1,996 research outputs found
Comparing Galaxies and Lyman Alpha Absorbers at Low Redshift
A scenario is explored in which Lyman alpha absorbers at low redshift arise
from lines of sight through extended galaxy disks, including those of dwarf and
low surface brightness galaxies. A population of galaxies is simulated based
upon observed distributions of galaxy properties, and the gas disks are modeled
using pressure and gravity confinement. Some parameter values are ruled out by
comparing simulation results with the observed galaxy luminosity function, and
constraints may be made on the absorbing cross sections of galaxies. Simulation
results indicate that it is difficult to match absorbers with particular
galaxies observationally since absorption typically occurs at high impact
parameters (>200 kpc) from luminous galaxies. Low impact parameter absorption
is dominated by low luminosity dwarfs. A large fraction of absorption lines is
found to originate from low surface brightness galaxies, so that the absorbing
galaxy is likely to be misidentified. Low redshift Lyman alpha absorber counts
can easily be explained by moderately extended galaxy disks when low surface
brightness galaxies are included, and it is easily possible to find a scenario
which is consistent with observed the galaxy luminosity function, with low
redshift Lyman limit absorber counts, and with standard nucleosynthesis
predictions of the baryon density, Omega_Baryon.Comment: 17 pages, 8 figures, accepted to the Astrophysical Journa
The Arecibo Legacy Fast ALFA Survey: VIII. HI Source Catalog of the Anti-Virgo Region at dec = +25 deg
We present a fourth catalog of HI sources from the Arecibo Legacy Fast ALFA
(ALFALFA) Survey. We report 541 detections over 136 deg2, within the region of
the sky having 22h < R.A. < 03h and 24 deg < Dec. < 26 deg . This complements a
previous catalog in the region 26 deg < Dec. < 28 deg (Saintonge et al. 2008).
We present here the detections falling into three classes: (a) extragalactic
sources with S/N > 6.5, where the reliability of the catalog is better than
95%; (b) extragalactic sources 5.0 < S/N < 6.5 and a previously measured
optical redshift that corroborates our detection; or (c) High Velocity Clouds
(HVCs), or subcomponents of such clouds, in the periphery of the Milky Way. Of
the 541 objects presented here, 90 are associated with High Velocity Clouds,
while the remaining 451 are identified as extragalactic objects. Optical
counterparts have been matched with all but one of the extragalactic objects.Comment: 26 pages, 5 figures, 1 table, accepted for publication in
Astrophysical Journal Supplement Serie
Massive, Absorption-selected Galaxies at Intermediate Redshifts
The nature of absorption-selected galaxies and their connection to the
general galaxy population have been open issues for more than three decades,
with little information available on their gas properties. Here we show, using
detections of carbon monoxide (CO) emission with the Atacama Large
Millimeter/submillimeter Array (ALMA), that five of seven high-metallicity,
absorption-selected galaxies at intermediate redshifts, ,
have large molecular gas masses, and high molecular gas fractions (. Their modest star
formation rates (SFRs), yr, then
imply long gas depletion timescales, Gyr. The
high-metallicity absorption-selected galaxies at appear
distinct from populations of star-forming galaxies at both ,
during the peak of star formation activity in the Universe, and lower
redshifts, . Their relatively low SFRs, despite the large
molecular gas reservoirs, may indicate a transition in the nature of star
formation at intermediate redshifts, .Comment: 8 pages, 3 figures; accepted for publication in Astrophysical Journal
Letters. Minor changes to match the version in press in ApJ
Mg II Absorber Number Density at z~0.05: Implications for Omega_DLA Evolution
An unbiased sample of 147 quasar/AGN spectra, obtained with the FOS/HST, has
been searched for intervening MgII absorbers over the redshift range 0<z<0.15.
The total redshift path searched is 18.8, with the survey being 80% complete to
a 5-sigma rest-frame equivalent width, W_r(2796), of 0.6 Ang. Main results of
this work are: [1] Four systems were found, with a mean redshift of =0.06,
yielding a redshift number density dN/dz=0.22(+0.12)(-0.09) for absorbers with
W_r(2796)>0.6 Ang. This is consistent with the value expected if these systems
do not evolve from higher redshifts (z=2.2). [2] No systems with W_r(2796)<0.6
Ang were found. It is a 2-sigma result to have a null detection of smaller
W_r(2796) systems. If this implies a turnover in the low W_r(2796) region of
the equivalent width distribution at z~0, then there is at least a 25%
reduction in the average galaxy gas cross section from z<0.2 galaxies. [3]
These systems have strong FeII absorption and are good candidates for damped
Ly-alpha absorbers DLAs (see Rao & Turnshek 2000, ApJS, 130, 1). This
translates to a redshift number density of dN/dz=0.08(+0.09)(-0.05) for DLAs at
z~0. In tandem with the data analyzed by Rao & Turnshek, these results indicate
that the redshift number density of DLAs does not evolve from z~4 to z~0. If
the HI mass function does not evolve from z~0.5 to z~0, then the cosmological
HI mass density is also deduced to not evolve from z~4 to z~0. These z~0
results for MgII absorption-selected DLAs are at odds with those based upon
21-cm emission from HI galaxies by a factor of five to six.Comment: 23 pages, 7 Figures, accepted to ApJ. Replaced version includes
additional figures and tables and substantial modifications to the tex
Modelling magnetic flux emergence in the solar convection zone
[Abridged] Bipolar magnetic regions are formed when loops of magnetic flux
emerge at the solar photosphere. Our aim is to investigate the flux emergence
process in a simulation of granular convection. In particular we aim to
determine the circumstances under which magnetic buoyancy enhances the flux
emergence rate (which is otherwise driven solely by the convective upflows). We
use three-dimensional numerical simulations, solving the equations of
compressible magnetohydrodynamics in a horizontally-periodic Cartesian domain.
A horizontal magnetic flux tube is inserted into fully developed hydrodynamic
convection. We systematically vary the initial field strength, the tube
thickness, the initial entropy distribution along the tube axis and the
magnetic Reynolds number. Focusing upon the low magnetic Prandtl number regime
(Pm<1) at moderate magnetic Reynolds number, we find that the flux tube is
always susceptible to convective disruption to some extent. However, stronger
flux tubes tend to maintain their structure more effectively than weaker ones.
Magnetic buoyancy does enhance the flux emergence rates in the strongest
initial field cases, and this enhancement becomes more pronounced when we
increase the width of the flux tube. This is also the case at higher magnetic
Reynolds numbers, although the flux emergence rates are generally lower in
these less dissipative simulations because the convective disruption of the
flux tube is much more effective in these cases. These simulations seem to be
relatively insensitive to the precise choice of initial conditions: for a given
flow, the evolution of the flux tube is determined primarily by the initial
magnetic field distribution and the magnetic Reynolds number.Comment: 12 pages, 15 figures, 2 tables. Accepted for publication in Astronomy
and Astrophysic
'Brussel' op afstand? Een verkenning naar mogelijkheden voor maatschappelijke diensten en ruimte voor zelfsturing bij agrarisch natuurbeheer in de Europese context
De landbouw speelt een belangrijke rol bij het in stand houden en ontwikkelen van een aantrekkelijk cultuurlandschap en de biodiversiteit in de Noordelijke Friese Wouden. Het behoud van deze waarden steunt op de inzet en de vele activiteiten van de Vereniging Noordelijke Friese Wouden, op de status van Nationaal Landschap en op een aantal subsidieregelingen vanuit de overheid. Voor het aanbrengen en beheren van landschapselementen kunnen agrariërs een vergoeding van de overheid krijgen. Dit project richt zich specifiek op zelfsturingarrangementen die gericht zijn op agrarisch natuur- en landschapsbeheer. Het verkent de mogelijkheden voor de Vereniging Noordelijke Friese Wouden om meer invloed te krijgen op en verantwoordelijkheid te nemen voor het verdelen, beheren en het controleren van de besteding van de overheidsgelden voor agrarisch natuur- en landschapsbeheer. Op het huidige moment zijn deze kaders onderhevig aan discussie binnen de Europese Unie. De hervorming van het Gemeenschappelijk Landbouwbeleid (GLB) - van waaruit een deel van deze vergoedingen betaald worden - en de vereenvoudiging van het GLB staan op de politieke agenda en discussies hierover zijn in volle gang
A Structural and Dynamical Study of Late-Type, Edge-On Galaxies: I. Sample Selection and Imaging Data
We present optical (B & R) and infrared (K_s) images and photometry for a
sample of 49 extremely late-type, edge-on disk galaxies selected from the Flat
Galaxy Catalog of Karenchentsev et al. (1993). Our sample was selected to
include galaxies with particularly large axial ratios, increading the
likelihood that the galaxies in the sample are truly edge-on. We have also
concentrated the sample on galaxies with low apparent surface brightness, in
order to increase the representation of intrinisically low surface brightness
galaxies. Finally, the sample was chosen to have no apprarent bulges or optical
warps so that the galaxies represent undisturbed, ``pure disk'' systems. The
resulting sample forms the basis for a much larger spectroscopic study designed
to place constraints on the physical quantities and processes which shape disk
galaxies. The imaging data presented in this paper has been painstakingly
reduced and calibrated to allow accurate surface photometry of features as
faint as 30 mag/sqr-arcsec in B and 29 mag/sqr-arcsec in R on scales larger
than 10 arcsec. Due to limitations in sky subtraction and flat fielding, the
infrared data can reach only to 22.5 mag/sqr-arcsec in K_s on comparable
scales. As part of this work, we have developed a new method for quantifying
the reliability of surface photometry, which provides useful diagnostics for
the presence of scattered light, optical emission from infrared cirrus, and
other sources of non-uniform sky backgrounds.Comment: scheduled to appear in the Astronomical Journal, LaTeX, 36 pages
including 7 pages of figures (fig 1-2,4). A low resolution version of Figure
3 is included in JPEG format; contours are seriously degraded. A full
resolution Postscript version of Figure 3 (10.6Mb,gzipped) is available
through anonymous ftp at
ftp://ftp.astro.washington.edu/pub/users/jd/FGC/dalcanton.f3.ps.g
Evolution of the cosmological mass density of neutral gas from Sloan Digital Sky Survey II - Data Release 7
We present the results of a search for damped Lyman-alpha (DLA) systems in
the Sloan Digital Sky Survey II (SDSS), Data Release 7. We use a fully
automatic procedure to identify DLAs and derive their column densities. The
procedure is checked against the results of previous searches for DLAs in SDSS.
We discuss the agreements and differences and show the robustness of our
procedure. For each system, we obtain an accurate measurement of the absorber's
redshift, the HI column density and the equivalent width of associated metal
absorption lines, without any human intervention. We find 1426 absorbers with
2.15 =20, out of which 937 systems have log N(HI)>=
20.3. This is the largest DLA sample ever built, made available to the
scientific community through the electronic version of this paper. In the
course of the survey, we discovered the intervening DLA with highest HI column
density known to date with log N(HI)=22.0+/-0.1. This single system provides a
strong constraint on the high-end of the N(HI) frequency distribution now
measured with high accuracy. We show that the presence of a DLA at the blue end
of a QSO spectrum can lead to important systematic errors and propose a method
to avoid them. This has important consequences for the measurement of the
cosmological mass density of neutral gas at z~2.2 and therefore on our
understanding of galaxy evolution over the past 10 billion years. [truncated]Comment: Accepted for publication in A&
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