's 'blue plume' stars reveals evidence for a young ($\sim$2 Gyr) stellar population which makes up $\sim$2\% of its stellar mass. This sprinkling of star formation, only detectable in this deep study, highlights the need for further imaging of the new Milky Way satellites along with theoretical work on the expected, detailed properties of these possible 'reionization fossils'.Comment: 26 pages, 14 figures, emulateapj format, ApJ accepted versio

Turning the Tides on the Ultra-Faint Dwarf Spheroidal Galaxies: Coma Berenices and Ursa Major II

We present deep CFHT/MegaCam photometry of the ultra-faint Milky Way satellite galaxies Coma Berenices (ComBer) and Ursa Major II (UMa II). These data extend to r~25, corresponding to three magnitudes below the main sequence turn-offs in these galaxies. We robustly calculate a total luminosity of M_V=-3.8 +/- 0.6 for ComBer and M_V=-3.9 +/- 0.5 for UMa II, in agreement with previous results. ComBer shows a fairly regular morphology with no signs of active tidal stripping down to a surface brightness limit of 32.4 magarcsec^-2. Using a maximum likelihood analysis, we calculate the half-light radius of ComBer to be r_half=74 +/- 4 pc (5.8 +/- 0.3 arcmin) and its ellipticity e=0.36 +/- 0.04. In contrast, UMa II shows signs of on-going disruption. We map its morphology down to mu_V=32.6 mag arcsec^-2 and found that UMa II is larger than previously determined, extending at least ~700 pc (1.2 deg on the sky) and it is also quite elongated with an ellipticity of e=0.50 +/- 0.2. However, our estimate for the half-light radius, 123 +/- 3 pc (14.1 +/- 0.3 arcmin) is similar to previous results. We discuss the implications of these findings in the context of potential indirect dark matter detections and galaxy formation. We conclude that while ComBer appears to be a stable dwarf galaxy, UMa II shows signs of on-going tidal interaction.Comment: Submitted to A

Willman 1 - a probable dwarf galaxy with an irregular kinematic distribution

We investigate the kinematic properties and stellar population of the Galactic satellite Willman 1 (Wil 1) by combining Keck/DEIMOS spectroscopy with KPNO mosaic camera imaging. Wil 1 is an ultra-low luminosity Milky Way companion. This object lies in a region of size-luminosity space (M_V ~ -2 mag, d ~ 38 kpc, r_half ~ 20 pc) also occupied by the Galactic satellites Bo\"otes II and Segue 1 and 2, but no other known old stellar system. We use kinematic and color-magnitude criteria to identify 45 stars as possible members of Wil 1. With a systemic velocity of v_helio = -12.8 +/- 1.0 km/s, Wil 1 stars have velocities similar to those of foreground Milky Way stars. Informed by Monte-Carlo simulations, we identify 5 of the 45 candidate member stars as likely foreground contaminants. We confirm a significant spread in the abundances of the likely Wil 1 red giant branch members ([Fe/H] = -1.73 +/- 0.12 and -2.65 +/- 0.12, [Ca/Fe] = -0.4 +/- 0.18 and +0.13 +/- 0.28). This spread supports the scenario that Wil 1 is an ultra-low luminosity dwarf galaxy rather than a star cluster. Wil 1's innermost stars move with radial velocities offset by 8 km/s from its outer stars and have a velocity dispersion consistent with 0 km/s, suggesting that Wil 1 may not be in dynamical equilibrium. The combination of the foreground contamination and unusual kinematic distribution make it difficult to robustly determine the dark matter mass of Wil 1. As a result, X-ray or gamma-ray observations of Wil 1 that attempt to constrain models of particle dark matter using an equilibrium mass model are strongly affected by the systematics in the observations presented here. We conclude that, despite the unusual features in the Wil 1 kinematic distribution, evidence indicates that this object is, or at least once was, a dwarf galaxy.Comment: AJ accepted version. The primary improvements are a detailed investigation of the membership probability (Section 3.4 and new Figures 6, 7 and 8) and the revised spectroscopic [Fe/H] and [Ca/Fe] measurements of the two brightest member stars. Conclusions are unchanged from the submitted versio

The Least Luminous Galaxy: Spectroscopy of the Milky Way Satellite Segue 1

We present Keck/DEIMOS spectroscopy of Segue 1, an ultra-low luminosity (M_V = -1.5) Milky Way satellite companion. While the combined size and luminosity of Segue 1 are consistent with either a globular cluster or a dwarf galaxy, we present spectroscopic evidence that this object is a dark matter-dominated dwarf galaxy. We identify 24 stars as members of Segue 1 with a mean heliocentric recession velocity of 206 +/- 1.3 kms. We measure an internal velocity dispersion of 4.3+/-1.2 kms. Under the assumption that these stars are in dynamical equilibrium, we infer a total mass of 4.5^{+4.7}_{-2.5} x 10^5 Msun in the case where mass-follow-light; using a two-component maximum likelihood model, we determine a similar mass within the stellar radius of 50 pc. This implies a mass-to-light ratio of ln(M/L_V) = 7.2^{+1.1}_{-1.2} or M/L_V = 1320^{+2680}_{-940}. The error distribution of the mass-to-light ratio is nearly log-normal, thus Segue 1 is dark matter-dominated at a high significance. Using spectral synthesis modeling, we derive a metallicity for the single red giant branch star in our sample of [Fe/H]=-3.3 +/- 0.2 dex. Finally, we discuss the prospects for detecting gamma-rays from annihilation of dark matter particles and show that Segue 1 is the most promising satellite for indirect dark matter detection. We conclude that Segue 1 is the least luminous of the ultra-faint galaxies recently discovered around the Milky Way, and is thus the least luminous known galaxy.Comment: 12 pages, 6 figures, ApJ accepte

Baryon Acoustic Oscillations in the Sloan Digital Sky Survey Data Release 7 Galaxy Sample

The spectroscopic Sloan Digital Sky Survey (SDSS) Data Release 7 (DR7) galaxy sample represents the final set of galaxies observed using the original SDSS target selection criteria. We analyse the clustering of galaxies within this sample, including both the Luminous Red Galaxy (LRG) and Main samples, and also include the 2-degree Field Galaxy Redshift Survey (2dFGRS) data. Baryon Acoustic Oscillations are observed in power spectra measured for different slices in redshift; this allows us to constrain the distance--redshift relation at multiple epochs. We achieve a distance measure at redshift z=0.275, of r_s(z_d)/D_V(0.275)=0.1390+/-0.0037 (2.7% accuracy), where r_s(z_d) is the comoving sound horizon at the baryon drag epoch, D_V(z)=[(1+z)^2D_A^2cz/H(z)]^(1/3), D_A(z) is the angular diameter distance and H(z) is the Hubble parameter. We find an almost independent constraint on the ratio of distances D_V(0.35)/D_V(0.2)=1.736+/-0.065, which is consistent at the 1.1sigma level with the best fit Lambda-CDM model obtained when combining our z=0.275 distance constraint with the WMAP 5-year data. The offset is similar to that found in previous analyses of the SDSS DR5 sample, but the discrepancy is now of lower significance, a change caused by a revised error analysis and a change in the methodology adopted, as well as the addition of more data. Using WMAP5 constraints on Omega_bh^2 and Omega_ch^2, and combining our BAO distance measurements with those from the Union Supernova sample, places a tight constraint on Omega_m=0.286+/-0.018 and H_0 = 68.2+/-2.2km/s/Mpc that is robust to allowing curvature and non-Lambda dark energy. This result is independent of the behaviour of dark energy at redshifts greater than those probed by the BAO and supernova measurements. (abridged)Comment: 22 pages, 16 figures, minor changes to match version published in MNRA

A Complete Spectroscopic Survey of the Milky Way Satellite Segue 1: The Darkest Galaxy

We present the results of a comprehensive Keck/DEIMOS spectroscopic survey of the ultra-faint Milky Way satellite galaxy Segue 1. We have obtained velocity measurements for 98.2% of the stars within 67 pc (10 arcmin, or 2.3 half-light radii) of the center of Segue 1 that have colors and magnitudes consistent with membership, down to a magnitude limit of r=21.7. Based on photometric, kinematic, and metallicity information, we identify 71 stars as probable Segue 1 members, including some as far out as 87 pc. After correcting for the influence of binary stars using repeated velocity measurements, we determine a velocity dispersion of 3.7^{+1.4}_{-1.1} km/s, with a corresponding mass within the half-light radius of 5.8^{+8.2}_{-3.1} x 10^5 Msun. The stellar kinematics of Segue 1 require very high mass-to-light ratios unless the system is far from dynamical equilibrium, even if the period distribution of unresolved binary stars is skewed toward implausibly short periods. With a total luminosity less than that of a single bright red giant and a V-band mass-to-light ratio of 3400 Msun/Lsun, Segue 1 is the darkest galaxy currently known. We critically re-examine recent claims that Segue 1 is a tidally disrupting star cluster and that kinematic samples are contaminated by the Sagittarius stream. The extremely low metallicities ([Fe/H] < -3) of two Segue 1 stars and the large metallicity spread among the members demonstrate conclusively that Segue 1 is a dwarf galaxy, and we find no evidence in favor of tidal effects. We also show that contamination by the Sagittarius stream has been overestimated. Segue 1 has the highest measured dark matter density of any known galaxy and will therefore be a prime testing ground for dark matter physics and galaxy formation on small scales.Comment: 24 pages, 4 tables, 11 figures (10 in color). Submitted for publication in ApJ. V3 revised according to comments from the refere

The clustering of galaxies in the SDSS-III Baryon Oscillation Spectroscopic Survey: Analysis of potential systematics

We analyze the density field of galaxies observed by the Sloan Digital Sky Survey (SDSS)-III Baryon Oscillation Spectroscopic Survey (BOSS) included in the SDSS Data Release Nine (DR9). DR9 includes spectroscopic redshifts for over 400,000 galaxies spread over a footprint of 3,275 deg^2. We identify, characterize, and mitigate the impact of sources of systematic uncertainty on large-scale clustering measurements, both for angular moments of the redshift-space correlation function and the spherically averaged power spectrum, P(k), in order to ensure that robust cosmological constraints will be obtained from these data. A correlation between the projected density of stars and the higher redshift (0.43 < z < 0.7) galaxy sample (the `CMASS' sample) due to imaging systematics imparts a systematic error that is larger than the statistical error of the clustering measurements at scales s > 120h^-1Mpc or k < 0.01hMpc^-1. We find that these errors can be ameliorated by weighting galaxies based on their surface brightness and the local stellar density. We use mock galaxy catalogs that simulate the CMASS selection function to determine that randomly selecting galaxy redshifts in order to simulate the radial selection function of a random sample imparts the least systematic error on correlation function measurements and that this systematic error is negligible for the spherically averaged correlation function. The methods we recommend for the calculation of clustering measurements using the CMASS sample are adopted in companion papers that locate the position of the baryon acoustic oscillation feature (Anderson et al. 2012), constrain cosmological models using the full shape of the correlation function (Sanchez et al. 2012), and measure the rate of structure growth (Reid et al. 2012). (abridged)Comment: Matches version accepted by MNRAS. Clarifications and references have been added. See companion papers that share the "The clustering of galaxies in the SDSS-III Baryon Oscillation Spectroscopic Survey:" titl

The Star Formation History and Extended Structure of the Hercules Milky Way Satellite

We present imaging of the recently discovered Hercules Milky Way satellite and its surrounding regions to study its structure, star formation history and to thoroughly search for signs of disruption. We robustly determine the distance, luminosity, size and morphology of Hercules utilizing a bootstrap approach to characterize our uncertainties. We derive a distance to Hercules of $133 \pm 6$ kpc via a comparison to empirical and theoretical isochrones. As previous studies have found, Hercules is very elongated, with $\epsilon=0.67\pm0.03$ and a half light radius of $r_{h} \simeq 230$ pc. Using the color magnitude fitting package StarFISH, we determine that Hercules is old ($>12$ Gyr) and metal poor ($[Fe/H]\sim-2.0$), with a spread in metallicity, in agreement with previous spectroscopic work. We infer a total absolute magnitude of $M_V=-5.3\pm0.4$. Our innovative search for external Hercules structure both in the plane of the sky and along the line of sight yields some evidence that Hercules is embedded in a larger stream of stars. A clear stellar extension is seen to the Northwest with several additional candidate stellar overdensities along the position angle of Hercules out to $\sim$35' ($\sim$1.3 kpc). While the association of any of the individual stellar overdensities with Hercules is difficult to determine, we do show that the summed color magnitude diagram of all three is consistent with Hercules' stellar population. Finally, we estimate that any change in the distance to Hercules across its face is at most $\sim$6 kpc; and the data are consistent with Hercules being at the same distance throughout.Comment: 50 pages, 15 figures, submitted to the Astrophysical Journa

Southeastern Association of Law Libraries Annual Meeting

The 2009 SEAALL Annual Meeting was held in Athens Georgia, April 16-18, 2009