1,219 research outputs found
Absorption Systems In Radio-Selected QSO Surveys
Radio-selected samples of quasars with complete optical identifications offer
an ideal dataset with which to investigate dust bias associated with
intervening absorption systems. Here, we review our work on the Complete
Optical and Radio Absorption Line System (CORALS) survey whose aim is to
quantify this bias and assess the impact of dust on absorber statistics. First,
we review previously published results on the number density and gas content of
high column density absorbers over the redshift range 0.6 < z < 3.5. We then
present the latest results from CORALS which focus on measuring the metal
content of our unbiased absorber sample and an investigation of their
optical--IR colours. Overall we find that although dust is unarguably present
in absorption galaxies, the level appears to be low enough that the statistics
of previous magnitude limited samples have not been severely affected and that
the subsequent reddening of background QSOs is small.Comment: Proceedings of IAUC199, Probing Galaxies through Quasar Absorption
Lines, P. R. Williams, C. Shu, and B. Menard, ed
Clustering-based Redshift Estimation: Comparison to Spectroscopic Redshifts
We investigate the potential and accuracy of clustering-based redshift
estimation using the method proposed by M\'enard et al. (2013). This technique
enables the inference of redshift distributions from measurements of the
spatial clustering of arbitrary sources, using a set of reference objects for
which redshifts are known. We apply it to a sample of spectroscopic galaxies
from the Sloan Digital Sky Survey and show that, after carefully controlling
the sampling efficiency over the sky, we can estimate redshift distributions
with high accuracy. Probing the full colour space of the SDSS galaxies, we show
that we can recover the corresponding mean redshifts with an accuracy ranging
from z=0.001 to 0.01. We indicate that this mapping can be used to
infer the redshift probability distribution of a single galaxy. We show how the
lack of information on the galaxy bias limits the accuracy of the inference and
show comparisons between clustering redshifts and photometric redshifts for
this dataset. This analysis demonstrates, using real data, that
clustering-based redshift inference provides a powerful data-driven technique
to explore the redshift distribution of arbitrary datasets, without any prior
knowledge on the spectral energy distribution of the sources.Comment: 13 pages. Submitted to MNRAS. Comments welcom
Supernova cosmology: legacy and future
The discovery of dark energy by the first generation of high-redshift
supernova surveys has generated enormous interest beyond cosmology and has
dramatic implications for fundamental physics. Distance measurements using
supernova explosions are the most direct probes of the expansion history of the
Universe, making them extremely useful tools to study the cosmic fabric and the
properties of gravity at the largest scales. The past decade has seen the
confirmation of the original results. Type Ia supernovae are among the leading
techniques to obtain high-precision measurements of the dark energy equation of
state parameter, and in the near future, its time dependence. The success of
these efforts depends on our ability to understand a large number of effects,
mostly of astrophysical nature, influencing the observed flux at Earth. The
frontier now lies in understanding if the observed phenomenon is due to vacuum
energy, albeit its unnatural density, or some exotic new physics. Future
surveys will address the systematic effects with improved calibration
procedures and provide thousands of supernovae for detailed studies.Comment: Invited review, Annual Review of Nuclear and Particle Science
(submitted version
Galaxy Morphology - Halo Gas Connections
We studied a sample of 38 intermediate redshift MgII absorption-selected
galaxies using (1) Keck/HIRES and VLT/UVES quasar spectra to measure the halo
gas kinematics from MgII absorption profiles and (2) HST/WFPC-2 images to study
the absorbing galaxy morphologies. We have searched for correlations between
quantified gas absorption properties, and host galaxy impact parameters,
inclinations, position angles, and quantified morphological parameters. We
report a 3.2-sigma correlation between asymmetric perturbations in the host
galaxy morphology and the MgII absorption equivalent width. We suggest that
this correlation may indicate a connection between past merging and/or
interaction events in MgII absorption-selected galaxies and the velocity
dispersion and quantity of gas surrounding these galaxies.Comment: 6 pages; 3 figures; contributed talk for IAU 199: Probing Galaxies
through Quasar Absorption Line
Modelled sensitivity of the snow regime to topography, shrub fraction and shrub height
Recent studies show that shrubs are colonizing higher latitudes and altitudes
in the Arctic. Shrubs affect the wind transport, accumulation and melt of
snow, but there have been few sensitivity studies of how shrub expansion
might affect snowmelt rates and timing. Here, a three-source energy balance model
(3SOM), which calculates vertical and horizontal energy fluxes – thus
allowing within-cell advection – between the atmosphere, snow, snow-free
ground and vegetation, is introduced. The three-source structure was specifically
adopted to investigate shrub–tundra processes associated with patchy
snow cover that single- or two-source models fail to address. The ability of
the model to simulate the snow regime of an upland tundra valley is
evaluated; a blowing snow transport and sublimation model is used to simulate
premelt snow distributions and 3SOM is used to simulate melt. Some success at
simulating turbulent fluxes in point simulations and broad spatial pattern in
distributed runs is shown even if the lack of advection between cells causes
melt rates to be underestimated. The models are then used to investigate the
sensitivity of the snow regime in the valley to varying shrub cover and
topography. Results show that, for domain average shrub fractional cover
≤0.4, topography dominates the pre- and early melt energy budget but
has little influence for higher shrub cover. The increase in domain average
sensible heat fluxes and net radiation with increasing shrub cover is more
marked without topography where shrubs introduce wind-induced spatial
variability of snow and snow-free patches. As snowmelt evolves, differences
in the energy budget between simulations with and without topography remain
relatively constant and are independent of shrub cover. These results suggest
that, to avoid overestimating the effect of shrub expansion on the energy
budget of the Arctic, future large-scale investigations should consider wind
redistribution of snow, shrub bending and emergence, and sub-grid topography
as they affect the variability of snow cover
Direct Observation of the Fourth Star in the Zeta Cancri System
Direct imaging of the zeta Cnc system has resolved the fourth star in the
system, which is in orbit around zeta Cnc C. The presence of the fourth star
has been inferred for many years from irregularities in the motion of star C,
and recently from C's spectroscopic orbit. However, its mass is close to that
of C, making its non-detection puzzling. Observing at wavelengths of 1.2, 1.7,
and 2.2 microns with the adaptive-optics system of the CFHT, we have obtained
images which very clearly reveal star D and show it to have the color of an M2
star. Its brightness is consonant with its being two M stars, which are not
resolved in our observations but are likely to be in a short-period orbit,
thereby accounting for the large mass and the difficulty of detection at
optical wavelengths, where the magnitude difference is much larger. The
positions and colors of all four stars in the system are reported and are
consistent with the most recent astrometric observations.Comment: 7 pages including 3 tables, 1 figure; To appear in PAS
Atomic line radiative transfer with MCFOST I. Code description and benchmarking
Aims. We present MCFOST-art, a new non-local thermodynamic equilibrium
radiative transfer solver for multilevel atomic systems. The code is embedded
in the 3D radiative transfer code MCFOST and is compatible with most of the
MCFOST modules. The code is versatile and designed to model the close
environment of stars in 3D. Methods. The code solves for the statistical
equilibrium and radiative transfer equations using the Multilevel Accelerated
Lambda Iteration (MALI) method. We tested MCFOST-art on spherically symmetric
models of stellar photospheres as well as on a standard model of the solar
atmosphere. We computed atomic level populations and outgoing fluxes and
compared these values with the results of the TURBOspectrum and RH codes.
Calculations including expansion and rotation of the atmosphere were also
performed. We tested both the pure local thermodynamic equilibrium and the
out-of-equilibrium problems. Results. In all cases, the results from all codes
agree within a few percent at all wavelengths and reach the sub-percent level
between RH and MCFOST-art. We still note a few marginal discrepancies between
MCFOST-art and TURBOspectrum as a result of different treatments of background
opacities at some critical wavelength ranges
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