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Title:
The Least-Luminous Galaxy: Spectroscopy of the Milky Way Satellite Segue 1
Authors:
Geha, Marla; Willman, Beth; Simon, Joshua D.; Strigari, Louis E.; Kirby, Evan N.; Law, David R.; Strader, Jay
Affiliation:
AA(Astronomy Department, Yale University, New Haven, CT 06520, USA ), AB(Haverford College, Department of Physics, 370 Lancaster Avenue, Haverford, PA 19041, USA ), AC(Department of Astronomy, California Institute of Technology, 1200 E. California Blvd., MS 105-24, Pasadena, CA 91125, USA ), AD(Kavli Institute for Particle Astrophysics & Cosmology, Physics Department, Stanford University, Stanford, CA 94305, USA ; Hubble Fellow.), AE(UCO/Lick Observatory, University of California, Santa Cruz, 1156 High Street, Santa Cruz, CA 95064, USA ), AF(Department of Astronomy, California Institute of Technology, 1200 E. California Blvd., MS 105-24, Pasadena, CA 91125, USA ), AG(Harvard-Smithsonian CfA, 60 Garden St., Cambridge, MA 02144, USA )
Publication:
The Astrophysical Journal, Volume 692, Issue 2, pp. 1464-1475 (2009). (ApJ Homepage)
Publication Date:
02/2009
Origin:
IOP
ApJ Keywords:
galaxies: dwarf, galaxies: individual: Segue 1, galaxies: kinematics and dynamics, Local Group
DOI:
10.1088/0004-637X/692/2/1464
Bibliographic Code:
2009ApJ...692.1464G

Abstract

We present Keck/DEIMOS spectroscopy of Segue 1, an ultra-low-luminosity (MV = -1.5+0.6 -0.8) 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 km s-1. Although Segue 1 spatially overlaps the leading arm of the Sagittarius stream, its velocity is 100 km s-1 different from that predicted for recent Sagittarius tidal debris at this position. We measure an internal velocity dispersion of 4.3 ± 1.2
km s-1. Under the assumption that these stars are gravitationally bound and in dynamical equilibrium, we infer a total mass of 4.5+4.7 -2.5 × 105 M sun in the mass-follow-light case; using a two-component maximum-likelihood model, we determine a mass within 50 pc of 8.7+13 -5.2 × 105 M sun. These imply mass-to-light (M/L) ratios of ln(M/L V ) = 7.2+1.1 -1.2 (M/L V = 1320+2680 -940) and M/L V = 2440+1580 -1775, respectively. The error distribution of the M/L is nearly lognormal, thus Segue 1 is dark matter-dominated at a high significance. Although we cannot rule out the possibility that Segue 1 has been tidally disrupted, we do not find kinematic evidence supporting tidal effects. 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.
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