[en] The Milky Way’s dwarf spheroidal (dSph) galaxies are among the best targets for the indirect detection of dark matter (DM) with γ-rays. The expected gamma-ray flux depends on the so-called ‘ J -factor’, the integral of the squared DM density along the line-of-sight. Using a large number of simulated dSphs, we have defined an optimized Jeans analysis setup for the reconstruction of the DM density with stellar-kinematic data. Employing this setup, we provide here estimates of astrophysical J -factors for twenty-two Galactic dSphs, including the newly discovered Reticulum II. We finally identify several criteria that may indicate a contamination of a kinematic dataset by interlopers, leading to unreliable J -factors. We find that the kinematic sample of Segue I, one of the closest dSph, might be affected by this issue. (paper)

[en] We present a new analysis of the relative detectability of dark matter annihilation in the Milky Way's eight 'classical' dwarf spheroidal (dSph) satellite galaxies. Ours is similar to previous analyses in that we use Markov-Chain Monte Carlo techniques to fit dark matter halo parameters to empirical velocity dispersion profiles via the spherical Jeans equation, but more general in the sense that we do not adopt priors derived from cosmological simulations. We show that even without strong constraints on the shapes of dSph dark matter density profiles (we require only that the inner profile satisfies -liM_r→0 dln ρ/dln r ≤ 1), we obtain a robust and accurate constraint on the astrophysical component of a prospective dark matter annihilation signal, provided that the integration angle is approximately twice the projected half-light radius of the dSph divided by distance to the observer, α_int ∼ 2r_h /d. Using this integration angle, which represents a compromise between maximizing prospective flux and minimizing uncertainty in the dSph's dark matter distribution, we calculate the relative detectability of the classical dSphs by ground- and space-based γ-ray observatories.

[en] We present a spectroscopic study of Leo V, a recently discovered satellite of the Milky Way (MW). From stellar spectra obtained with the MMT/Hectochelle spectrograph we identify seven likely members of Leo V. Five cluster near the Leo V center (R < 3') and have a velocity dispersion of 2.4^+2.4_-1.4 km s^-1. The other two likely members lie near each other but far from the center (R ∼ 13' ∼ 700 pc) and inflate the global velocity dispersion to 3.7^+2.3_-1.4 km s^-1. Assuming the five central members are bound, we obtain a dynamical mass of M = 3.3^+9.1_-2.5 x 10⁵ M _sun (M/L_V = 75⁺²³⁰_-58[M/L_V ]_sun). From the stacked spectrum of the five central members we estimate a mean metallicity of [Fe/H]=-2.0 ± 0.2 dex. Thus, with respect to dwarf spheroidals of similar luminosity, Leo V is slightly less massive and slightly more metal rich. Since we resolve the central velocity dispersion only marginally, we do not rule out the possibility that Leo V is a diffuse star cluster devoid of dark matter. The wide separation of its two outer members implies Leo V is losing mass; however, its large distance (D ∼ 180 kpc) is difficult to reconcile with MW tidal stripping unless the orbit is very radial.

[en] We report the discovery of two new Milky Way satellites in the neighboring constellations of Pisces and Pegasus identified in data from the Sloan Digital Sky Survey. Pisces II, an ultra-faint dwarf galaxy lies at the distance of ∼180 kpc, some 15 deg. away from the recently detected Pisces I. Segue 3, an ultra-faint star cluster lies at the distance of 16 kpc. We use deep follow-up imaging obtained with the 4-m Mayall Telescope at Kitt Peak National Observatory to derive their structural parameters. Pisces II has a half-light radius of ∼60 pc, while Segue 3 is 20 times smaller at only 3 pc.

[en] We develop, implement, and characterize an enhanced data reduction approach which delivers precise, accurate, radial velocities from moderate resolution spectroscopy with the fiber-fed VLT/FLAMES+GIRAFFE facility. This facility, with appropriate care, delivers radial velocities adequate to resolve the intrinsic velocity dispersions of the very faint dwarf spheroidal (dSph) galaxies. Importantly, repeated measurements let us reliably calibrate our individual velocity errors (0.2 kms^-1 ≤ δ_V ≤ 5 km s^-1) and directly detect stars with variable radial velocities. We show, by application to the Booetes I dSph, that the intrinsic velocity dispersion of this system is significantly below 6.5 km s^-1 reported by previous studies. Our data favor a two-population model of Booetes I, consisting of a majority 'cold' stellar component, with velocity dispersion 2.4^+0.9_-0.5 km s^-1, and a minority 'hot' stellar component, with velocity dispersion ∼9 km s^-1, although we cannot completely rule out a single component distribution with velocity dispersion 4.6^0.8_-0.6 km s^-1. We speculate that this complex velocity distribution actually reflects the distribution of velocity anisotropy in Booetes I, which is a measure of its formation processes.

[en] In this paper, we report the discovery of a Milky Way satellite in the constellation of Antlia. The Antlia 2 dwarf galaxy is located behind the Galactic disc at a latitude of b ~ 11° and spans 1.26°, which corresponds to ~2.9 kpc at its distance of 130 kpc. While similar in spatial extent to the Large Magellanic Cloud, Antlia 2 is orders of magnitude fainter at M_V = -9 mag, making it by far the lowest surface brightness system known (at ~31.9 mag arcsec^-2), ~100 times more diffuse than the so-called ultra diffuse galaxies. The satellite was identified using a combination of astrometry, photometry, and variability data from Gaia Data Release 2, and its nature confirmed with deep archival DECam imaging, which revealed a conspicuous BHB signal. We have also obtained follow-up spectroscopy using AAOmega on the AAT, identifying 159 member stars, and we used them to measure the dwarf’s systemic velocity, 290.9 ± 0.5 km s^-1, its velocity dispersion, 5.7 ± 1.1 km s^-1, and mean metallicity, [Fe/H] = -1.4. From these properties we conclude that Antlia 2 inhabits one of the least dense dark matter (DM) haloes probed to date. Dynamical modelling and tidal-disruption simulations suggest that a combination of a cored DM profile and strong tidal stripping may explain the observed properties of this satellite. The origin of this core may be consistent with aggressive feedback, or may even require alternatives to cold dark matter (such as ultra-light bosons).