Browsing by Subject "milky-way tomography"
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Item Halo Streams In The Seventh Sloan Digital Sky Survey Data Release(2009-06) Klement, R.; Rix, Hans-Walter; Flynn, C.; Fuchs, B.; Beers, Timothy C.; Prieto, Carlos Allende; Bizyaev, Dmitry; Brewington, Howard; Lee, Y. S.; Malanushenko, Elena; Malanushenko, Viktor; Oravetz, Dan; Pan, K.; Fiorentin, P. R.; Simmons, Audrey; Snedden, Stephanie; Prieto, Carlos AllendeWe have detected stellar halo streams in the solar neighborhood using data from the seventh public data release of the Sloan Digital Sky Survey (SDSS), which includes the directed stellar program Sloan Extension For Galactic Understanding and Exploration (SEGUE). In order to derive distances to each star, we used the metallicity-dependent photometric parallax relation from Ivezic et al. We examine and quantify the accuracy of this relation by applying it to a set of globular and open clusters observed by the SDSS/SEGUE and comparing the resulting sequence to the fiducial cluster sequences obtained by An et al. Our final sample consists of 22,321 nearby (d <= 2 kpc), metal-poor ([Fe/H] <= -0.5) main-sequence stars with six-dimensional estimates of position and space velocity ((r) over bar, (v) over bar). We characterize the orbits of these stars through suitable kinematic proxies for their "effective" integrals of motion, angular momentum, eccentricity, and orbital polar angle and compare the observed distribution to expectations from a smooth distribution in four [Fe/H] bins. The metallicities provide an additional dimension in parameter space that is well suited to distinguish tidal streams from those of dynamical origin. On this basis, we identify at least five significant "phase-space overdensities" of stars on very similar orbits in the solar neighborhood to which we can assign unambiguously peaked [Fe/H] distributions. Three of them have been identified previously, including the halo stream discovered by Helmi et al. at a significance level of sigma = 12.0. In addition, we find at least two new genuine halo streams, judged by their kinematics and [Fe/H], at sigma = 2.9 and 4.8, respectively. For one stream the stars even show coherence in the configuration space, matching a spatial overdensity of stars found by Juric et al. at (R, z) approximate to (9.5, 0.8) kpc. Our results demonstrate the practical power of our search method to detect substructure in the phase-space distribution of nearby stars without making a priori assumptions about the detailed form of the gravitational potential.Item Identifying Contributions To The Stellar Halo From Accreted, Kicked-Out, And In Situ Populations(2012-12) Sheffield, Allyson A.; Majewski, Steven R.; Johnston, Kathryn V.; Cunha, Katia; Smith, Verne V.; Cheung, Andrew M.; Hampton, Christina M.; David, T. J.; Wagner-Kaiser, Rachel.; Johnson, Marshall C.; Kaplan, Evan; Miller, Jacob; Patterson, Richard J.; Johnson, Marshall C.We present a medium-resolution spectroscopic survey of late-type giant stars at mid-Galactic latitudes of (30 degrees < | b| < 60 degrees), designed to probe the properties of this population to distances of similar to 9 kpc. Because M giants are generally metal-rich and we have limited contamination from thin disk stars by the latitude selection, most of the stars in the survey are expected to be members of the thick disk (<[Fe/H]> similar to -0.6) with some contribution from the metal-rich component of the nearby halo. Here we report first results for 1799 stars. The distribution of radial velocity (RV) as a function of l for these stars shows (1) the expected thick disk population and (2) local metal-rich halo stars moving at high speeds relative to the disk, which in some cases form distinct sequences in RV-l space. High-resolution echelle spectra taken for 34 of these "RV outliers" reveal the following patterns across the [Ti/Fe]-[Fe/H] plane: 17 of the stars have abundances reminiscent of the populations present in dwarf satellites of the Milky Way, 8 have abundances coincident with those of the Galactic disk and a more metal-rich halo, and 9 of the stars fall on the locus defined by the majority of stars in the halo. The chemical abundance trends of the RV outliers suggest that this sample consists predominantly of stars accreted from infalling dwarf galaxies. A smaller fraction of stars in the RV outlier sample may have been formed in the inner Galaxy and subsequently kicked to higher eccentricity orbits, but the sample is not large enough to distinguish conclusively between this interpretation and the alternative that these stars represent the tail of the velocity distribution of the thick disk. Our data do not rule out the possibility that a minority of the sample could have formed from gas in situ on their current orbits. These results are consistent with scenarios where the stellar halo, at least as probed by M giants, arises from multiple formation mechanisms; however, when taken at face value, our results for metal-rich halo giants suggest a much higher proportion to be accreted than found by Carollo et al. and more like the fraction suggested in the analysis by Nissen & Schuster and Schuster et al. We conclude that M giants with large RVs can provide particularly fruitful samples to mine for accreted structures and that some of the velocity sequences may indeed correspond to real physical associations resulting from recent accretion events.