We present a measurement of the systemic proper motion of the Large Magellanic Cloud (LMC) from astrometry with the High Resolution Camera (HRC) of the Advanced Camera for Surveys (ACS) on the Hubble ...Space Telescope (HST). We observed LMC fields centered on 21 background QSOs that were discovered from their optical variability in the MACHO database. The QSOs are distributed homogeneously behind the central few degrees of the LMC. With two epochs of HRC data and a 62 yr baseline, we determine the proper motion of the LMC to better than 5% accuracy: k sub(W) = -2.03 c 0.08 mas yr super(-1), and k sub(N) = 0.44 c 0.05 mas yr super(-1). This is the most accurate proper-motion measurement for any Milky Way satellite thus far. When combined with H I data from the Magellanic Stream, this should provide new constraints on both the mass distribution of the Galactic halo and models of the Stream.
A change of pace: sudden hypotension Yuan, Neal, M.D; Geha, Rabih M., M.D; Schiller, Nelson B., M.D
The American journal of medicine,
07/2017, Letnik:
130, Številka:
7
Journal Article
We report the discovery of a new ultra-faint globular cluster in the constellation of Ursa Minor, based on stellar photometry from the MegaCam imager at the Canada-France-Hawaii Telescope. We find ...that this cluster, Munoz 1, is located at a distance of 45 + or - 5 kpc and at a projected distance of only 45' from the center of the Ursa Minor dwarf spheroidal galaxy. Using a maximum-likelihood technique we measure a half-light radius of 0'.5, or equivalently 7 pc, and an ellipticity consistent with being zero. We estimate its absolute magnitude to be M sub(V) = -0.4 + or - 0.9, which corresponds to L sub(V) = (ProQuest: Formulae and/or non-USASCII text omitted) L sub(middot in circle) and we measure a heliocentric radial velocity of -137 + or - 4 km s super(-1) based on Keck/DEIMOS spectroscopy. This new satellite is separate from Ursa Minor by ~30 kpc and 110 km s super(-1) suggesting the cluster is not obviously associated with the dSph, despite the very close angular separation. Based on its photometric properties and structural parameters we conclude that Munoz 1 is a new ultra-faint stellar cluster. Along with Segue 3 this is one of the faintest stellar clusters known to date.
As part of the SPLASH survey of the Andromeda (M31) system, we have obtained Keck/DEIMOS spectra of the compact elliptical (cE) satellite M32. This is the first resolved-star kinematical study of any ...cE galaxy. In contrast to most previous kinematical studies that extended out to r <, ~ 30" ~ 1 (ProQuest: Formulae and/or non-USASCII text omitted) ~ 100 pc, we measure the rotation curve and velocity dispersion profile out to r ~ 250" and higher order Gauss-Hermite moments out to r ~ 70". We achieve this by combining integrated-light spectroscopy at small radii (where crowding/blending are severe) with resolved stellar spectroscopy at larger radii, using spatial and kinematical information to account statistically for M31 contamination. The rotation curve and velocity dispersion profile extend well beyond the radius (r ~ 150") where the isophotes are distorted. Unlike NGC 205, another close dwarf companion of M31, M32's kinematics appear regular and symmetric and do not show obvious sharp gradients across the region of isophotal elongation and twists. We interpret M31's kinematics using three-integral axisymmetric dynamical equilibrium models constructed using Schwarzschild's orbit superposition technique. Models with a constant mass-to-light ratio can fit the data remarkably well. However, since such a model requires an increasing tangential anisotropy with radius, invoking the presence of an extended dark halo may be more plausible. Such an extended dark halo is definitely required to bind a half-dozen fast-moving stars observed at the largest radii, but these stars may not be an equilibrium component of M32.
NGC 205 is the nearest example of a dwarf elliptical galaxy and the prototype of this enigmatic galaxy class. Photometric evidence suggests that NGC 205, a close satellite of the M31 galaxy, is ...tidally interacting with its parent galaxy. We present stellar radial velocity measurements out to a projected radius of 20' (5 kpc) in NGC 205 based on Keck DEIMOS multislit spectroscopic observations of 725 individual red giant branch stars. Our kinematic measurements extend from the center out to 6 times the effective radius of NGC 205, well past the expected tidal radius. The contamination in our kinematic sample from M31 field stars is estimated to be a few percent based on maximum likelihood fits to the distribution of stars in position-velocity space. We measure a maximum major-axis rotation speed for the body of NGC 205 of 11 ± 5 km s-1 and note that this is based on observing a definite turnover in the rotation curve; this is the first dE galaxy in which the maximum rotation velocity has been measured. Combined with the velocity dispersion, we conclude that NGC 205 is supported by a combination of rotation and anisotropic velocity dispersion. At a major-axis distance of 45 (1 kpc), the velocity profile of NGC 205 turns over; stars beyond this radius are moving counter to the rotation of the inner part of the galaxy. The turnover radius is coincident with the onset of isophotal twisting and the estimated tidal radius, suggesting that the outer kinematics of NGC 205 is dominated by gravitational interactions with the nearby M31 galaxy. The motion of stars beyond a radius of ~45 implies that NGC 205 is in a prograde encounter with its parent galaxy, M31.
A Measured Solution: Klinefelter Syndrome Geha, Rabih M.; Manesh, Reza
The American journal of medicine,
June 2018, 2018-06-00, 20180601, Letnik:
131, Številka:
6
Journal Article
We present Keck DEIMOS spectroscopy of stars in eight of the newly discovered ultra-faint dwarf galaxies around the Milky Way. We measure the velocity dispersions of Canes Venatici I, Canes Venatici ...II, Coma Berenices, Hercules, Leo IV, Leo T, Ursa Major I, and Ursa Major II from the velocities of 18-214 stars in each galaxy and find dispersions ranging from 3.3 to 7.6 km s super(-1). The six galaxies with absolute magnitudes Mv < -4 are highly dark matter dominated, with mass-to-light ratios approaching 1000 M unk/L unkv. For the fainter galaxies we find tentative evidence for tidal disruption. The measured velocity dispersions of the ultra-faint dwarfs are correlated with their luminosities, indicating that a minimum mass for luminous galactic systems may not yet have been reached. We also measure the metallicities of the observed stars and find that the new dwarfs have mean metallicities of Fe/H unk -2.0 to -2.3; these galaxies represent some of the most metal-poor stellar systems known. The six brightest of the ultra-faint dwarfs extend the luminosity-metallicity relationship followed by more luminous dwarfs by a factor of similar to 30 in luminosity. We detect metallicity spreads of up to 0.5 dex in several objects, suggesting multiple star formation epochs. UMa II and Com, despite their exceptionally low luminosities, have higher metallicities that suggest they may once have been much more massive. Having established the masses of the ultra-faint dwarfs, we re-examine the missing satellite problem. After correcting for the sky coverage of the Sloan Digital Sky Survey, we find that the ultra-faint dwarfs substantially alleviate the discrepancy between the predicted and observed numbers of satellites around the Milky Way, but there are still a factor of similar to 4 too few dwarf galaxies over a significant range of masses. We show that if galaxy formation in low-mass dark matter halos is strongly suppressed after reionization, the simulated circular velocity function of CDM subhalos can be brought into approximate agreement with the observed circular velocity function of Milky Way satellite galaxies.
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