1,101 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
Smooth Boundaries to Cosmological HII Regions from Galaxy Clustering
The HII regions around quasars and galaxies at redshifts beyond the epoch of
reionisation will provide prime targets for upcoming 21cm campaigns using a new
generation of low-frequency radio observatories. Here we show that the
boundaries of these HII regions will not be sharp. Rather, the clustering of
sources near massive galaxies results in a neutral fraction that rises
gradually towards large radii from an interior value near zero. A neutral
fraction corresponding to the global background value is typically reached at a
distance of 2-5 times the radius of the HII region around the central massive
galaxy.Comment: 5 Pages, 3 figures. Submitted to MNRA
Very Massive Stars in High-Redshift Galaxies
A significant fraction of Lyman Alpha (Lya) emitting galaxies (LAEs) at z>
5.7 have rest-frame equivalent widths (EW) greater than ~100 Angstrom. However
only a small fraction of the Lya flux produced by a galaxy is transmitted
through the IGM, which implies intrinsic Lya EWs that are in excess of the
maximum allowed for a population-II stellar population having a Salpeter mass
function. In this paper we study characteristics of the sources powering Lya
emission in high redshift galaxies. We propose a simple model for Lya emitters
in which galaxies undergo a burst of very massive star formation that results
in a large intrinsic EW, followed by a phase of population-II star formation
with a lower EW. We confront this model with a range of high redshift
observations and find that the model is able to simultaneously describe the
following eight properties of the high redshift galaxy population with
plausible values for parameters like the efficiency and duration of star
formation: i-iv) the UV and Lya luminosity functions of LAEs at z=5.7 and 6.5,
v-vi) the mean and variance of the EW distribution of Lya selected galaxies at
z=5.7, vii) the EW distribution of i-drop galaxies at z~6, and viii) the
observed correlation of stellar age with EW. Our modeling suggests that the
observed anomalously large intrinsic equivalent widths require a burst of very
massive star formation lasting no more than a few to ten percent of the
galaxies star forming lifetime. This very massive star formation may indicate
the presence of population-III star formation in a few per cent of i-drop
galaxies, and in about half of the Lya selected galaxies.Comment: 11 pages, 7 figures, MNRAS in press, comments by referee included,
references added+update
Fluctuations in 21cm Emission After Reionization
The fluctuations in the emission of redshifted 21cm photons from neutral
inter-galactic hydrogen will provide an unprecedented probe of the reionization
era. Conventional wisdom assumes that this 21cm signal disappears as soon as
reionization is complete, when little atomic hydrogen is left through most of
the volume of the inter-galactic medium (IGM). However observations of damped
Ly-alpha absorbers indicate that the fraction of hydrogen in its neutral form
is significant by mass at all redshifts. Here we use a physically-motivated
model to show that residual neutral gas, confined to dense regions in the IGM
with a high recombination rate, will generate a significant post-reionization
21cm signal. We show that the power-spectrum of fluctuations in this signal
will be detectable by the first generation of low-frequency observatories at a
signal-to-noise that is comparable to that achievable in observations of the
reionization era. The statistics of 21cm fluctuations will therefore probe not
only the pre-reionization IGM, but rather the entire process of HII region
overlap, as well as the appearance of the diffuse ionized IGM.Comment: 10 pages, 5 figures, submitted to MNRA
The stellar mass function and star formation rate-stellar mass relation of galaxies at z ~ 4 - 7
We investigate the evolution of the star formation rate-stellar mass relation
(SFR-M*) and Galaxy Stellar Mass Function (GSMF) of z ~ 4-7 galaxies, using
cosmological simulations run with the smoothed particle hydrodynamics code
P-GADGET3(XXL). We explore the effects of different feedback prescriptions
(supernova driven galactic winds and AGN feedback), initial stellar mass
functions and metal cooling. We show that our fiducial model, with strong
energy-driven winds and early AGN feedback, is able to reproduce the observed
stellar mass function obtained from Lyman-break selected samples of star
forming galaxies at redshift 6 < z < 7. At z ~ 4, observed estimates of the
GSMF vary according to how the sample was selected. Our simulations are more
consistent with recent results from K-selected samples, which provide a better
proxy of stellar masses and are more complete at the high mass end of the
distribution. We find that in some cases simulated and observed SFR-M*
relations are in tension, and this can lead to numerical predictions for the
GSMF in excess of the GSMF observed. By combining the simulated SFR(M*)
relationship with the observed star formation rate function at a given
redshift, we argue that this disagreement may be the result of the uncertainty
in the SFR-M* (Luv-M*) conversion. Our simulations predict a population of
faint galaxies not seen by current observations.Comment: 23 Pages, 13 figures, modified to match accepted version to MNRA
Luminosity Functions of Lyman Alpha Emitting Galaxies and Cosmic Reionization of Hydrogen
Recent observations imply that the observed number counts of Lya Emitters
(LAEs) evolved significantly between z=5.7 and z=6.5. It has been suggested
that this was due to a rapid evolution in the ionisation state, and hence
transmission of the IGM which caused Lya flux from z=6.5 galaxies to be more
strongly suppressed. In this paper we consider the joint evolution of the Lya
and UV luminosity functions (LFs) and show that the IGM transmission evolved
between z=6.5 and z=5.7 by a factor 1.1 <R < 1.8 (95% CL). This result is
insensitive to the underlying model of the Lya LF (as well as cosmic variance).
Using a model for IGM transmission, we find that the evolution of the mean IGM
density through cosmic expansion alone may result in a value for the ratio of
transmissions as high as R=1.3. Thus, the existing LFs do not provide evidence
for overlap. Furthermore, the constraint R<1.8 suggests that the Universe at
z=6.5 was more than half ionised by volume, i.e. x_i,V>0.5.Comment: MNRAS in press. Constraints from rest-frame UV LF added. Discussion
added on cosmic variance. Lower limit on x_i,V lowered to 0.5 (from 0.8
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