463 research outputs found
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
The Imprint of Cosmic Reionization on Galaxy Clustering
We consider the effect of reionization on the clustering properties of galaxy
samples at intermediate redshifts (z~0.3-5.5). Current models for the
reionization of intergalactic hydrogen predict that overdense regions will be
reionized early, thus delaying the build up of stellar mass in the progenitors
of massive lower-redshift galaxies. As a result, the stellar populations
observed in intermediate redshift galaxies are somewhat younger and hence
brighter in overdense regions of the Universe. Galaxy surveys would therefore
be sensitive to galaxies with a somewhat lower dark matter mass in overdense
regions. The corresponding increase in the observed number density of galaxies
can be parameterized as a galaxy bias due to reionization. We model this
process using merger trees combined with a stellar synthesis code. Our model
demonstrates that reionization has a significant effect on the clustering
properties of galaxy samples that are selected based on their star-formation
properties. The bias correction in Lyman-break galaxies (including those in
proposed baryonic oscillation surveys at z<1) is at the level of 10-20% for a
halo mass of 10^12 solar masses, leading to corrections factors of 1.5-2 in the
halo mass inferred from measurements of clustering length. The reionization of
helium could also lead to a sharp increase in the amplitude of the galaxy
correlation function at z~3. We find that the reionization bias is
approximately independent of scale and halo mass. However since the traditional
galaxy bias is mass dependent, the reionization bias becomes relatively more
important for lower mass systems. The correction to the bias due to
reionization is very small in surveys of luminous red galaxies at z<1.Comment: 17 pages, 6 figures. Submitted to MNRA
Application of the Contouring Method to Extended Microlensed Sources
The method devised by Lewis et al. (1993) for calculating the light curve of
a microlensed point source is expanded to two dimensions to enable the
calculation of light curves of extended sources. This method is significantly
faster than the ray shooting method that has been used in the past. The
increased efficiency is used to obtain much higher resolution light curves over
increased timescales. We investigate the signatures arising from different
source geometries in a realistic microlensing model. We show that a large
fraction of high magnification events (HMEs) in image A of Q2237+0305 involve
only one caustic, and could therefore yield information on the structure of the
quasar continuum through the recognition of a characteristic event shape. In
addition, the cataloguing of HMEs into morphological type will, in theory,
enable the direction of the transverse motion, as well as the source size to be
obtained from long term monitoring.Comment: 10 pages including 4 figures. Accepted for publication in M.N.R.A.
The relation between star formation rate and stellar mass of galaxies at z 1-4
The relation between the Star Formation Rate (SFR) and stellar mass () of galaxies represents a fundamental constraint on galaxy
formation and has been studied extensively both in observations and
cosmological simulations. However, the observed amplitude has not been
successfully reproduced in simulations, indicating either that the halo
accretion history and baryonic physics are poorly modeled or that observations
contain biases. We examine the evolution of the SFR relation
of galaxies and display the inconsistency between observed
relations that are obtained using different techniques. We employ cosmological
hydrodynamic simulations and compare these with a range of observed SFR relations. We find that numerical results are consistent with
observations that use Spectral Energy Distribution (SED) techniques to estimate
star formation rates and dust corrections. On the contrary, simulations are not
able to reproduce results that were obtained by combining only UV and IR
luminosities. These imply SFRs at a fixed stellar mass that are larger almost
by a factor of 5 than those of SED measurements for . Furthermore,
we find remarkable agreement between the numerical results from various authors
who have employed different cosmological codes and run simulations with
different resolutions. This is interesting for two reasons. A) simulations can
produce realistic populations of galaxies within representative cosmological
volumes even at relatively modest resolutions. B) It is likely that current
numerical codes that rely on similar subgrid multiphase Inter-Stellar Medium
(ISM) models and are tuned to reproduce statistical properties of galaxies,
produce similar results for the SFR relation by construction,
regardless of resolution, box size and, to some extent, the adopted feedback
prescriptions.Comment: 16 pages, 6 figures, 3 tables, accepted for publication in PAS
Constraining the Quasar Contribution to the Reionisation of Cosmic Hydrogen
Absorption spectra of high redshift quasars suggest that the reionisation of
cosmic hydrogen was complete near z~6. The dominant sources of ionising photons
responsible for this reionisation are generally thought to be stars and
quasars. In this paper we make a quantitative estimate of the relative
contributions made by these sources. Our approach is to compute the evolution
of the post overlap ionising background radiation by combining semi-analytic
descriptions of reionisation in a clumpy medium with a model for the quasar
luminosity function. Our overall model has two free parameters, the star
formation efficiency and the minimum quasar luminosity. By adjusting these
parameters, we constrain the relative contributions made by stars and quasars
through comparison with reported observations (Fan et al. 2005). We find that
the relative quasar contribution (at z=5.7) to the ionising background was
between 1.4% and 14.5%. The range of uncertainty is dominated by the unknown
minimum quasar luminosity.Comment: 8 pages, 2 figures. Accepted for publication in MNRA
Intensity mapping cross-correlations II: HI halo models including shot noise
HI intensity mapping data traces the large-scale structure matter
distribution using the integrated emission of neutral hydrogen gas (HI). The
cross-correlation of the intensity maps with optical galaxy surveys can
mitigate foreground and systematic effects, but has been shown to significantly
depend on galaxy evolution parameters of the HI and the optical sample.
Previously, we have shown that the shot noise of the cross-correlation scales
with the HI content of the optical samples, such that the shot noise estimation
infers the average HI masses of these samples. In this article, we present an
adaptive framework for the cross-correlation of HI intensity maps with galaxy
samples using our implementation of the halo model formalism (Murray et al
2018, in prep) which utilises the halo occupation distribution of galaxies to
predict their power spectra. We compare two HI population models, tracing the
spatial halo and the galaxy distribution respectively, and present their auto-
and cross-power spectra with an associated galaxy sample. We find that the
choice of the HI model and the distribution of the HI within the galaxy sample
have minor significance for the shape of the auto- and cross-correlations, but
highly impact the measured shot noise amplitude of the estimators, a finding we
confirm with simulations. We demonstrate parameter estimation of the HI halo
occupation models and advocate this framework for the interpretation of future
experimental data, with the prospect of determining the HI masses of optical
galaxy samples via the cross-correlation shot noise.Comment: 15 pages, 8 figures, 3 tables. Comments welcom
Determining the HI content of galaxies via intensity mapping cross-correlations
We propose an innovative method for measuring the neutral hydrogen (HI)
content of an optically-selected spectroscopic sample of galaxies through
cross-correlation with HI intensity mapping measurements. We show that the
HI-galaxy cross-power spectrum contains an additive shot noise term which
scales with the average HI brightness temperature of the optically-selected
galaxies, allowing constraints to be placed on the average HI mass per galaxy.
This approach can estimate the HI content of populations too faint to directly
observe through their 21cm emission over a wide range of redshifts. This
cross-correlation, as a function of optical luminosity or colour, can be used
to derive HI-scaling relations. We demonstrate that this signal will be
detectable by cross-correlating upcoming Australian SKA Pathfinder (ASKAP)
observations with existing optically-selected samples. We also use
semi-analytic simulations to verify that the HI mass can be successfully
recovered by our technique in the range M_HI > 10^8 M_solar, in a manner
independent of the underlying power spectrum shape. We conclude that this
method is a powerful tool to study galaxy evolution, which only requires a
single intensity mapping dataset to infer complementary HI gas information from
existing optical and infra-red observations.Comment: 8 pages, 4 figures, submitted to MNRA
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.
Interpretation of the OGLE Q2237+0305 microlensing light-curve
The four bright images of the gravitationally lensed quasar Q2237+0305 are
being monitored from the ground (eg. OGLE collaboration, Apache Point
Observatory) in the hope of observing a high magnification event (HME). Over
the past three seasons (1997-1999) the OGLE collaboration has produced
microlensing light-curves with unprecedented coverage. These demonstrate
smooth, independent (therefore microlensing) variability between the images
(Wozniak et al. 2000a,b; OGLE web page). We have retrospectively compared
probability functions for high-magnification event parameters with several
observed light-curve features. We conclude that the 1999 image C peak was due
to the source having passed outside of a cusp rather than to a caustic
crossing. In addition, we find that the image C light-curve shows evidence for
a caustic crossing between the 1997 and 1998 observing seasons involving the
appearance of new critical images. Our models predict that the next image C
event is most likely to arrive 500 days following the 1999 peak, but with a
large uncertainty (100-2000 days). Finally, given the image A light-curve
derivative at the end of the 1999 observing season, our modelling suggests that
a caustic crossing will occur between the 1999 and 2000 observing seasons,
implying a minimum for the image A light-curve ~1-1.5 magnitudes fainter than
the November 1999 level.Comment: 11 pages, 15 figures. Accepted for publication in M.N.R.A.
Limits on the microlens mass function of Q2237+0305
Gravitational microlensing at cosmological distances is potentially a
powerful tool for probing the mass functions of stars and compact objects in
other galaxies. In the case of multiply-imaged quasars, microlensing data has
been used to determine the average microlens mass. However the measurements
have relied on an assumed transverse velocity for the lensing galaxy. Since the
measured mass scales with the square of the transverse velocity, published mass
limits are quite uncertain. In the case of Q2237+0305 we have properly
constrained this uncertainty. The distribution of light curve derivatives
allows quantitative treatment of the relative rates of microlensing due to
proper motions of microlenses, the orbital stream motion of microlenses and the
bulk galactic transverse velocity. By demanding that the microlensing rate due
to the motions of microlenses is the minimum that should be observed we
determine lower limits for the average mass of stars and compact objects in the
bulge of Q2237+0305. If microlenses are assumed to move in an orbital stream
the lower limit ranges between 0.005 and 0.023 solar masses where the the
systematic dependence is due to the fraction of smooth matter and the size of
photometric error assumed for published monitoring data. However, if the
microlenses are assumed to move according to an isotropic velocity dispersion
then a larger lower limit of 0.019-0.11 solar masses is obtained. A significant
contribution of Jupiter mass compact objects to the mass distribution of the
galactic bulge of Q2237+0305 is therefore unambiguously ruled out.Comment: 10 pages, 5 figures. Accepted for publication in Monthly Notices of
the Royal Astronomical Society. New version has improved presentatio
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