169 research outputs found
Different Velocity Dependences of Physical Conditions of High- and Low-Ionization Lines in Broad-Line Regions
We present results from a study of high- and low-ionization emission line
ratios as a function of projected velocity for a sample of eight active
galactic nuclei (AGNs). Our results are based on analysis of high
signal-to-noise optical and Hubble Space Telescope (HST) UV spectra. Comparing
the emission line ratios to those predicted by photoionization models indicates
that the physical conditions responsible for the high-ionization emission lines
are consistent with a wind, whereas those of the low-ionization lines are
consistent with a virialized disk.Comment: 5 pages, 2 figures in AASTeX. To appear in "AGN Physics with the
Sloan Digital Sky Survey", ASP Conference Series Vol. 311, 2004, G. T.
Richards and P. B. Hall, ed
The Case for Optically-Thick High Velocity Broad Line Region Gas in Active Galactic Nuclei
A combined analysis of the profiles of the main broad quasar emission lines
in both Hubble Space Telescope and optical spectra shows that while the
profiles of the strong UV lines are quite similar, there is frequently a strong
increase in the Ly-alpha/H-alpha ratio in the high-velocity gas. We show that
the suggestion that the high velocity gas is optically-thin presents many
problems. We show that the relative strengths of the high velocity wings arise
naturally in an optically-thick BLR component. An optically-thick model
successfully explains the equivalent widths of the lines, the Ly-alpha/H-alpha
ratios and flatter Balmer decrements in the line wings, the strengths of CIII]
and the lambda 1400 blend, and the strong variability of high-velocity,
high-ionization lines (especially HeII and HeI).Comment: 34 pages in AASTeX, including 10 pages of figures. Submitted to
Astrophysical Journa
Discovery of New Ultracool White Dwarfs in the Sloan Digital Sky Survey
We report the discovery of five very cool white dwarfs in the Sloan Digital
Sky Survey (SDSS). Four are ultracool, exhibiting strong collision induced
absorption (CIA) from molecular hydrogen and are similar in color to the three
previously known coolest white dwarfs, SDSS J1337+00, LHS 3250 and LHS 1402.
The fifth, an ultracool white dwarf candidate, shows milder CIA flux
suppression and has a color and spectral shape similar to WD 0346+246. All five
new white dwarfs are faint (g > 18.9) and have significant proper motions. One
of the new ultracool white dwarfs, SDSS J0947, appears to be in a binary system
with a slightly warmer (T_{eff} ~ 5000K) white dwarf companion.Comment: 15 pages, 3 figures, submitted to ApJL. Higher resolution versions of
finding charts are available at
http://astro.uchicago.edu/~gates/findingchart
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 Quasar Without Broad Lyman-alpha Emission
The z=3.02 quasar SDSS J095253.83+011421.9 exhibits broad metal-line emission
(CIV FWHM=9000 km/s), but broad Ly-alpha emission is not present. Instead, only
a narrow Ly-alpha line is observed (FWHM=1140 km/s). The large CIV/Ly-alpha
ratio in the broad-line region (BLR) emission from this object can be matched
most closely by a BLR dominated by gas at very high densities (10^15 cm^-3),
which suppresses the Ly-alpha emission, and illuminated by an incident
power-law extending to ~200 micron, which yields increased emission from purely
collisionally excited coolant lines (such as CIV, NV and OVI) but not from
recombination lines like Ly-alpha. However, the strong CIII emission predicted
by this model is not observed, and the observed broad CIII] emission must come
from a lower-density BLR component and should be accompanied by broad Ly-alpha
emission which is not observed. The least unlikely explanation for this
spectrum seems to be that any intrinsic broad Ly-alpha emission is removed by
smooth NV absorption in the red wing of the Ly-alpha emission line and by
smooth Ly-alpha absorption in the blue wing of the Ly-alpha emission line. This
postulated smooth absorption would be in addition to the strong, associated,
narrow absorption seen in numerous ions. Smooth absorption in Ly-alpha, NV and
OVI but not in CIV would be unusual, but not impossible, although it is
suspicious that the postulated absorption must almost exactly cancel the
postulated intrinsic broad emission. We conclude that the spectrum of SDSS
J0952+0114 appears unique (among ~3600 SDSS spectra of quasars at z>2.12)
because of some_combination_ of unusual parameters, and we discuss possible
observations to determine the combination of circumstances responsible for the
spectrum.Comment: 9 pages, 4 figures, accepted by AJ for the August 2004 issu
Chandra X-Ray Observations of Two Unusual BAL Quasars
We report sensitive Chandra X-ray non-detections of two unusual, luminous
Iron Low-Ionization Broad Absorption Line Quasars (FeLoBALs). The observations
do detect a non-BAL, wide-binary companion quasar to one of the FeLoBAL
quasars. We combine X-ray-derived column density lower limits (assuming solar
metallicity) with column densities measured from ultraviolet spectra and CLOUDY
photoionization simulations to explore whether constant density slabs at broad
line region densities can match the physical parameters of these two BAL
outflows, and find that they cannot. In the "overlapping-trough" object SDSS
J0300+0048, we measure the column density of the X-ray absorbing gas to be N_H
>= 1.8 x 1024 cm-2. From the presence of Fe II UV78 absorption but lack of Fe
II UV195/UV196 absorption, we infer the density in that part of the absorbing
region to be n_e ~ 106 cm-3. We do find that a slab of gas at that density
might be able to explain this object's absorption. In the Fe III-dominant
object SDSS J2215-0045, the X-ray absorbing column density of N_H >= 3.4 x 1024
cm-2 is consistent with the Fe III-derived N_H >= 2 x 1022 cm-2 provided the
ionization parameter is log U > 1.0 for both the n_e = 1011 cm-3 and n_e = 1012
cm-3 scenarios considered (such densities are required to produce Fe III
absorption without Fe II absorption). However, the velocity width of the
absorption rules out its being concentrated in a single slab at these
densities. Instead, this object's spectrum can be explained by a low density,
high ionization and high temperature disk wind that encounters and ablates
higher density, lower ionization Fe III-emitting clumps.Comment: 18 pages, 6 figure
Hdelta-Selected Galaxies in the Sloan Digital Sky Survey I: The Catalog
[Abridged] We present here a new and homogeneous sample of 3340 galaxies
selected from the Sloan Digital Sky Survey (SDSS) based solely on the observed
strength of their Hdelta absorption line. These galaxies are commonly known as
``post-starburst'' or ``E+A'' galaxies, and the study of these galaxies has
been severely hampered by the lack of a large, statistical sample of such
galaxies. In this paper, we rectify this problem by selecting a sample of
galaxies which possess an absorption Hdelta equivalent width of EW(Hdelta_max)
- Delta EW(Hdelta_max) > 4A from 106682 galaxies in the SDSS. We have performed
extensive tests on our catalog including comparing different methodologies of
measuring the Hdelta absorption and studying the effects of stellar absorption,
dust extinction, emission-filling and measurement error. The measured abundance
of our Hdelta-selected (HDS) galaxies is 2.6 +/- 0.1% of all galaxies within a
volume-limited sample of 0.05<z<0.1 and M(r*)<-20.5, which is consistent with
previous studies of such galaxies in the literature. We find that only 25 of
our HDS galaxies in this volume-limited sample (3.5+/-0.7%) show no evidence
for OII and Halpha emission, thus indicating that true E+A (or k+a) galaxies
are extremely rare objects at low redshift, i.e., only 0.09+/-0.02% of all
galaxies in this volume-limited sample are true E+A galaxies. In contrast,
89+/-5% of our HDS galaxies in the volume-limited sample have significant
detections of the OII and Halpha emission lines. We find 27 galaxies in our
volume-limited HDS sample that possess no detectable OII emission, but do
however possess detectable Halpha emission. These galaxies may be dusty
star-forming galaxies. We provide the community with this new catalog of
Hdelta-selected galaxies to aid in the understanding of these galaxies.Comment: Submitted to PASJ. Catalog of galaxies available at
http://astrophysics.phys.cmu.edu/~tomo/ea
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
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
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