148 research outputs found
The quasar Q0957+561: Lensed CO emission from a disk at z~1.4?
In recent years large efforts have been made to detect molecular gas towards
high redshifted objects. Up to now the literature reports on only two cases of
CO-detection in quasars at a redshift between 1 and 2 - Q0957+561, a
gravitationally lensed system at z=1.41 (Planesas et al. 1999), and HR10 at
z=1.44 (Andreani et al. 2000). According to Planesas et al. (1999), 12CO(2-1)
emission was detected towards both the lensed images of Q0957+561 with the IRAM
Plateau de Bure Interferometer (PdBI). In contrast to the optical spectra of
the two images which support the idea that they are images of one and the same
object, the CO-spectra were surprisingly different: the southern image (named
CO-B) shows a single blueshifted line whereas a double-peaked line profile with
a blue- and a redshifted part appears towards the northern image (CO-A). Based
on the observations and on simulations with a gravitational lens program, we
are tempted to argue that the line profile traces the presence of molecular gas
of a disk in the host galaxy around the quasar. We have now new observations
with the PdBI providing the necessary sensitivity to corroborate our disk
model.Comment: 4 pages, 1 figure, to appear in "Proceedings of the 4th
Cologne-Bonn-Zermatt-Symposium", ed. S. Pfalzner, C. Kramer, C. Straubmeier,
and A. Heithausen (Springer Verlag
X-ray synchrotron emission from the oblique shock in the jet of the powerful radio galaxy 3C 346
We report the first detection, with Chandra, of X-ray emission from the jet
of the powerful narrow-line radio galaxy 3C 346. X-rays are detected from the
bright radio and optical knot at which the jet apparently bends by about 70
degrees. The Chandra observation also reveals a bright galaxy-scale atmosphere
within the previously-known cluster, and provides a good X-ray spectrum for the
bright core of 3C 346. The X-ray emission from the knot is synchrotron
radiation, as seen in lower-power sources. In common with these sources, there
is evidence of morphological differences between the radio/optical and X-ray
structures, and the spectrum is inconsistent with a one-component
continuous-injection model. We suggest that the X-ray-bright knot is associated
with a strong oblique shock in a moderately relativistic, light jet, at about
20 degrees to the line of sight, and that this shock is caused by the jet
interacting with the wake in the cluster medium behind 3C 346's companion
galaxy. The general jet curvature can result from pressure gradients in the
cluster atmosphere.Comment: Accepted for publication in the MNRAS. 9 page
The Hercules-Aquila Cloud
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 about 40 degrees and extends above and below the Galactic
plane by at least 50 degrees. 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 about 80 degrees in longitude. Its
heliocentric distance lies between 10 and 20 kpc so that the extent of the
Cloud in projection is roughly 20 kpc by 15 kpc. It has an absolute magnitude
of -13 and its stellar population appears to be comparable to, but somewhat
more metal-rich than, M92.Comment: ApJ (Letters), in pres
A Catalog of Spectroscopically Confirmed White Dwarfs from the Sloan Digital Sky Survey Data Release 4
We present a catalog of 9316 spectroscopically confirmed white dwarfs from
the Sloan Digital Sky Survey Data Release 4. We have selected the stars through
photometric cuts and spectroscopic modeling, backed up by a set of visual
inspections. Roughly 6000 of the stars are new discoveries, roughly doubling
the number of spectroscopically confirmed white dwarfs. We analyze the stars by
performing temperature and surface gravity fits to grids of pure hydrogen and
helium atmospheres. Among the rare outliers are a set of presumed helium-core
DA white dwarfs with estimated masses below 0.3 Msun, including two candidates
that may be the lowest masses yet found. We also present a list of 928 hot
subdwarfs.Comment: Accepted by the Astrophysical Journal Supplements, 25 pages, 24
figures, LaTeX. The electronic catalog, as well as diagnostic figures and
links to the spectra, is available at http://das.sdss.org/wdcat/dr4
A New Giant Stellar Structure in the Outer Halo of M31
The Sloan Digital Sky Survey has revealed an overdensity of luminous red
giant stars ~ 3 degrees (40 projected kpc) to the northeast of M31, which we
have called Andromeda NE. The line-of-sight distance to Andromeda NE is within
approximately 50 kpc of M31; Andromeda NE is not a physically unrelated
projection. Andromeda NE has a g-band absolute magnitude of ~ -11.6 and central
surface brightness of ~ 29 mag/sq.arcsec, making it nearly two orders of
magnitude more diffuse than any known Local Group dwarf galaxy at that
luminosity. Based on its distance and morphology, Andromeda NE is likely
undergoing tidal disruption. Andromeda NE's red giant branch color is unlike
that of M31's present-day outer disk or the stellar stream reported by Ibata et
al. (2001), arguing against a direct link between Andromeda NE and these
structures. However, Andromeda NE has a red giant branch color similar to that
of the G1 clump; it is possible that these structures are both material torn
off of M31's disk in the distant past, or that these are both part of one
ancient stellar stream.Comment: 11 pages, 3 figures; ApJ Letters accepted versio
Discovery of an Unusual Dwarf Galaxy in the Outskirts of the Milky Way
In this Letter, we announce the discovery of a new dwarf galaxy, Leo T, in
the Local Group. It was found as a stellar overdensity in the Sloan Digital Sky
Survey Data Release 5 (SDSS DR5). The color-magnitude diagram of Leo T shows
two well-defined features, which we interpret as a red giant branch and a
sequence of young, massive stars. As judged from fits to the color-magnitude
diagram, it lies at a distance of about 420 kpc and has an intermediate-age
stellar population with a metallicity of [Fe/H]= -1.6, together with a young
population of blue stars of age of 200 Myr. There is a compact cloud of neutral
hydrogen with mass roughly 10^5 solar masses and radial velocity 35 km/s
coincident with the object visible in the HIPASS channel maps. Leo T is the
smallest, lowest luminosity galaxy found to date with recent star-formation. It
appears to be a transition object similar to, but much lower luminosity than,
the Phoenix dwarf.Comment: Ap J (Letters) in press, the subject of an SDSS press release toda
A Strategy for Finding Near Earth Objects with the SDSS Telescope
We present a detailed observational strategy for finding Near Earth Objects
(NEOs) with the Sloan Digital Sky Survey (SDSS) telescope. We investigate
strategies in normal, unbinned mode as well as binning the CCDs 2x2 or 3x3,
which affects the sky coverage rate and the limiting apparent magnitude. We
present results from 1 month, 3 year and 10 year simulations of such surveys.
For each cadence and binning mode, we evaluate the possibility of achieving the
Spaceguard goal of detecting 90% of 1 km NEOs (absolute magnitude H <= 18 for
an albedo of 0.1). We find that an unbinned survey is most effective at
detecting H <= 20 NEOs in our sample. However, a 3x3 binned survey reaches the
Spaceguard Goal after only seven years of operation. As the proposed large
survey telescopes (PanStarss; LSST) are at least 5-10 years from operation, an
SDSS NEO survey could make a significant contribution to the detection and
photometric characterization of the NEO population.Comment: Accepted by AJ -- 12 pages, 11 figure
Cats and Dogs, Hair and A Hero: A Quintet of New Milky Way Companions
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 last two years, a total of ten
new Milky Way satellites with effective surface brightness mu_v >~ 28 mag/sq.
arcsec have been discovered in SDSS data. They are less luminous, more
irregular and appear to be 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 ~40 pc and ~ 100 pc. We conjecture that this may
represent the division between star clusters and dwarf galaxies.Comment: 10 pages, 8 figures, submitted to the Astrophysical Journa
New Low Accretion-Rate Magnetic Binary Systems and their Significance for the Evolution of Cataclysmic Variables
Discoveries of two new white dwarf plus M star binaries with striking optical
cyclotron emission features from the Sloan Digital Sky Survey (SDSS) brings to
six the total number of X-ray faint, magnetic accretion binaries that accrete
at rates < 10^{-13} Msun/yr, or <1% of the values normally encountered in
cataclysmic variables. This fact, coupled with donor stars that underfill their
Roche lobes and very cool white dwarfs, brand the binaries as post
common-envelope systems whose orbits have not yet decayed to the point of
Roche-lobe contact. They are pre-magnetic CVs, or pre-Polars. The systems
exhibit spin/orbit synchronism and apparently accrete by efficient capture of
the stellar wind from the secondary star, a process that has been dubbed a
``magnetic siphon''. Because of this, period evolution of the binaries will
occur solely by gravitational radiation, which is very slow for periods >3 hr.
Optical surveys for the cyclotron harmonics appear to be the only means of
discovery, so the space density of pre-Polars could rival that of Polars, and
the binaries provide an important channel of progenitors (in addition to the
asynchronous Intermediate Polars). Both physical and SDSS observational
selection effects are identified that may help to explain the clumping of all
six systems in a narrow range of magnetic field strength around 60 MG.Comment: 25 pages, 13 figures, Accepted to Ap
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