18,473 research outputs found
Flux-ratio anomalies from discs and other baryonic structures in the Illustris simulation
The flux ratios in the multiple images of gravitationally lensed quasars can
provide evidence for dark matter substructure in the halo of the lensing galaxy
if the flux ratios differ from those predicted by a smooth model of the lensing
galaxy mass distribution. However, it is also possible that baryonic structures
in the lensing galaxy, such as edge-on discs, can produce flux-ratio anomalies.
In this work, we present the first statistical analysis of flux-ratio anomalies
due to baryons from a numerical simulation perspective. We select galaxies with
various morphological types in the Illustris simulation and ray-trace through
the simulated halos, which include baryons in the main lensing galaxies but
exclude any substructures, in order to explore the pure baryonic effects. Our
ray-tracing results show that the baryonic components can be a major
contribution to the flux-ratio anomalies in lensed quasars and that edge-on
disc lenses induce the strongest anomalies. We find that the baryonic
components increase the probability of finding high flux-ratio anomalies in the
early-type lenses by about 8% and by about 10 - 20% in the disc lenses. The
baryonic effects also induce astrometric anomalies in 13% of the mock lenses.
Our results indicate that the morphology of the lens galaxy becomes important
in the analysis of flux-ratio anomalies when considering the effect of baryons,
and that the presence of baryons may also partially explain the discrepancy
between the observed (high) anomaly frequency and what is expected due to the
presence of subhalos as predicted by the CDM simulations.Comment: 16 pages, 11 figures, accepted by MNRA
Modelling the Molecular Gas in NGC 6240
We present the first observations of HCN, HCO
and SiO in NGC\,6240, obtained with the IRAM PdBI. Combining a Markov
Chain Monte Carlo (MCMC) code with Large Velocity Gradient (LVG) modelling, and
with additional data from the literature, we simultaneously fit three gas
phases and six molecular species to constrain the physical condition of the
molecular gas, including massluminosity conversion factors. We find
of dense molecular gas in cold, dense clouds (\,K, \,cm) with a volume filling factor
, embedded in a shock heated molecular medium (\,K,
\,cm), both surrounded by an extended diffuse
phase (\,K, \,cm). We
derive a global with gas masses
, dominated by the
dense gas. We also find , which traces the
cold, dense gas. The [C]/[C] ratio is only slightly elevated
(), contrary to the very high [CO]/[CO] ratio (300-500)
reported in the literature. However, we find very high [HCN]/[HCN] and
[HCO]/[HCO] abundance ratios which we
attribute to isotope fractionation in the cold, dense clouds.Comment: 27 pages, 17 figures, 9 tables. Accepted in Ap
Towards the first detection of strongly lensed HI emission
We report interferometric observations tuned to the redshifted neutral
hydrogen (HI) 21cm emission line in three strongly lensed galaxies at with the Giant Metrewave Radio Telescope (GMRT). One galaxy spectrum
(J1106+5228 at z=0.407) shows evidence of a marginal detection with an
integrated signal-to-noise ratio of 3.8, which, if confirmed by follow-up
observations, would represent the first strongly lensed and most distant
individual galaxy detected in HI emission. Two steps are performed to
transcribe the lensed integrated flux measurements into HI mass measurements
for all three target galaxies. First, we calculate the HI magnification factor
by applying general relativistic ray-tracing to a physical model of the
source-lens system. The HI magnification generally differs from the optical
magnification and depends largely on the intrinsic HI mass due to
the HI mass-size relation. Second, we employ a Bayesian formalism to convert
the integrated flux, amplified by the -dependent magnification
factor , into a probability density for , accounting for the
asymmetric uncertainty due to the declining HI mass function (Eddington bias).
In this way, we determine a value of for J1106+5228, consistent with the estimate of
from the optical properties of this galaxy. The HI mass of the other two
sources are consistent with zero within a 95 per cent confidence interval
however we still provide upper limits for both sources and a lower
limit for J1250-0135 using the same formalism.Comment: Accepted by MNRAS, no changes made. 12 pages, 7 figure
Dark matter voids in the SDSS galaxy survey
What do we know about voids in the dark matter distribution given the Sloan
Digital Sky Survey (SDSS) and assuming the model? Recent
application of the Bayesian inference algorithm BORG to the SDSS Data Release 7
main galaxy sample has generated detailed Eulerian and Lagrangian
representations of the large-scale structure as well as the possibility to
accurately quantify corresponding uncertainties. Building upon these results,
we present constrained catalogs of voids in the Sloan volume, aiming at a
physical representation of dark matter underdensities and at the alleviation of
the problems due to sparsity and biasing on galaxy void catalogs. To do so, we
generate data-constrained reconstructions of the presently observed large-scale
structure using a fully non-linear gravitational model. We then find and
analyze void candidates using the VIDE toolkit. Our methodology therefore
predicts the properties of voids based on fusing prior information from
simulations and data constraints. For usual void statistics (number function,
ellipticity distribution and radial density profile), all the results obtained
are in agreement with dark matter simulations. Our dark matter void candidates
probe a deeper void hierarchy than voids directly based on the observed
galaxies alone. The use of our catalogs therefore opens the way to
high-precision void cosmology at the level of the dark matter field. We will
make the void catalogs used in this work available at
http://www.cosmicvoids.net.Comment: 15 pages, 6 figures, matches JCAP published version, void catalogs
publicly available at http://www.cosmicvoids.ne
Star Formation at z=2.481 in the Lensed Galaxy SDSS J1110+6459, I: Lens Modeling and Source Reconstruction
Using the combined resolving power of the Hubble Space Telescope and
gravitational lensing, we resolve star-forming structures in a z~2.5 galaxy on
scales much smaller than the usual kiloparsec diffraction limit of HST. SGAS
J111020.0+645950.8 is a clumpy, star forming galaxy lensed by the galaxy
cluster SDSS J1110+6459 at z = 0.659, with a total magnification ~30x across
the entire arc. We use a hybrid parametric/non-parametric strong lensing mass
model to compute the deflection and magnification of this giant arc,
reconstruct the light distribution of the lensed galaxy in the source plane,
and resolve the star formation into two dozen clumps. We develop a
forward-modeling technique to model each clump in the source plane. We ray
trace the model to the image plane, convolve with the instrumental point spread
function (PSF), and compare with the GALFIT model of the clumps in the image
plane, which decomposes clump structure from more extended emission. This
technique has the advantage, over ray tracing, by accounting for the asymmetric
lensing shear of the galaxy in the image plane and the instrument PSF. At this
resolution, we can begin to study star formation on a clump-by-clump basis,
toward the goal of understanding feedback mechanisms and the buildup of
exponential disks at high redshift.Comment: 19 pages, 12 figures, accepted to Ap
An improved SPH scheme for cosmological simulations
We present an implementation of smoothed particle hydrodynamics (SPH) with
improved accuracy for simulations of galaxies and the large-scale structure. In
particular, we combine, implement, modify and test a vast majority of SPH
improvement techniques in the latest instalment of the GADGET code. We use the
Wendland kernel functions, a particle wake-up time-step limiting mechanism and
a time-dependent scheme for artificial viscosity, which includes a high-order
gradient computation and shear flow limiter. Additionally, we include a novel
prescription for time-dependent artificial conduction, which corrects for
gravitationally induced pressure gradients and largely improves the SPH
performance in capturing the development of gas-dynamical instabilities. We
extensively test our new implementation in a wide range of hydrodynamical
standard tests including weak and strong shocks as well as shear flows,
turbulent spectra, gas mixing, hydrostatic equilibria and self-gravitating gas
clouds. We jointly employ all modifications; however, when necessary we study
the performance of individual code modules. We approximate hydrodynamical
states more accurately and with significantly less noise than standard SPH.
Furthermore, the new implementation promotes the mixing of entropy between
different fluid phases, also within cosmological simulations. Finally, we study
the performance of the hydrodynamical solver in the context of radiative galaxy
formation and non-radiative galaxy cluster formation. We find galactic disks to
be colder, thinner and more extended and our results on galaxy clusters show
entropy cores instead of steadily declining entropy profiles. In summary, we
demonstrate that our improved SPH implementation overcomes most of the
undesirable limitations of standard SPH, thus becoming the core of an efficient
code for large cosmological simulations.Comment: 21 figures, 2 tables, accepted to MNRA
Consistency check of {\Lambda}CDM phenomenology
The standard model of cosmology LCDM assumes general relativity, flat space,
and the presence of a positive cosmological constant. We relax these
assumptions allowing spatial curvature, time-dependent effective dark energy
equation of state, as well as modifications of the Poisson equation for the
lensing potential, and modifications of the growth of linear matter density
perturbations in alternate combinations. Using six parameters characterizing
these relations, we check LCDM for consistency utilizing cosmic microwave
background anisotropies, cross correlations thereof with high-redshift galaxies
through the integrated Sachs-Wolfe effect, the Hubble constant, supernovae and
baryon acoustic oscillation distances, as well as the relation between weak
gravitational lensing and galaxy flows. In all scenarios, we find consistency
of the concordance model at the 95% confidence level. However, we emphasize
that constraining supplementary background parameters and parametrizations of
the growth of large-scale structure separately may lead to a priori exclusion
of viable departures from the concordance model.Comment: 15 pages, 14 figures, 4 tables; revision with minor change
Capturing in-situ Feelings and Experiences of Public Transit Riders Using Smartphones
High-density urban environments are susceptible to ever-growing traffic congestion issues, which speaks to the importance of implementing and maintaining effective and sustainable transportation networks. While transit oriented developments offer the potential to help mitigate traffic congestion issues, transit networks ought to be safe and reliable for ideal transit-user communities. As such, it is imperative to capture meaningful data regarding transit experiences, and deduce how transit networks can be enhanced or modified to continually maintain ideal transit experiences. Historically speaking, it has been relatively tricky to measure how people feel whilst using public transportation, without leaning on recall memory to explain such phenomena. Recall memory can be vague and is often less detailed than recording in-situ observations of the transit-user community. This thesis explores the feasibility of using smartphones to capture meaningful in-situ data to leverage the benefits of the Experience Sampling Method (ESM), while also addressing some limitations. Students travelled along Grand River Transit bus routes in Waterloo, Ontario from Wilfrid Laurier University to Conestoga Mall and back using alternate routes. The mobile survey captured qualitative and quantitative data from 145 students to explore variations in wellbeing, and the extent to which environmental variables can influence transit experiences. There were many findings to consider for future research, especially the overall role anxiety played on transit experiences. In addition, the results indicate that the methodology is appropriate for further research, and can be applied to a wide range of research topics. In particular, it is recommended that a similar study be applied to a much larger, and more representative sample of the transit-user community. Future considerations are discussed as key considerations to leverage the benefits of ESM research, and the promise it can bring towards the enhancement of transit experiences and the cohesion of transit-user communities
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