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

Geha, Marla and Willman, Beth and Simon, Joshua D. and Strigari, Louis E. and Kirby, Evan N. and Law, David R. and Strader, Jay (2009) The Least-Luminous Galaxy: Spectroscopy of the Milky Way Satellite Segue 1. Astrophysical Journal, 692 (2). pp. 1464-1475. ISSN 0004-637X.

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We present Keck/DEIMOS spectroscopy of Segue 1, an ultra-low-luminosity (M_V = –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) × 10^5 M_☉ 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) × 10^5 M_☉ . 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.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Geha, Marla0000-0002-7007-9725
Willman, Beth0000-0003-2892-9906
Kirby, Evan N.0000-0001-6196-5162
Law, David R.0000-0002-9402-186X
Strader, Jay0000-0002-1468-9668
Additional Information:© 2009. The American Astronomical Society. Received 2008 August 3, accepted for publication 2008 October 10. Published 2009 February 24. We acknowledge and appreciate conversations regarding this work with James Bullock, Raja Guhathakurta, Manoj Kaplinghat, Shane Walsh, and Adi Zolotov. E.N.K. acknowledges the support of an NSF Graduate Research Fellowship. J.D.S. gratefully acknowledges the support of a Millikan Fellowship provided by Caltech. L.E.S. by NASA through Hubble Fellowship grant HF-01225.01 awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS 5-26555.
Funding AgencyGrant Number
NSF Graduate Research FellowshipUNSPECIFIED
Caltech Millikan FellowshipUNSPECIFIED
NASA Hubble FellowshipHF-01225.01
NASANAS 5-26555
Subject Keywords:galaxies: dwarf; galaxies: individual (Segue 1); galaxies: kinematics and dynamics; Local Group
Issue or Number:2
Record Number:CaltechAUTHORS:20090827-110153354
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Official Citation:The Least-Luminous Galaxy: Spectroscopy of the Milky Way Satellite Segue 1 Marla Geha, Beth Willman, Joshua D. Simon, Louis E. Strigari, Evan N. Kirby, David R. Law, and Jay Strader 2009 ApJ 692 1464-1475 doi: 10.1088/0004-637X/692/2/1464
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:15354
Deposited By: Ruth Sustaita
Deposited On:11 Sep 2009 22:18
Last Modified:09 Mar 2020 13:18

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