308 research outputs found
Gravitational lensing by elliptical galaxies
The fraction of high-redshift sources which are multiply-imaged by
intervening galaxies is strongly dependent on the cosmological constant, and so
can be a useful probe of the cosmological model. However its power is limited
by various systematic (and random) uncertainties in the calculation of lensing
probabilities, one of the most important of which is the dynamical
normalisation of elliptical galaxies. Assuming ellipticals' mass distributions
can be modelled as isothermal spheres, the mass normalisation depends on: the
velocity anisotropy; the luminosity density; the core radius; and the area over
which the velocity dispersion is measured. The differences in the lensing
probability and optical depth produced by using the correct normalisation can
be comparable to the differences between even the most extreme cosmological
models. The existing data is not sufficient to determine the correct
normalisation with enough certainty to allow lensing statistics to be used to
their full potential. However, as the correct lensing probability is almost
certainly higher than is usually assumed, upper bounds on the cosmological
constant are not weakened by these possibilities.Comment: MNRAS, in press; 13 pages, 22 figure
Gravitational lensing in galaxy redshift surveys
Gravitationally-lensed quasars should be discovered as a by-product of large
galaxy redshift surveys, being discovered spectroscopically when a low-redshift
galaxy exhibits high-redshift quasar emission lines. The number of lenses
expected is higher than previously estimated, mainly due to the fact that the
presence of the quasar images brings faint deflector galaxies above the survey
limit. Thus the a posteriori likelihood of the discovery of Q 2237+0305 in the
Center for Astrophysics redshift survey is approximately 0.03. In the future,
the 2 degree Field survey should yield at least 10 lensed quasars, and the
Sloan Digitial Sky Survey up to 100.Comment: Gravitational Lensing: Recent Progress and Future Goals, C.S.
Kochanek & T.G. Brainerd, eds., in press; 2 pages, 1 figur
Using the 2dF galaxy redshift survey to detect gravitationally-lensed quasars
Galaxy redshift surveys can be used to detect gravitationally-lensed quasars
if the spectra obtained are searched for the quasars' emission lines. Previous
investigations of this possibility have used simple models to show that the 2
degree Field (2dF) redshift survey could yield several tens of new lenses, and
that the larger Sloan Digital Sky Survey should contain an order of magnitude
more. However the particular selection effects of the samples were not included
in these calculations, limiting the robustness of the predictions; thus a more
detailed simulation of the 2dF survey was undertaken here. The use of an
isophotal magnitude limit reduces both the depth of the sample and the expected
number of lenses, but more important is the Automatic Plate Measuring survey's
star-galaxy separation algorithm, used to generate the 2dF input catalogue. It
is found that most quasar lenses are classed as merged stars, with only the few
lenses with low-redshift deflectors likely to be classified as galaxies.
Explicit inclusion of these selection effects implies that the 2dF survey
should contain 10 lenses on average. The largest remaining uncertainty is the
lack of knowledge of the ease with which any underlying quasars can be
extracted from the survey spectra.Comment: MNRAS, in press; 14 pages, 19 figure
Smooth matter and source size in microlensing simulations of gravitationally lensed quasars
Several gravitationally lensed quasars are observed with anomalous
magnifications in pairs of images that straddle a critical curve. Simple
theoretical arguments suggest that the magnification of these images should be
approximately equivalent, whereas one image is observed to be significantly
demagnified. Microlensing provides a possible explanation for this discrepancy.
There are two key parameters when modelling this effect. The first, the
fraction of smooth matter in the lens at the image positions, has been explored
by Schechter and Wambsganss (2002). They have shown that the anomalous flux
ratio observed in the lensed quasar MG 0414+0534 is a priori a factor of 5 more
likely if the assumed smooth matter content in the lens model is increased from
0% to 93%. The second parameter, the size of the emission region, is explored
in this paper, and shown to be more significant. We find that the broadening of
the magnification probability distributions due to smooth matter content is
washed out for source sizes that are predicted by standard models for quasars.
We apply our model to the anomalous lensed quasar MG 0414+0534, and find a 95%
upper limit of 2.62 x 10^(16) h^(-1/2) (M/Msun)^(1/2) cm on the radius of the
I-band emission region. The smooth matter percentage in the lens is
unconstrained.Comment: 6 pages, 6 figures. To be published in MNRA
Multi-object spectroscopy of the field surrounding PKS 2126-158: Discovery of a z=0.66 galaxy group
The high-redshift radio-loud quasar PKS 2126-158 is found to have a large
number of red galaxies in close apparent proximity. We use the Gemini
Multi-Object Spectrograph (GMOS) on Gemini South to obtain optical spectra for
a large fraction of these sources. We show that there is a group of galaxies at
, coincident with a metal-line absorption system seen in the
quasar's optical spectrum. The multiplexing capabilities of GMOS also allow us
to measure redshifts of many foreground galaxies in the field surrounding the
quasar.
The galaxy group has five confirmed members, and a further four fainter
galaxies are possibly associated. All confirmed members exhibit early-type
galaxy spectra, a rare situation for a Mg II absorbing system. We discuss the
relationship of this group to the absorbing gas, and the possibility of
gravitational lensing of the quasar due to the intervening galaxies.Comment: Monthly Notices of the Royal Astronomical Society, in press. 10
pages, 8 figure
Host Galaxy Contribution to the Colours of `Red' Quasars
We describe an algorithm that measures self-consistently the relative galaxy
contribution in a sample of radio-quasars from their optical spectra alone.
This is based on a spectral fitting method which uses the size of the
characteristic 4000\AA~ feature of elliptical galaxy SEDs. We apply this method
to the Parkes Half-Jansky Flat Spectrum sample of Drinkwater et al. (1997) to
determine whether emission from the host galaxy can significantly contribute to
the very red optical-to-near-infrared colours observed. We find that at around
confidence, most of the reddening in unresolved (mostly quasar-like)
sources is unlikely to be due to contamination by a red stellar component.Comment: 11 pages, 11 figures. Accepted for Publication in Monthly Notices of
the Royal Astronomical Societ
Evolution of damped Lyman alpha kinematics and the effect of spatial resolution on 21-cm measurements
We have investigated the effect of spatial resolution on determining
pencil-beam like velocity widths and column densities in galaxies. Three 21-cm
datasets are used, the HIPASS galaxy catalogue, a subset of HIPASS galaxies
with ATCA maps and a high-resolution image of the LMC. Velocity widths measured
from 21-cm emission in local galaxies are compared with those measured in
intermediate redshift Damped Lyman alpha (DLA) absorbers. We conclude that
spatial resolution has a severe effect on measuring pencil-beam like velocity
widths in galaxies. Spatial smoothing by a factor of 240 is shown to increase
the median velocity width by a factor of two. Thus any difference between
velocity widths measured from global profiles or low spatial resolution 21-cm
maps at z=0 and DLAs at z>1 cannot unambiguously be attributed to galaxy
evolution. The effect on column density measurements is less severe and the
values of dN/dz from local low-resolution 21-cm measurements are expected to be
overestimated by only ~10 per cent.Comment: 5 pages, 6 figures, accepted for publication in MNRAS letter
Dissecting a galaxy: mass distribution of 2237+0305
We determine the mass distribution of a spiral galaxy, 2237+0305 using both
gravitational lensing and dynamical constraints. We find that lensing can break
the disc-halo degeneracy. 2237+0305 has a sub-maximal disc, contributing 57+/-3
per cent of the rotational support at the disc maximum. The disc mass-to-light
ratio is 1.1+/-0.2 in the I-band and the bulge, 2.9+/-0.5. The dark matter
halo, modelled as a softened isothermal sphere, has a large core radius
(13.4+/-0.4 kpc, equivalent to 1.4r_d) to high accuracy for the best-fitting
solution. The image positions are reasonably well fitted, but require further
rotation information to obtain a unique solution.Comment: 11 pages, 6 figures, accepted by MNRAS, in pres
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