537 research outputs found
HyRec: A fast and highly accurate primordial hydrogen and helium recombination code
We present a state-of-the-art primordial recombination code, HyRec, including
all the physical effects that have been shown to significantly affect
recombination. The computation of helium recombination includes simple analytic
treatments of hydrogen continuum opacity in the He I 2 1P - 1 1S line, the He
I] 2 3P - 1 1S line, and treats feedback between these lines within the
on-the-spot approximation. Hydrogen recombination is computed using the
effective multilevel atom method, virtually accounting for an infinite number
of excited states. We account for two-photon transitions from 2s and higher
levels as well as frequency diffusion in Lyman-alpha with a full radiative
transfer calculation. We present a new method to evolve the radiation field
simultaneously with the level populations and the free electron fraction. These
computations are sped up by taking advantage of the particular sparseness
pattern of the equations describing the radiative transfer. The computation
time for a full recombination history is ~2 seconds. This makes our code well
suited for inclusion in Monte Carlo Markov chains for cosmological parameter
estimation from upcoming high-precision cosmic microwave background anisotropy
measurements.Comment: Version accepted by PRD. Numerical integration switches adapted to be
well behaved for a wide range of cosmologies (Sec. V E). HyRec is available
at http://www.tapir.caltech.edu/~yacine/hyrec/hyrec.htm
Environment assisted electron capture
Electron capture by {\it isolated} atoms and ions proceeds by
photorecombination. In this process a species captures a free electron by
emitting a photon which carries away the excess energy. It is shown here that
in the presence of an {\it environment} a competing non-radiative electron
capture process can take place due to long range electron correlation. In this
interatomic (intermolecular) process the excess energy is transferred to
neighboring species. The asymptotic expression for the cross section of this
process is derived. We demonstrate by explicit examples that under realizable
conditions the cross section of this interatomic process can clearly dominate
that of photorecombination
HI ``Tails'' from Cometary Globules in IC1396
IC 1396 is a relatively nearby (750 pc), large (>2 deg), HII region ionized
by a single O6.5V star and containing bright-rimmed cometary globules. We have
made the first arcmin resolution images of atomic hydrogen toward IC 1396, and
have found remarkable ``tail''-like structures associated with some of the
globules and extending up to 6.5 pc radially away from the central ionizing
star. These HI ``tails'' may be material which has been ablated from the
globule through ionization and/or photodissociation and then accelerated away
from the globule by the stellar wind, but which has since drifted into the
``shadow'' of the globules.
This report presents the first results of the Galactic Plane Survey Project
recently begun by the Dominion Radio Astrophysical Observatory.Comment: 11 pages, 5 postscript figures, uses aaspp4.sty macros, submitted in
uuencoded gzipped tar format, accepted for publication in Astrophysical
Journal Letters, colour figures available at
http://www.drao.nrc.ca/~schieven/news_sep95/ic1396.htm
Intrinsic Absorption Lines in Seyfert 1 Galaxies. I. Ultraviolet Spectra from the Hubble Space Telescope
We present a study of the intrinsic absorption lines in the ultraviolet
spectra of Seyfert 1 galaxies. We find that the fraction of Seyfert 1 galaxies
that show absorption associated with their active nuclei is more than one-half
(10/17), which is much higher than previous estimates (3 - 10%) . There is a
one-to-one correspondence between Seyferts that show intrinsic UV absorption
and X-ray ``warm absorbers''. The intrinsic UV absorption is generally
characterized by high ionization: C IV and N V are seen in all 10 Seyferts with
detected absorption (in addition to Ly-alpha), whereas Si IV is present in only
four of these Seyferts, and Mg II absorption is only detected in NGC 4151. The
absorption lines are blueshifted (or in a few cases at rest) with respect to
the narrow emission lines, indicating that the absorbing gas is undergoing net
radial outflow. At high resolution, the absorption often splits into distinct
kinematic components that show a wide range in widths (20 - 400 km/s FWHM),
indicating macroscopic motions (e.g., radial velocity subcomponents or
turbulence) within a component. The strong absorption components have cores
that are much deeper than the continuum flux levels, indicating that the
regions responsible for these components lie completely outside of the broad
emission-line regions. The covering factor of the absorbing gas in the line of
sight, relative to the total underlying emission, is C > 0.86, on average. The
global covering factor, which is the fraction of emission intercepted by the
absorber averaged over all lines of sight, is C > 0.5.Comment: 56 pages, Latex, includes 4 figures (encapsulated postscript), Fig. 1
has 2 parts and Fig. 2 has 3 parts, to appear in the Astrophysical Journa
Sequential and Spontaneous Star Formation Around the Mid-Infrared Halo HII Region KR 140
We use 2MASS and MSX infrared observations, along with new molecular line
(CO) observations, to examine the distribution of young stellar objects (YSOs)
in the molecular cloud surrounding the halo HII region KR 140 in order to
determine if the ongoing star-formation activity in this region is dominated by
sequential star formation within the photodissociation region (PDR) surrounding
the HII region. We find that KR 140 has an extensive population of YSOs that
have spontaneously formed due to processes not related to the expansion of the
HII region. Much of the YSO population in the molecular cloud is concentrated
along a dense filamentary molecular structure, traced by C18O, that has not
been erased by the formation of the exciting O star. Some of the previously
observed submillimetre clumps surrounding the HII region are shown to be sites
of recent intermediate and low-mass star formation while other massive starless
clumps clearly associated with the PDR may be the next sites of sequential star
formation.Comment: Accepted for publication in MNRAS, 8 pages, 10 figure
Cosmic Reionisation by Stellar Sources: Population II Stars
We study the reionisation of the Universe by stellar sources using a
numerical approach that combines fast 3D radiative transfer calculations with
high resolution hydrodynamical simulations. Ionising fluxes for the sources are
derived from intrinsic star formation rates computed in the underlying
hydrodynamical simulations. Our mass resolution limit for sources is M~ 4.0 x
10^7 h^-1 M_sol, which is roughly an order of magnitude smaller than in
previous studies of this kind. Our calculations reveal that the reionisation
process is sensitive to the inclusion of dim sources with masses below ~10^9
h^-1 M_sol. We present the results of our reionisation simulation assuming a
range of escape fractions for ionising photons and make statistical comparisons
with observational constraints on the neutral fraction of hydrogen at z~6
derived from the z=6.28 SDSS quasar of Becker and coworkers. Our best fitting
model has an escape fraction of ~20% and causes reionisation to occur by z~8,
although the IGM remains fairly opaque until z~6. In order to simultaneously
match the observations from the z=6.28 SDSS quasar and the optical depth
measurement from WMAP with the sources modeled here, we require an evolving
escape fraction that rises from f_esc=0.20 near z~6 to f_esc>~10 at z~18.Comment: 42 pages, 13 figure
Near-IR spectroscopy of PKS1549-79: a proto-quasar revealed?
We present a near-IR spectrum of the nearby radio galaxy PKS1549-79 (z=0153).
These data were taken with the aim of testing the idea that this object
contains a quasar nucleus that is moderately extinguished, despite evidence
that its radio jet points close to our line-of-sight. We detect broad Paschen
Alpha emission (FWHM ~1745 km/s), relatively bright continuum emission, and a
continuum slope consistent with a reddened quasar spectrum (3.1 < Av < 7.3),
all emitted by an unresolved point source. Therefore we conclude that we have,
indeed, detected a hidden quasar nucleus in PKS1549-79. Combined with previous
results, these observations are consistent with the idea that PKS1549-79 is a
young radio source in which the cocoon of debric left over from the triggering
events has not yet been swept aside by circumnuclear outflows.Comment: 6 pages, 4 figures, accepted for publication in MNRA
Polarization and kinematics in Cygnus A
From optical spectropolarimetry of Cygnus A we conclude that the scattering
medium in the ionization cones in Cygnus A is moving outward at a speed of
170+-34 km/s, and that the required momentum can be supplied by the radiation
pressure of an average quasar. Such a process could produce a structure
resembling the observed ionization cones, which are thought to result from
shadowing by a circumnuclear dust torus. We detect a polarized red wing in the
[O III] emission lines arising from the central kiloparsec of Cygnus A. This
wing is consistent with line emission created close to the boundary of the
broad-line region.Comment: 5 pages, accepted for publication in MNRAS letter
Spatially resolved spectroscopy of Coma cluster early-type galaxies - II:the minor axis dataset
We present minor axis, off set major axis and one diagonal long slit spectra for 10 E and S0 galaxies of the Coma cluster drawn from a magnitude-limited sample studied before. We derive rotation curves, velocity dispersion profiles and the H-3 and H-4 coefficients of the Hermite decomposition of the line of sight velocity distribution. Moreover, we derive the line index profiles of Mg, Fe and Hbeta line indices and assess their errors. The data will be used to construct dynamical models of the galaxies and study their stellar populations
Radio Properties of Low Redshift Broad Line Active Galactic Nuclei
The question as to whether the distribution of radio-loudness in active
galactic nuclei (AGN) is actually bimodal has been discussed extensively in the
literature. Futhermore, there have been claims that radio-loudness depends on
black hole mass and Eddington ratio. We investigate these claims using the low
redshift broad line AGN sample of Greene & Ho (2007), which consists of 8434
objects at z < 0.35 from the Sloan Digital Sky Survey Fourth Data Release (SDSS
DR4). We obtained radio fluxes from the Very Large Array Faint Images of the
Radio Sky at Twenty-Centimeters (FIRST) survey for the SDSS AGN. Out of the
8434 SDSS AGN, 821 have corresponding observed radio fluxes in the FIRST
survey. We calculated the radio-loudness parameter (R) for all objects above
the FIRST detection limit (1 mJy), and an upper limit to R for the undetected
objects. Using these data, the question of radio bimodality is investigated for
different subsets of the total sample. We find no clear demarcation between the
radio-loud (RL, R > 10) and radio-quiet (RQ, R < 10) objects, but instead fill
in a more radio-intermediate population in a continuous fashion for all
subsamples. We find that 4.7% of the AGN in the flux-limited subsample are RL
based on core radio emission alone. We calculate the radio-loud fraction (RLF)
as both a function of black hole mass and Eddington ratio. The RLF decreases
(from 13% to 2%) as Eddington ratio increases over 2.5 order of magnitude. The
RLF is nearly constant (~5%) over 4 decades in black hole mass, except for an
increase at masses greater than 10^8 solar masses. We find for the FIRST
detected subsample that 367 of the RL AGN have black hole masses less than 10^8
solar masses, a large enough number to indicate that RL AGN are not a product
of only the most massive black holes in the local universe.Comment: 28 pages, 14 figures, accepted to A
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