* Astronomy

Title: The Leo IV dwarf spheroidal galaxy: colour-magnitude diagram and pulsating stars
Authors: M. I. Moretti, M. Dall'Ora, V. Ripepi, G. Clementini, L. Di Fabrizio, H. Smith, N. De Lee, C. Kuehn, M. Catelan, M. Marconi, I. Musella, T. Beers, K. Kinemuchi

We present the first V, B-V colour-magnitude diagram of the Leo IV dwarf spheroidal galaxy, a faint Milky Way satellite recently discovered by the Sloan Digital Sky Survey. We have obtained B,V time-series photometry reaching about half a magnitude below the Leo IV turnoff, which we detect at V= 24.7 mag, and have performed the first study of the variable star population. We have identified three RR Lyrae stars (all fundamental-mode pulsators, RRab) and one SX Phoenicis variable in the galaxy. In the period-amplitude diagram the Leo IV RR Lyrae stars are located close to the loci of Oosterhoff type I systems and the evolved fundamental-mode RR Lyrae stars in the Galactic globular cluster M3. However, their mean pulsation period, <P{ab}>=0.655 days, would suggest an Oosterhoff type II classification for this galaxy. The RR Lyrae stars trace very well the galaxy's horizontal branch, setting its average magnitude at <V_{RR}>= 21.48 ±0.03 mag (standard deviation of the mean). This leads to a distance modulus of \mu_{0}=20.94 ±0.07 mag, corresponding to a distance of 154 ±5 kpc, by adopting for the Leo IV dSph a reddening E(B-V) = 0.04 ±0.01 mag and a metallicity of [Fe/H] = -2.31 ± 0.10.

Read more  (290kb, PDF)

Title: Tracing Sagittarius Structure with SDSS and SEGUE Imaging and Spectroscopy
Authors: Brian Yanny, Heidi Jo Newberg, Jennifer A. Johnson, Young Sun Lee, Timothy C. Beers, Dmitry Bizyaev, Howard Brewington, Paola Re Fiorentin, Paul Harding, Elena Malanushenko, Viktor Malanushenko, Dan Oravetz, Kaike Pan, Audrey Simmons, Stephanie Snedden
(Version v2)

We show that the Sagittarius dwarf tidal stream can be traced with very red K/M-giant stars selected from SDSS photometry. A subset of these stars are spectroscopically confirmed with SEGUE and SDSS spectra, and the distance scale of 2MASS and SDSS M giants is calibrated to the RR Lyrae distance scale. The absolute g band magnitude of the K/M-giant stars at the tip of the giant branch is M_g=-1.0. The line-of-sight velocities of the M giant and BHB stars that are spatially coincident with the Sgr dwarf tidal stream are consistent with those of previous authors, reinforcing the need for new models that can explain all of the Sgr tidal debris stream observations. We estimate stellar densities along the tidal tails that can be used to help constrain future models. The K/M-giant, BHB, and F-turnoff stars in the lower surface brightness tidal stream that is adjacent to the main leading Sgr dwarf tidal tail have velocities and metallicities that are similar to those of the stars in the leading tidal tail. The ratio of K/M giants to BHBs and BHBs to F-turnoff stars are also similar for both branches of the leading tidal tail. We show that there is an additional low-metallicity tidal stream near the Sgr trailing tidal tail.

Read more (981kb, PDF)

Title: An Orbit Fit for the Grillmair Dionatos Cold Stellar Stream
Authors: Benjamin A. Willett, Heidi Jo Newberg, Haotong Zhang, Brian Yanny, Timothy C. Beers

We use velocity and metallicity information from SDSS and SEGUE stellar spectroscopy to fit an orbit to the narrow 63° stellar stream of Grillmair and Dionatos. The stars in the stream have a retrograde orbit with eccentricity e = 0.33 (perigalacticon of 14.4 kpc and apogalacticon of 28.7 kpc) and inclination approximately i ~ 35°. In the region of the orbit which is detected, it has a distance of about 7 to 11 kpc from the Sun. Assuming a standard disk plus bulge and logarithmic halo potential for the Milky Way stars plus dark matter, the stream stars are moving with a large space velocity of approximately 276
m km s^{-1} at perigalacticon. Using this stream alone, we are unable to determine if the dark matter halo is oblate or prolate. The metallicity of the stream is [Fe/H] = -2.1 ±0.1. Observed proper motions for individual stream members above the main sequence turnoff are consistent with the derived orbit. None of the known globular clusters in the Milky Way have positions, radial velocities, and metallicities that are consistent with being the progenitor of the GD-1 stream.

Read more (1068kb, PDF)

Astronomers have identified what appears to be the dimmest galaxy in the universe. Although it shines with the brightness of only a few hundred Suns, it seems to be full of dark matter, making it an ideal candidate to search for evidence of the mysterious material, they say.
The galaxy, known as Segue 1, is one of roughly two dozen dwarf galaxies that orbit the Milky Way. It sits close to the Sagittarius stream, a river of stellar debris torn from another dwarf galaxy.

Read more