916 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
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
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
The Connections between QSO Absorption Systems and Galaxies: Low-Redshift Observations
Quasar absorption lines have long been recognized to be a sensitive probe of
the abundances, physical conditions, and kinematics of gas in a wide variety of
environments including low-density intergalactic regions that probably cannot
be studied by any other means. While some pre-Hubble Space Telescope (HST)
observations indicated that Mg II absorption lines arise in gaseous galactic
halos with a large covering factor, many early QSO absorber studies were
hampered by a lack of information about the context of the absorbers and their
connections with galaxies. By providing access to crucial ultraviolet resonance
lines at low redshifts, deployment of HST and the Far Ultraviolet Spectroscopic
Explorer enabled detailed studies of the relationships between QSO absorbers
and galaxies. The advent of large surveys such as the Sloan Digital Sky Survey
(SDSS) has also advanced the topic by greatly improving the size of absorber
and galaxy samples. This paper briefly reviews some observational results on
absorber-galaxy connections that have been obtained in the HST/SDSS era,
including Mg II absorbers, the low-z Lyman alpha forest, Lyman limit and damped
Lyman alpha absorbers, and O VI systems.Comment: Review paper presented at IAU Colloquium 199, Probing Galaxies
through Quasar Absorption Lines, eds. P. R. Williams, C. Shu, and B. Menard.
19 pages, 10 figure
The impact of skull bone intensity on the quality of compressed CT neuro images
International audienceThe increasing use of technologies such as CT and MRI, along with a continuing improvement in their resolution, has contributed to the explosive growth of digital image data being generated. Medical communities around the world have recognized the need for efficient storage, transmission and display of medical images. For example, the Canadian Association of Radiologists (CAR) has recommended compression ratios for various modalities and anatomical regions to be employed by lossy JPEG and JPEG2000 compression in order to preserve diagnostic quality. Here we investigate the effects of the sharp skull edges present in CT neuro images on JPEG and JPEG2000 lossy compression. We conjecture that this atypical effect is caused by the sharp edges between the skull bone and the background regions as well as between the skull bone and the interior regions. These strong edges create large wavelet coefficients that consume an unnecessarily large number of bits in JPEG2000 compression because of its bitplane coding scheme, and thus result in reduced quality at the interior region, which contains most diagnostic information in the image. To validate the conjecture, we investigate a segmentation based compression algorithm based on simple thresholding and morphological operators. As expected, quality is improved in terms of PSNR as well as the structural similarity (SSIM) image quality measure, and its multiscale (MS-SSIM) and informationweighted (IW-SSIM) versions. This study not only supports our conjecture, but also provides a solution to improve the performance of JPEG and JPEG2000 compression for specific types of CT images
Tomographic Magnification of Lyman Break Galaxies in The Deep Lens Survey
Using about 450,000 galaxies in the Deep Lens Survey, we present a detection
of the gravitational magnification of z > 4 Lyman Break Galaxies by massive
foreground galaxies with 0.4 < z < 1.0, grouped by redshift. The magnification
signal is detected at S/N greater than 20, and rigorous checks confirm that it
is not contaminated by any galaxy sample overlap in redshift. The inferred
galaxy mass profiles are consistent with earlier lensing analyses at lower
redshift. We then explore the tomographic lens magnification signal by
splitting our foreground galaxy sample into 7 redshift bins. Combining
galaxy-magnification cross-correlations and galaxy angular auto-correlations,
we develop a bias-independent estimator of the tomographic signal. As a
diagnostic of magnification tomography, the measurement of this estimator
rejects a flat dark matter dominated Universe at > 7.5{\sigma} with a fixed
\sigma_8 and is found to be consistent with the expected redshift-dependence of
the WMAP7 {\Lambda}CDM cosmology.Comment: 12 pages, 9 figures, Accepted to MNRA
Evolution from protoplanetary to debris discs: The transition disc around HD 166191
HD 166191 has been identified by several studies as hosting a rare and
extremely bright warm debris disc with an additional outer cool disc component.
However, an alternative interpretation is that the star hosts a disc that is
currently in transition between a full gas disc and a largely gas-free debris
disc. With the help of new optical to mid-IR spectra and Herschel imaging, we
argue that the latter interpretation is supported in several ways: i) we show
that HD 166191 is co-moving with the ~4 Myr-old Herbig Ae star HD 163296,
suggesting that the two have the same age, ii) the disc spectrum of HD 166191
is well matched by a standard radiative transfer model of a gaseous
protoplanetary disc with an inner hole, and iii) the HD 166191 mid-IR silicate
feature is more consistent with similarly primordial objects. We note some
potential issues with the debris disc interpretation that should be considered
for such extreme objects, whose lifetime at the current brightness is mush
shorter than the stellar age, or in the case of the outer component requires a
mass comparable to the solid component of the Solar nebula. These aspects
individually and collectively argue that HD 166191 is a 4-5 Myr old star that
hosts a gaseous transition disc. Though it does not argue in favour of either
scenario, we find strong evidence for 3-5 um disc variability. We place HD
166191 in context with discs at different evolutionary stages, showing that it
is a potentially important object for understanding the protoplanetary to
debris disc transition.Comment: accepted to MNRAS, fixed typos in abstract and axis labe
Protoplanet Magnetosphere Interactions
In this paper, we study a simple model of an orbiting protoplanet in a
central magnetospheric cavity, the entry into such a cavity having been
proposed as a mechanism for halting inward orbital migration. We have
calculated the gravitational interaction of the protoplanet with the
magnetosphere using a local model and determined the rate of evolution of the
orbit. The interaction is found to be determined by the outward flux of MHD
waves and thus the possibility of the existence of such waves in the cavity is
significant. The estimated orbital evolution rates due to gravitational and
other interactions with the magnetosphere are unlikely to be significant during
protoplanetary disk lifetimes.Comment: Accepted for publication in A&
DZ Cha: a bona fide photoevaporating disc
DZ Cha is a weak-lined T Tauri star (WTTS) surrounded by a bright
protoplanetary disc with evidence of inner disc clearing. Its narrow \Ha line
and infrared spectral energy distribution suggest that DZ Cha may be a
photoevaporating disc. We aim to analyse the DZ Cha star + disc system to
identify the mechanism driving the evolution of this object. We have analysed
three epochs of high resolution optical spectroscopy, photometry from the UV up
to the sub-mm regime, infrared spectroscopy, and J-band imaging polarimetry
observations of DZ Cha. Combining our analysis with previous studies we find no
signatures of accretion in the \Ha line profile in nine epochs covering a
time baseline of years. The optical spectra are dominated by
chromospheric emission lines, but they also show emission from the forbidden
lines [SII] 4068 and [OI] 6300 that indicate a disc outflow. The
polarized images reveal a dust depleted cavity of au in radius and two
spiral-like features, and we derive a disc dust mass limit of
M_\mathrm{dust}
80 \MJup) companions are detected down to 0\farcs07 ( au,
projected). The negligible accretion rate, small cavity, and forbidden line
emission strongly suggests that DZ Cha is currently at the initial stages of
disc clearing by photoevaporation. At this point the inner disc has drained and
the inner wall of the truncated outer disc is directly exposed to the stellar
radiation. We argue that other mechanisms like planet formation or binarity
cannot explain the observed properties of DZ Cha. The scarcity of objects like
this one is in line with the dispersal timescale ( yr) predicted
by this theory. DZ Cha is therefore an ideal target to study the initial stages
of photoevaporation.Comment: A&A in press, language corrections include
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