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Register a new userBootes II ReBooted: An MMT/MegaCam Study of An Ultra-Faint Milky Way Satellite
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[abridged] We present MMT/Megacam imaging in Sloan $g$ and $r$ of the extremely low luminosity Bootes II Milky Way companion. We use a bootstrap approach to perform robust measurements of, and uncertainties on, Bootes IIs distance, luminosity, size, and morphology. We show that Bootes IIs stellar population is old and metal-poor ([Fe/H] $lta$ -2). Assuming a stellar population like that of M92, Bootes II is at a distance of 42 $pm$ 2 kpc, closer than the initial published estimate of 60 $pm$ 10 kpc. This distance revision, combined with a more robust measurement of Bootes IIs structure with a Plummer model (exponential model) results in a more compact half-light size of $r_hsimeq 36 (33) pm 9 (10)$ pc and lower luminosity of $M_Vsimeq-2.4 (-2.2) pm 0.7 (0.7)$ mag. This revised size and luminosity move Bootes II into a region of size-luminosity space not previously known to be occupied by old stellar populations, but also occupied by the recently discovered Milky Way satellites Willman 1 and SEGUE 1. We show that the apparently distorted morphology of Bootes II is not statistically significant given the present data. We use a tidal argument to support a scenario where Bootes II is a dwarf galaxy (dark matter dominated) rather than a globular cluster (not dark matter dominated). However, we can not rule out that Bootes II is a star cluster on the verge of disruption, such as Palomar 5.
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In this Letter, we announce the discovery of a new satellite of the Milky Way in the constellation of Bootes at a distance of 60 kpc. It was found in a systematic search for stellar overdensities in the North Galactic Cap using Sloan Digital Sky Survey Data Release 5 (SDSS DR5). The color-magnitude diagram shows a well-defined turn-off, red giant branch, and extended horizontal branch. Its absolute magnitude is -5.8, which makes it one of the faintest galaxies known. The half-light radius is 220 pc. The isodensity contours are elongated and have an irregular shape, suggesting that Boo may be a disrupted dwarf spheroidal galaxy.
Pegasus III (Peg III) is one of the few known ultra-faint stellar systems in the outer halo of the Milky Way. We present the results from a follow-up campaign with Magellan/IMACS and Keck/DEIMOS. Deep stellar photometry down to $r_0approx 25$ mag at 50% completeness level has allowed accurate measurements of its photometric and structural properties. The color-magnitude diagram of Peg III confirms that the stellar system is well described by an old ($gtrsim12$ Gyr) and metal-poor ([Fe/H]$lesssim-2.0$ dex) stellar population at a heliocentric distance of $215pm12$ kpc. The revised half-light radius $r_{h}=53pm14$ pc, ellipticity $epsilon=0.38^{+0.22}_{-0.38}$, and total luminosity $M_{V}=-3.4pm0.4$ are in good agreement with the values quoted in our previous paper. We further report on the spectroscopic identification of seven, possibly eight member stars of Peg III. The Ca II triplet lines of the brightest member stars indicate that Peg III contains stars with metallicity as low as [Fe/H]=$-2.55pm0.15$ dex. Peg III has a systemic velocity of $-222.9 pm 2.6$ km s$^{-1}$ and a velocity dispersion of $5.4^{+3.0}_{-2.5}$ km s$^{-1}$. The inferred dynamical mass within the half-light radius is $1.4^{+3.0}_{-1.1} times 10^6rm{M_{odot}}$ and the mass-to-light ratio $rm{M/L}$$_{V} = 1470^{+5660}_{-1240}$ $rm{M_{odot}/L_{odot}}$, providing further evidence that Peg III is a dwarf galaxy satellite. We find that Peg III and another distant dwarf satellite Pisces II lie relatively close to each other ($Delta d_{spatial}=43pm19$ kpc) and share similar radial velocities in the Galactic standard-of-rest frame ($Delta v_{GSR}=12.3pm3.7$ km s$^{-1}$). This suggests that they may share a common origin.
We use new kinematic data from the ultra-faint Milky Way satellite Segue 1 to model its dark matter distribution and derive upper limits on the dark matter annihilation cross-section. Using gamma-ray flux upper limits from the Fermi satellite and MAGIC, we determine cross-section exclusion regions for dark matter annihilation into a variety of different particles including charged leptons. We show that these exclusion regions are beginning to probe the regions of interest for a dark matter interpretation of the electron and positron fluxes from PAMELA, Fermi, and HESS, and that future observations of Segue 1 have strong prospects for testing such an interpretation. We additionally discuss prospects for detecting annihilation with neutrinos using the IceCube detector, finding that in an optimistic scenario a few neutrino events may be detected. Finally we use the kinematic data to model the Segue 1 dark matter velocity dispersion and constrain Sommerfeld enhanced models.
We present the star formation histories (SFHs) of 20 faint M31 satellites ($-12 lesssim M_V lesssim -6$) that were measured by modeling sub-horizontal branch (HB) depth color-magnitude diagrams constructed from Hubble Space Telescope (HST) imaging. Reinforcing previous results, we find that virtually all galaxies quenched between 3 and 9 Gyr ago, independent of luminosity, with a notable concentration $3-6$ Gyr ago. This is in contrast to the Milky Way (MW) satellites, which are generally either faint with ancient quenching times or luminous with recent ($<3$ Gyr) quenching times. We suggest that systematic differences in the quenching times of M31 and MW satellites may be a reflection of the varying accretion histories of M31 and the MW. This result implies that the formation histories of low-mass satellites may not be broadly representative of low-mass galaxies in general. Among the M31 satellite population we identify two distinct groups based on their SFHs: one with exponentially declining SFHs ($tau sim 2$ Gyr) and one with rising SFHs with abrupt quenching. We speculate how these two groups could be related to scenarios for a recent major merger involving M31. The Cycle 27 HST Treasury survey of M31 satellites will provide well-constrained ancient SFHs to go along with the quenching times we measure here. The discovery and characterization of M31 satellites with $M_V gtrsim -6$ would help quantify the relative contributions of reionization and environment to quenching of the lowest-mass satellites.
We present Magellan/IMACS spectroscopy of the recently-discovered Milky Way satellite Eridanus II (Eri II). We identify 28 member stars in Eri II, from which we measure a systemic radial velocity of $v_{rm hel} = 75.6 pm 1.3~mbox{(stat.)} pm 2.0~mbox{(sys.)}~mathrm{km,s^{-1}}$ and a velocity dispersion of $6.9^{+1.2}_{-0.9}~mathrm{km,s^{-1}}$. Assuming that Eri~II is a dispersion-supported system in dynamical equilibrium, we derive a mass within the half-light radius of Eri II is $1.2^{+0.4}_{-0.3} times 10^{7}~mathrm{M_odot}$, indicating a mass-to-light ratio of $420^{+210}_{-140}~mathrm{M_odot}/mathrm{L_odot}$ and confirming that it is a dark matter-dominated dwarf galaxy. From the equivalent width measurements of the CaT lines of 16 red giant member stars, we derive a mean metallicity of ${rm [Fe/H]} = -2.38 pm 0.13$ and a metallicity dispersion of $sigma_{rm [Fe/H]} = 0.47 ^{+0.12}_{-0.09}$. The velocity of Eri II in the Galactic Standard of Rest frame is $v_{rm GSR} = -66.6~mathrm{km,s^{-1}}$, indicating that either Eri II is falling into the Milky Way potential for the first time or it has passed the apocenter of its orbit on a subsequent passage. At a Galactocentric distance of $sim$370 kpc, Eri II is one of the Milky Ways most distant satellites known. Additionally, we show that the bright blue stars previously suggested to be a young stellar population are not associated with Eri II. The lack of gas and recent star formation in Eri II is surprising given its mass and distance from the Milky Way, and may place constraints on models of quenching in dwarf galaxies and on the distribution of hot gas in the Milky Way halo. Furthermore, the large velocity dispersion of Eri II can be combined with the existence of a central star cluster to constrain MACHO dark matter with mass $gtrsim10~mathrm{M_odot}$.
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