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

Abstract

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-) 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.

The early days of the Sculptor dwarf spheroidal galaxy

“…Mass is the major structural difference between Sculptor (or classical dSphs) and the UFDs: while Sculptor has a V-magnitude M V ∼ −11.2, and a velocity dispersion σ ∼ 9−10 km s −1 (Battaglia et al 2008a;Walker et al 2009), the UFDs have much smaller masses with σ between ∼3 and 6 km s −1 and have a sparse stellar population, M V < −4 mag (Simon & Geha 2007;Geha et al 2009). The probability of producing Sr and Ba seems to correlate with the number of massive stars formed in the systems.…”

Cosmic ray anomalies from the MSSM?

“…Discovered relatively recently in analyses of SDSS data [101], Segue 1 is known to have a mass-to-light ratio M/L ∼ 1000(M /L ), and a preliminary analysis has been carried out to estimate its DM content using the kinematics of 24 of its member stars [102]. We will use the analysis presented in [103], appropriate to the FERMI-LAT observations of Segue 1, in which the line-of-sight halo integral was estimated by marginalizing over halo distributions, including both cusped and cored types.…”

The extremely low-metallicity tail of the Sculptor dwarf spheroidal galaxy

“…Although these galaxies have very old stellar populations and a low average metallicity, no extremely metal-poor candidates were found from initial Ca II triplet (CaT, around 8500 Å) surveys, in contrast to the relative numbers of extremely metal-poor stars discovered in the Galactic halo (Helmi et al 2006). Various extremely metal-poor candidates were however A&A 549, A88 (2013) discovered and followed-up using different selection methods, mainly in the ultra-faint dwarf galaxies (e.g., Kirby et al 2008;Koch et al 2008;Geha et al 2009;Kirby et al 2009;Norris et al 2010a,b;Frebel et al 2010b;Simon et al 2011;François et al 2012). The question thus arose if the empirical CaT method, extrapolated at [Fe/H] < -2.2 due to a lack of globular cluster measurement to calibrate, was biased in the metal-poor regime.…”