We derive new constraints on the mass of the Milky Way's dark matter halo, based on 2401 rigorously selected blue horizontal-branch halo stars from SDSS DR6. This sample enables construction of the ...full line-of-sight velocity distribution at different galactocentric radii. To interpret these distributions, we compare them to matched mock observations drawn from two different cosmological galaxy formation simulations designed to resemble the Milky Way. This procedure results in an estimate of the Milky Way's circular velocity curve to image60 kpc, which is found to be slightly falling from the adopted value of 220 km s super(-1) at the Sun's location, and implies image M sub(image). The radial dependence of image, derived in statistically independent bins, is found to be consistent with the expectations from an NFW dark matter halo with the established stellar mass components at its center. If we assume that an NFW halo profile of characteristic concentration holds, we can use the observations to estimate the virial mass of the Milky Way's dark matter halo, image M sub(image), which is lower than many previous estimates. We have checked that the particulars of the cosmological simulations are unlikely to introduce systematics larger than the statistical uncertainties. This estimate implies that nearly 40% of the baryons within the virial radius of the Milky Way's dark matter halo reside in the stellar components of our Galaxy. A value for image of only image M sub(image) also (re)opens the question of whether all of the Milky Way's satellite galaxies are on bound orbits.
We use Sloan Digital Sky Survey (SDSS) Data Release 5 (DR5) u, g, r, i, z photometry to study Milky Way halo substructure in the area around the north Galactic cap. A simple color cut (g - r < 0.4) ...reveals the tidal stream of the Sagittarius dwarf spheroidal galaxy, as well as a number of other stellar structures in the field. Two branches (A and B) of the Sagittarius stream are clearly visible in an RGB composite image created from three magnitude slices, and there is also evidence for a still more distant wrap behind the A branch. A comparison of these data with numerical models suggests that the shape of the Galactic dark halo is close to spherical.
We use A-type stars selected from Sloan Digital Sky Survey data release 9 photometry to measure the outer slope of the Milky Way stellar halo density profile beyond 50 kpc. A likelihood-based ...analysis is employed that models the ugr photometry distribution of blue horizontal branch and blue straggler stars. In the magnitude range 18.5 < g < 20.5, these stellar populations span a heliocentric distance range of: 10 lap less than D sub(BS)/kpc lap less than 75, 40 lap less than D sub(BHB)/kpc lap less than 100. Contributions from contaminants, such as QSOs, and the effect of photometric uncertainties, are also included in our modeling procedure.We find evidence for a very steep outer halo profile, with power-law index alpha ~ 6 beyond Galactocentric radii r = 50 kpc, and even steeper slopes favored (alpha ~ 6-10) at larger radii. This result holds true when stars belonging to known overdensities, such as the Sagittarius stream, are included or excluded. We show that, by comparison to numerical simulations, stellar halos with shallower slopes at large distances tend to have more recent accretion activity. Thus, it is likely that the Milky Way has undergone a relatively quiet accretion history over the past several gigayears. Our measurement of the outer stellar halo profile may have important implications for dynamical mass models of the Milky Way, where the tracer density profile is strongly degenerate with total mass estimates.
We present five new satellites of the Milky Way discovered in Sloan Digital Sky Survey (SDSS) imaging data, four of which were followed up with either the Subaru or the Isaac Newton Telescopes. They ...include four probable new dwarf galaxies--one each in the constellations of Coma Berenices, Canes Venatici, Leo, and Hercules--together with one unusually extended globular cluster, Segue 1. We provide distances, absolute magnitudes, half-light radii, and color-magnitude diagrams for all five satellites. The morphological features of the color-magnitude diagrams are generally well described by the ridge line of the old, metal-poor globular cluster M92. In the past two years, a total of 10 new Milky Way satellites with effective surface brightness k sub(u) 28 mag arcsec super(-2) have been discovered in SDSS data. They are less luminous, more irregular, and apparently more metal-poor than the previously known nine Milky Way dwarf spheroidals. The relationship between these objects and other populations is discussed. We note that there is a paucity of objects with half-light radii between 640 and 6100 pc. We conjecture that this may represent the division between star clusters and dwarf galaxies.
Using the photometric parallax method we estimate the distances to similar to 48 million stars detected by the Sloan Digital Sky Survey (SDSS) and map their three-dimensional number density ...distribution in the Galaxy. The currently available data sample the distance range from 100 pc to 20 kpc and cover 6500 deg super(2) of sky, mostly at high Galactic latitudes ( degree 'b degree ' > 25). These stellar number density maps allow an investigation of the Galactic structure with no a priori assumptions about the functional form of its components. The data show strong evidence for a Galaxy consisting of an oblate halo, a disk component, and a number of localized overdensities. The number density distribution of stars as traced by M dwarfs in the solar neighborhood (D < 2 kpc) is well fit by two exponential disks (the thin and thick disk) with scale heights and lengths, bias corrected for an assumed 35% binary fraction, of H sub(1) = 300 pc and L sub(1) = 2600 pc, and H sub(2) = 900 pc and L sub(2) = 3600 pc, and local thick-to-thin disk density normalization P sub(thick) (R unk)/ unk (R unk) = 12%. we use the stars near main-sequence turnoff to measure the shape of the Galactic halo. We find a strong preference for oblate halo models, with best-fit axis ratio c/a = 0.64, PH proportional to r super(-2.8) power-law profile, and the local halo-to-thin disk normalization of 0.5%. Based on a series of Monte Carlo simulations, we estimate the errors of derived model parameters not to be larger than similar to 20% for the disk scales and similar to 10% for the density normalization, with largest contributions to error coming from the uncertainty in calibration of the photometric parallax relation and poorly constrained binary fraction. While generally consistent with the above model, the measured density distribution shows a number of statistically significant localized deviations. In addition to known features, such as the Monoceros stream, we detect two overdensities in the thick disk region at cylindrical galactocentric radii and heights (R, Z) similar to (6.5, 1.5) kpc and (R, Z) similar to (9.5, 0.8) kpc and a remarkable density enhancement in the halo covering over 1000 deg super(2) of sky toward the constellation of Virgo, at distances of similar to 6-20 kpc. Compared to counts in a region symmetric with respect to the l = 0 degree line and with the same Galactic latitude, the Virgo overdensity is responsible for a factor of 2 number density excess and may be a nearby tidal stream or a low-surface brightness dwarf galaxy merging with the Milky Way. The u-g color distribution of stars associated with it implies metallicity lower than that of thick disk stars and consistent with the halo metallicity distribution. After removal of the resolved overdensities, the remaining data are consistent with a smooth density distribution; we detect no evidence of further unresolved clumpy substructure at scales ranging from similar to 50 pc in the disk to similar to 1-2 kpc in the halo.
The Hercules-Aquila Cloud Belokurov, V; Evans, N. W; Bell, E. F ...
The Astrophysical journal,
03/2007, Letnik:
657, Številka:
2
Journal Article
Recenzirano
We present evidence for a substantial overdensity of stars in the direction of the constellations of Hercules and Aquila. The cloud is centered at a Galactic longitude of l - 40 and extends above and ...below the Galactic plane by at least 50. Given its off-centeredness and height, it is unlikely that the Hercules-Aquila cloud is related to the bulge or thick disk. More likely, this is a new structural component of the Galaxy that passes through the disk. The cloud stretches 680 in longitude. Its heliocentric distance lies between 10 and 20 kpc so that the extent of the cloud in projection is 620 kpc by 615 kpc. It has an absolute magnitude of Mu = -13, and its stellar population appears to be comparable to, but somewhat more metal-rich than, M92.
We report the discovery of SDSS J131326.89−001941.4, an ultra iron-poor red giant star (Fe/H ≃ −4.3) with a very high carbon abundance (C/Fe ≃ +2.5). This object is the fifth star in this rare class, ...and the combination of a fairly low effective temperature (Teff ≃ 5300 K), which enhances line absorption, with its brightness (g = 16.9), makes it possible to measure the abundances of calcium, carbon and iron using a low-resolution spectrum from the Sloan Digital Sky Survey. We examine the carbon and iron abundance ratios in this star and other similar objects in the light of predicted yields from metal-free massive stars, and conclude that they are consistent. By way of comparison, stars with similarly low iron abundances but lower carbon-to-iron ratios deviate from the theoretical predictions.
We announce the discovery of a new satellite of the Milky Way in the constellation of Bootes at a distance of 660 kpc. It was found in a systematic search for stellar overdensities in the north ...Galactic cap using Sloan Digital Sky Survey Data Release 5. The color-magnitude diagram shows a well-defined turnoff, red giant branch, and extended horizontal branch. Its absolute magnitude is M sub(v) 6 -5.8 mag, which makes it one of the faintest galaxies known. The half-light radius is 6220 pc. The isodensity contours are elongated and have an irregular shape, suggesting that Boo may be a disrupted dwarf spheroidal galaxy.
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