118,758 research outputs found
Properties of Gamma-Ray Burst Time Profiles Using Pulse Decomposition Analysis
The time profiles of many gamma-ray bursts consist of distinct pulses, which
offers the possibility of characterizing the temporal structure of these bursts
using a relatively small set of pulse shape parameters. This pulse
decomposition analysis has previously been performed on a small sample of
bright long bursts using binned data from BATSE, which comes in several data
types, and on a sample of short bursts using the BATSE Time-Tagged Event (TTE)
data type. We have developed an interactive pulse-fitting program using the
phenomenological pulse model of Norris, et al. and a maximum-likelihood fitting
routine. We have used this program to analyze the Time-to-Spill (TTS) data for
all bursts observed by BATSE up through trigger number 2000, in all energy
channels for which TTS data is available. We present statistical information on
the attributes of pulses comprising these bursts, including relations between
pulse characteristics in different energy channels and the evolution of pulse
characteristics through the course of a burst. We carry out simulations to
determine the biases that our procedures may introduce. We find that pulses
tend to have shorter rise times than decay times, and tend to be narrower and
peak earlier at higher energies. We also find that pulse brightness, pulse
width, and pulse hardness ratios do not evolve monotonically within bursts, but
that the ratios of pulse rise times to decay times tend to decrease with time
within bursts.Comment: 40 pages, 19 figures. Submitted to Astrophysical Journal. PostScript
and PDF with un-bitmapped figures available at
http://www.slac.stanford.edu/pubs/slacpubs/8000/slac-pub-8364.html .
Accompanying paper astro-ph/0002218 available at
http://www.slac.stanford.edu/pubs/slacpubs/8000/slac-pub-8365.htm
Very metal-poor galaxies: ionized gas kinematics in nine objects
The study of ionized gas morphology and kinematics in nine eXtremely
Metal-Deficient (XMD) galaxies with the scanning Fabry-Perot interferometer on
the SAO 6-m telescope is presented. Some of these very rare objects (with
currently known range of O/H of 7.12 < 12+log(O/H) < 7.65, or Zo/35 < Z <
Zo/10) are believed to be the best proxies of `young' low-mass galaxies in the
high-redshift Universe. One of the main goals of this study is to look for
possible evidence of star formation (SF) activity induced by external
perturbations. Recent results from HI mapping of a small subsample of XMD
star-forming galaxies provided confident evidence for the important role of
interaction-induced SF. Our observations provide complementary or new
information that the great majority of the studied XMD dwarfs have strongly
disturbed gas morphology and kinematics or the presence of detached components.
We approximate the observed velocity fields by simple models of a rotating
tilted thin disc, which allow us the robust detection of non-circular gas
motions. These data, in turn, indicate the important role of current/recent
interactions and mergers in the observed enhanced star formation. As a
by-product of our observations, we obtained data for two LSB dwarf galaxies:
Anon J012544+075957 that is a companion of the merger system UGC 993, and SAO
0822+3545 which shows off-centre, asymmetric, low SFR star-forming regions,
likely induced by the interaction with the companion XMD dwarf HS 0822+3542.Comment: Accepted to MNRAS, 20 pages, 3 tables, 7 figure
The ACS Fornax Cluster Survey. VI. The Nuclei of Early-Type Galaxies in the Fornax Cluster
The Advanced Camera for Surveys (ACS) Fornax Cluster Survey is a Hubble Space
Telescope program to image 43 early-type galaxies in the Fornax cluster, using
the F475W and F850LP bandpasses of the ACS. We employ both 1D and 2D techniques
to characterize the properties of the stellar nuclei in these galaxies, defined
as the central "luminosity excesses" relative to a Sersic model fitted to the
underlying host. We find 72+/-13% of our sample (31 galaxies) to be nucleated,
with only three of the nuclei offset by more than 0.5" from their galaxy
photocenter, and with the majority of nuclei having colors bluer than their
hosts. The nuclei are observed to be larger, and brighter, than typical Fornax
globular clusters, and to follow different structural scaling relations. A
comparison of our results to those from the ACS Virgo Cluster Survey reveals
striking similarities in the properties of the nuclei belonging to these
different environments. We briefly review a variety of proposed formation
models and conclude that, for the low-mass galaxies in our sample, the most
important mechanism for nucleus growth is probably infall of star clusters
through dynamical friction, while for higher mass galaxies, gas accretion
triggered by mergers, accretions and tidal torques is likely to dominate, with
the relative importance of these two processes varying smoothly as a function
of galaxy mass. Some intermediate-mass galaxies in our sample show a complexity
in their inner structure that may be the signature of "hybrid nuclei" that
arose through parallel formation channels.Comment: 34 pages, 27 figures, accepted for publication in ApJ
The AGN Outflow in the HDFS Target QSO J2233-606 from a High-Resolution VLT/UVES Spectrum
We present a detailed analysis of the intrinsic UV absorption in the central
HDFS target QSO J2233-606, based on a high-resolution, high S/N (~25 -- 50)
spectrum obtained with VLT/UVES. This spectrum samples the cluster of intrinsic
absorption systems outflowing from the AGN at radial velocities v ~ -5000 --
-3800 km/s in the key far-UV diagnostic lines - the lithium-like CNO doublets
and H I Lyman series. We fit the absorption troughs using a global model of all
detected lines to solve for the independent velocity-dependent covering factors
of the continuum and emission-line sources and ionic column densities. This
reveals increasing covering factors in components with greater outflow
velocity. Narrow substructure is revealed in the optical depth profiles,
suggesting the relatively broad absorption is comprised of a series of multiple
components. We perform velocity-dependent photoionization modeling, which
allows a full solution to the C, N, and O abundances, as well as the velocity
resolved ionization parameter and total column density. The absorbers are found
to have supersolar abundances, with [C/H] and [O/H] ~0.5 -- 0.9, and [N/H] ~
1.1 -- 1.3, consistent with enhanced nitrogen production expected from
secondary nucleosynthesis processes. Independent fits to each kinematic
component give consistent results for the abundances. The lowest-ionization
material in each of the strong absorbers is modeled with similar ionization
parameters. Components of higher-ionization (indicated by stronger O VI
relative to C IV and N V) are present at velocities just redward of each
low-ionization absorber. We explore the implications of these results for the
kinematic-geometric-ionization structure of the outflow.Comment: 12 pages, 10 figures, emulateapj, accepted for publication in Ap
WFPC2 Observations of Compact Star Cluster Nuclei in Low Luminosity Spiral Galaxies
We have used the Wide Field Planetary Camera 2 aboard the Hubble Space
Telescope to image the compact star cluster nuclei of the nearby, late-type,
low-luminosity spiral galaxies NGC 4395, NGC 4242, and ESO 359-029. We also
analyze archival WFPC2 observations of the compact star cluster nucleus of M33.
A comparative analysis of the structural and photometric properties of these
four nuclei is presented. All of the nuclei are very compact, with luminosity
densities increasing at small radii to the resolution limit of our data. NGC
4395 contains a Seyfert 1 nucleus with a distinct bipolar structure and bright
associated filaments which are likely due to [OIII] emission. The M33 nucleus
has a complex structure, with elongated isophotes and possible signatures of
weak activity, including a jet-like component. The other two nuclei are not
known to be active, but share similar physical size scales and luminosities to
the M33 and NGC 4395 nuclei. The circumnuclear environments of all four of our
program galaxies are extremely diffuse, have only low-to-moderate star
formation, and appear to be devoid of large quantities of dust. The central
gravitational potentials of the galaxies are also quite shallow, making the
origin of these types of `naked' nuclei problematic.Comment: to appear in the July 1999 Astronomical Journal; 38 pages (Latex), 5
tables (postscript), 21 figures (gif); postscript versions of the figures may
be obtained via anonymous ftp at
ftp://ftp.cv.nrao.edu/NRAO-staff/lmatthew/lanl-nucle
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