207 research outputs found
Tracing the Sagittarius stream with blue horizontal branch stars
The definitive version is available at www.blackwell-synergy.com--Copyright Blackwell Publishing DOI : 10.1111/j.1365-2966.2006.10110.xPeer reviewe
The TOOT00 Redshift Survey of Radio Sources
We present first results from the study of the TOOT00 region consisting of 47
radio sources brighter than 100 mJy at 151 MHz. We have 81% spectroscopic
redshift completeness. From the K-z diagram we deduce that the host galaxies
are similar to ~3 L* passively evolved elliptical galaxies and thus estimate
the redshifts of the 9 sources without a secure spectroscopic redshift yielding
a median redshift of 1.287. Above the RLF break we have a quasar fraction f ~
0.3 although the quasars appear reddened; below the RLF break f -> 0 if we
exclude flat-spectrum radio sources. We present a histogram of the number of
TOOT00 radio sources versus their redshift which looks broadly like the Willott
et al. (2001) prediction for TOOT, although the observed ratio of high to low
redshift objects is somewhat lower than the prediction.Comment: 4 pages, 5 figures, to be published in AN (proceedings of the Granada
Workshop on High Redshift Radio Galaxies
The Century Survey Galactic Halo Project III: A Complete 4300 deg^2 Survey of Blue Horizontal Branch Stars in the Metal-Weak Thick Disk and Inner Halo
We present a complete spectroscopic survey of 2414 2MASS-selected blue
horizontal branch (BHB) candidates selected over 4300 deg^2 of the sky. We
identify 655 BHB stars in this non-kinematically selected sample. We calculate
the luminosity function of field BHB stars and find evidence for very few hot
BHB stars in the field. The BHB stars located at a distance from the Galactic
plane |Z|<4 kpc trace what is clearly a metal-weak thick disk population, with
a mean metallicity of [Fe/H]= -1.7, a rotation velocity gradient of
dv_{rot}/d|Z|= -28+-3.4 km/s in the region |Z|<6 kpc, and a density scale
height of h_Z= 1.26+-0.1 kpc. The BHB stars located at 5<|Z|<9 kpc are a
predominantly inner-halo population, with a mean metallicity of [Fe/H]= -2.0
and a mean Galactic rotation of -4+-31 km/s. We infer the density of halo and
thick disk BHB stars is 104+-37 kpc^-3 near the Sun, and the relative
normalization of halo to thick-disk BHB stars is 4+-1% near the Sun.Comment: 12 pages in emulateapj format, accepted for publication in February
A
Hypervelocity Stars. I. The Spectroscopic Survey
We discuss our targeted search for hypervelocity stars (HVSs), stars
traveling with velocities so extreme that dynamical ejection from a massive
black hole is their only suggested origin. Our survey, now half complete, has
successfully identified a total of four probable HVSs plus a number of other
unusual objects. Here we report the most recently discovered two HVSs: SDSS
J110557.45+093439.5 and possibly SDSS J113312.12+010824, traveling with
Galactic rest-frame velocities at least +508+-12 and +418+-10 km/s,
respectively. The other late B-type objects in our survey are consistent with a
population of post main-sequence stars or blue stragglers in the Galactic halo,
with mean metallicity [Fe/H]=-1.3 and velocity dispersion 108+-5 km/s.
Interestingly, the velocity distribution shows a tail of objects with large
positive velocities that may be a mix of low-velocity HVSs and high-velocity
runaway stars. Our survey also includes a number of DA white dwarfs with
unusually red colors, possibly extremely low mass objects. Two of our objects
are B supergiants in the Leo A dwarf, providing the first spectroscopic
evidence for star formation in this dwarf galaxy within the last ~30 Myr.Comment: 10 pages, uses emulateapj, accepted by Ap
Stellar Population Variations in the Milky Way's Stellar Halo
If the stellar halos of disk galaxies are built up from the disruption of
dwarf galaxies, models predict highly structured variations in the stellar
populations within these halos. We test this prediction by studying the ratio
of blue horizontal branch stars (BHB stars; more abundant in old, metal-poor
populations) to main-sequence turn-off stars (MSTO stars; a feature of all
populations) in the stellar halo of the Milky Way using data from the Sloan
Digital Sky Survey. We develop and apply an improved technique to select BHB
stars using ugr color information alone, yielding a sample of ~9000 g<18
candidates where ~70% of them are BHB stars. We map the BHB/MSTO ratio across
~1/4 of the sky at the distance resolution permitted by the absolute magnitude
distribution of MSTO stars. We find large variations of BHB/MSTO star ratio in
the stellar halo. Previously identified, stream-like halo structures have
distinctive BHB/MSTO ratios, indicating different ages/metallicities. Some halo
features, e.g., the low-latitude structure, appear to be almost completely
devoid of BHB stars, whereas other structures appear to be rich in BHB stars.
The Sagittarius tidal stream shows an apparent variation in BHB/MSTO ratio
along its extent, which we interpret in terms of population gradients within
the progenitor dwarf galaxy. Our detection of coherent stellar population
variations between different stellar halo substructures provides yet more
support to cosmologically motivated models for stellar halo growth.Comment: Astronomical Journal, in press. 10 pages, 5 color figures. Much
better printed in colo
Identifying Blue Horizontal Branch Stars Using the z Filter
In this paper we present a new method for selecting blue horizontal branch
(BHB) candidates based on color-color photometry. We make use of the Sloan
Digital Sky Survey z band as a surface gravity indicator and show its value for
selecting BHB stars from quasars, white dwarfs and main sequence A type stars.
Using the g, r, i, and z bands, we demonstrate that extraction accuracies on
par with more traditional u, g, and r photometric selection methods may be
achieved. We also show that the completeness necessary to probe major Galactic
structure may be maintained. Our new method allows us to efficiently select BHB
stars from photometric sky surveys that do not include a u band filter such as
the Panoramic Survey Telescope and Rapid Response System.Comment: Accepted for publication in The Astronomical Journa
Kinematic Structure in the Galactic Halo at the North Galactic Pole: RR Lyrae and BHB Stars show different kinematics
Space motions are given for 38 RR Lyrae (RRL) stars and 79 blue horizontal
branch (BHB) stars in a ~200 deg2 area around the North Galactic Pole (NGP)
using a homogeneous distance scale consistent with (m-M)0=18.52 for the LMC.
The kinematics of the 26 RRL and 52 BHB stars in the 10.4 cubic kpc volume
that have Z<8 kpc are not homogeneous. Our BHB sample (like that of Sirko et
al. 2004b) has a zero galactic rotation (V_phi) and roughly isotropic velocity
dispersions. The RRL sample shows a definite retrograde rotation (V_phi =
-95+/-29 km/s) and non-isotropic velocity dispersions. The combined BHB and RRL
sample has a retrograde galactic rotation (V) that is similar to that found by
Majewski (1992) for his sample of subdwarfs in SA 57. The velocity dispersion
of the RRL stars that have a positive W motion is significantly smaller than
the dispersion of those "streaming down" with a negative W.
One component of our sample (rich in RRL's) shows retrograde rotation and the
streaming motion that we associate with the accretion process. The other
(traced by the BHB stars) shows essentially no rotation and less evidence of
streaming. These two components have HB morphologies that suggest that they may
be the field star equivalents of the young and old halo globular clusters
respectively.Comment: Accepted for publication on MNRAS. 20 pages, 7 figures, 12 table
The Structure of the Sagittarius Stellar Stream as Traced by Blue Horizontal Branch Stars
We use a sample of blue horizontal branch (BHB) stars from the Sloan Digital
Sky Survey Data Release 7 to explore the structure of the tidal tails from the
Sagittarius Dwarf Galaxy. We use a method yielding BHB star candidates with up
to ~70% purity from photometry alone. The resulting sample has a distance
precision of roughly 5% and can probe distances in excess of 100 kpc. Using
this sample, we identify a possible extension to the trailing arm at distances
of 60-80 kpc from the Sun with an estimated significance of at least 3.8 sigma.
Current models predict that a distant `returning' segment of the debris stream
should exist, but place it substantially closer to the Sun where no debris is
observed in our data. Exploiting the distance precision of our tracers, we
estimate the mean line-of-sight thickness of the leading arm to be ~3 kpc, and
show that the two `bifurcated' branches of the debris stream differ by only 1-2
kpc in distance. With a spectroscopic very pure BHB star subsample, we estimate
the velocity dispersion in the leading arm, 37 km s^-1, which is in reasonable
agreement with models of Sgr disruption. We finally present a sample of
high-probability Sgr BHB stars in the leading arm of Sgr, selected to have
distances and velocities consistent with Sgr membership, to allow further
study.Comment: Astrophysical Journal, in press. 17 pages, 15 figure
- âŠ