460 research outputs found
The Ray Bundle method for calculating weak magnification by gravitational lenses
We present here an alternative method for calculating magnifications in
gravitational lensing calculations -- the Ray Bundle method. We provide a
detailed comparison between the distribution of magnifications obtained
compared with analytic results and conventional ray-shooting methods. The Ray
Bundle method provides high accuracy in the weak lensing limit, and is
computationally much faster than (non-hierarchical) ray shooting methods to a
comparable accuracy.
The Ray Bundle method is a powerful and efficient technique with which to
study gravitational lensing within realistic cosmological models, particularly
in the weak lensing limit.Comment: 9 pages Latex, 8 figures, submitted to MNRA
The distribution of microlensed light curve derivatives: the relationship between stellar proper motions and transverse velocity
We present a method for computing the probability distribution of microlensed
light curve derivatives both in the case of a static lens with a transverse
velocity, and in the case of microlensing that is produced through stellar
proper motions. The distributions are closely related in form, and can be
considered equivalent after appropriate scaling of the input transverse
velocity. The comparison of the distributions in this manner provides a
consistent way to consider the relative contribution to microlensing (both
large and small fluctuations) of the two classes of motion, a problem that is
otherwise an extremely expensive computational exercise. We find that the
relative contribution of stellar proper motions to the microlensing rate is
independent of the mass function assumed for the microlenses, but is a function
of optical depth and shear. We find that stellar proper motions produce a
higher overall microlensing rate than a transverse velocity of the same
magnitude. This effect becomes more pronounced at higher optical depth. With
the introduction of shear, the relative rates of microlensing become dependent
on the direction of the transverse velocity. This may have important
consequences in the case of quadruply lensed quasars such as Q2237+0305, where
the alignment of the shear vector with the source trajectory varies between
images.Comment: 12 pages, including 9 figures. Submitted to M.N.R.A.S. Revised
version includes a short section on the applicability of the metho
Weak redshift discretisation in the Local Group of Galaxies?
We discuss the distribution of radial velocities of galaxies belonging to the
Local Group. Two independent samples of galaxies as well as several methods of
reduction from the heliocentric to the galactocentric radial velocities are
explored. We applied the power spectrum analysis using the Hann function as a
weighting method, together with the jackknife error estimation. We performed a
detailed analysis of this approach. The distribution of galaxy redshifts seems
to be non-random. An excess of galaxies with radial velocities of and is detected, but the effect
is statistically weak. Only one peak for radial velocities of seems to be confirmed at the confidence level of 95%.Comment: 23 pages 7 figures Astronomische Nachrichten accepte
A small source in Q2237+0305 ?
Microlensing in Q2237+0305 between 1985 and 1995 (eg. Irwin et al. 1989;
Corrigan et al. 1991; Ostensen et al. 1996) has been interpreted in two
different ways; as microlensing by stellar mass objects of a continuum source
having dimensions significantly smaller than the microlens Einstein radius (ER)
(eg. Wambsganss, Paczynski & Schneider 1990; Rauch & Blandford 1991), and as
microlensing by very low mass objects of a source as large as 5 ER (Refsdal &
Stabell 1993; Haugan 1996). In this paper we present evidence in favour of a
small source. Limits on the source size (in units of ER) are obtained from the
combination of limits on the number of microlens Einstein radii crossed by the
source during the monitoring period with two separate light-curve features.
Firstly, recently published monitoring data (Wozniak et al. 2000; OGLE web
page) show large variations (~0.8-1.5 magnitudes) between image brightnesses
over a period of 700 days or ~15% of the monitoring period. Secondly, the 1988
peak in the image A light-curve had a duration that is a small fraction (<0.02)
of the monitoring period. Such rapid microlensing rises and short microlensing
peaks only occur for small sources. We find that the observed large-rapid
variation limits the source size to be <0.2 ER (95% confidence). The width of
the light-curve peak provides a stronger constraint of <0.02 ER (99%
confidence). The Einstein radius (projected into the source plane) of the
average microlens mass (m) in Q2237+0305 is ER ~ 10^{17}\sqrt{m} cm. The
interpretation that stars are responsible for microlensing in Q2237+0305
therefore results in limits on the continuum source size that are consistent
with current accretion disc theory.Comment: 8 pages, 3 figures, accepted for publication in M.N.R.A.
Using a complete spectroscopic survey to find red quasars and test the KX method
We present an investigation of quasar colour-redshift parameter space in
order to search for radio-quiet red quasars and to test the ability of a
variant of the KX quasar selection method to detect quasars over a full range
of colour without bias. This is achieved by combining IRIS2 imaging with the
complete Fornax Cluster Spectroscopic Survey to probe parameter space
unavailable to other surveys. We construct a new sample of 69 quasars with
measured bJ - K colours. We show that the colour distribution of these quasars
is significantly different from that of the Large Bright Quasar Survey's
quasars at a 99.9% confidence level. We find 11 of our sample of 69 quasars
have signifcantly red colours (bJ - K >= 3.5) and from this, we estimate the
red quasar fraction of the K <= 18.4 quasar population to be 31%, and robustly
constrain it to be at least 22%. We show that the KX method variant used here
is more effective than the UVX selection method, and has less colour bias than
optical colour-colour selection methods.Comment: 11 pages, 14 figures, accepted for publication in MNRA
The multiple quasar Q2237+0305 under a microlensing caustic
We use the high magnification event seen in the 1999 OGLE campaign light
curve of image C of the quadruply imaged gravitational lens Q2237+0305 to study
the structure of the quasar engine. We have obtained g'- and r'-band photometry
at the Apache Point Observatory 3.5m telescope where we find that the event has
a smaller amplitude in the r'-band than in the g'- and OGLE V-bands. By
comparing the light curves with microlensing simulations we obtain constraints
on the sizes of the quasar regions contributing to the g'- and r'-band flux.
Assuming that most of the surface mass density in the central kiloparsec of the
lensing galaxy is due to stars and by modeling the source with a Gaussian
profile, we obtain for the Gaussian width 1.20 x 10^15 sqrt(M/0.1M_sun)cm <
sigma_g' < 7.96 x 10^15 sqrt(M/0.1Msun) cm, where M is the mean microlensing
mass, and a ratio sigma_r'/sigma_g'=1.25^{+0.45}_{-0.15}. With the limits on
the velocity of the lensing galaxy from Gil-Merino et al. (2005) as our only
prior, we obtain 0.60 x 10^15 sqrt(M/0.1Msun) cm < sigma_g' < 1.57 x 10^15
sqrt(M/0.1Msun) cm and a ratio sigma_r'/sigma_g'=1.45^{+0.90}_{-0.25} (all
values at 68 percent confidence). Additionally, from our microlensing
simulations we find that, during the chromatic microlensing event observed, the
continuum emitting region of the quasar crossed a caustic at >72 percent
confidence.Comment: Accepted for publication in A&A, 8 pages, 4 figures. Slightly
modified compared to the original version: qualitative results unchanged,
constraints on the r'/g' source size ratio now tighter due to correction of
an error in the numerical treatment of the simulated light curve
Interpreting microlensing signal in QSO 2237+0305: Stars or planets?
The multiply imaged, gravitationally lensed quasar, QSO 2237+0305, has been
the subject of recent optical monitoring campaigns, with its light curves
displaying uncorrelated variability attributed to gravitational microlensing by
masses in the foreground galaxy. Based on these light curves, it has been
claimed that the dominant microlensing population must be a population of
free-floating Jupiter-like objects; such a conclusion is not new, with several
similar claims in the literature. Historically, however, it has been shown that
such conclusions are flawed, with an incorrect interpretation of the complex
caustic network that arises at significant optical depth. This paper examines
this more recent claim, illustrating that it too is flawed.Comment: 4 pages, 2 figures, A&A Letters accepted; a response to
astro-ph/050301
ImpZ: a new photometric redshift code for galaxies and quasars
We present a combined galaxy-quasar approach to template-fitting photometric
redshift techniques and show the method to be a powerful one. The code (ImpZ)
is presented, developed and applied to two spectroscopic redshift catalogues,
namely the Isaac Newton Telescope Wide Angle Survey ELAIS N1 and N2 fields and
the Chandra Deep Field North. In particular, optical size information is used
to improve the redshift determination. The success of the code is shown to be
very good with Delta z/(1+z) constrained to within 0.1 for 92 per cent of the
galaxies in our sample. The extension of template-fitting to quasars is found
to be reasonable with Delta z/(1+z) constrained to within 0.25 for 68 per cent
of the quasars in our sample. Various template extensions into the far-UV are
also tested.Comment: 21 pages. MNRAS in press. Minor alterations to match MNRAS final
proo
An Extragalactic HI Cloud with No Optical Counterpart?
We report the discovery, from the HI Parkes All-Sky Survey (HIPASS), of an
isolated cloud of neutral hydrogen which we believe to be extragalactic. The HI
mass of the cloud (HIPASS J1712-64) is very low, 1.7 x 10^7 Msun, using an
estimated distance of ~3.2 Mpc. Most significantly, we have found no optical
companion to this object to very faint limits (mu(B)~ 27 mag arcsec^-2). HIPASS
J1712-64 appears to be a binary system similar to, but much less massive than,
HI 1225+01 (the Virgo HI Cloud) and has a size of at least 15 kpc. The mean
velocity dispersion, measured with the Australia Telescope Compact Array
(ATCA), is only 4 km/s for the main component and because of the weak or
non-existent star-formation, possibly reflects the thermal linewidth (T<2000 K)
rather than bulk motion or turbulence. The peak column density for HIPASS
J1712-64, from the combined Parkes and ATCA data, is only 3.5 x 10^19 cm^-2,
which is estimated to be a factor of two below the critical threshold for star
formation. Apart from its significantly higher velocity, the properties of
HIPASS J1712-64 are similar to the recently recognised class of Compact High
Velocity Clouds. We therefore consider the evidence for a Local Group or
Galactic origin, although a more plausible alternative is that HIPASS J1712-64
was ejected from the interacting Magellanic Cloud/Galaxy system at
perigalacticon ~ 2 x 10^8 yr ago.Comment: 23 pages, 7 figures, AJ accepte
Practicing convict criminology: lessons learned from British academic activism
Joanne Belknapâs recent ASC presidential address included a critique of Convict Criminologyâs activism. A number of concerns were provided, although of particular importance here are, first, Belknapâs concerns regarding the absence of âmarginalized voicesâ in the Convict Criminology network. Second, the issue of defining how non-con academics function as Convict Criminology group members. This paper responds to these criticisms. Specifically, we discuss the question of ârepresentationâ in BCC and our attempts to remedy this issue. We also draw attention to the academic activism that British Convict Criminology is conducting in Europe. This includes a detailed discussion of the collaborative research-activist activities that involve non-con as well as ex-con academic network members. We demonstrate how these collaborations explain the vital group membership role that non-con academics assume in the activism of Convict Criminology
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