29 research outputs found
The morphology of HII regions during reionization
It is possible that the properties of HII regions during reionization depend
sensitively on many poorly constrained quantities (the nature of the ionizing
sources, the clumpiness of the gas in the IGM, the degree to which
photo-ionizing feedback suppresses the abundance of low mass galaxies, etc.),
making it extremely difficult to interpret upcoming observations of this epoch.
We demonstrate that the actual situation is more encouraging, using a suite of
radiative transfer simulations, post-processed on outputs from a 1024^3, 94 Mpc
N-body simulation. Analytic prescriptions are used to incorporate small-scale
structures that affect reionization, yet remain unresolved in the N-body
simulation. We show that the morphology of the HII regions is most dependent on
the global ionization fraction x_i. This is not to say that the bubble
morphology is completely independent of all parameters besides x_i. The next
most important dependence is that of the nature of the ionizing sources. The
rarer the sources, the larger and more spherical the HII regions become. The
typical bubble size can vary by as much as a factor of 4 at fixed x_i between
different possible source prescriptions. The final relevant factor is the
abundance of minihalos or of Lyman-limit systems. These systems suppress the
largest bubbles from growing, and the magnitude of this suppression depends on
the thermal history of the gas as well as the rate at which minihalos are
photo-evaporated. We find that neither source suppression owing to
photo-heating nor gas clumping significantly affect the large-scale structure
of the HII regions. We discuss how observations of the 21cm line with MWA and
LOFAR can constrain properties of reionization, and we study the effect patchy
reionization has on the statistics of Lyman-alpha emitting galaxies. [abridged]Comment: 23 pages, 18 figure
The Persistence of Warps in Spiral Galaxies with Massive Halos
We study the persistence of warps in galactic discs in the presence of
massive halos. A disc is approximated by a set of massive rings, while a halo
is represented by a conventional n-body simulation. We confirm the conclusion
of Nelson & Tremaine (1995) that a halo responds strongly to an embedded
precessing disc. This response invalidates the approximations made by in the
derivation of classical `modified tilt' modes. We show that the response of the
halo causes the line of nodes of a disc that starts from a modified tilt mode
to wind up within a few dynamical times. We explain this finding in terms of
the probable spectrum of true normal modes of a combined disc-halo system.Comment: 8 pages, 6 figures, 1 table, submitted to monthly notice on July 23,
199
Probing the Neutral Fraction of the IGM with GRBs during the Epoch of Reionization
We show that near-infrared observations of the red side of the Ly-alpha line
from a single gamma ray burst (GRB) afterglow cannot be used to constrain the
global neutral fraction of the intergalactic medium (IGM), x_H, at the GRB's
redshift to better than ~0.3. Some GRB sight-lines will encounter more neutral
hydrogen than others at fixed x_H owing to the patchiness of reionisation. GRBs
during the epoch of reionization will often bear no discernible signature of a
neutral IGM in their afterglow spectra. We discuss the constraints on x_H from
the z = 6.3 burst, GRB050904, and quantify the probability of detecting a
neutral IGM using future spectroscopic observations of high-redshift,
near-infrared GRB afterglows. Assuming an observation with signal-to-noise
similar to the Subaru FOCAS spectrum of GRB050904 and that the column density
distribution of damped Ly-alpha absorbers is the same as measured at lower
redshifts, a GRB from an epoch when x_H = 0.5 can be used to detect a partly
neutral IGM at 98% confidence level 10% of the time (and, for an observation
with three times the sensitivity, 30% of the time).Comment: 10 pages, 8 figures, submitted to MNRA
A Measurement of Small Scale Structure in the 2.2 < z < 4.2 Lyman-alpha Forest
The amplitude of fluctuations in the Ly-a forest on small spatial scales is
sensitive to the temperature of the IGM and its spatial fluctuations. The
temperature of the IGM and its spatial variations contain important information
about hydrogen and helium reionization. We present a new measurement of the
small-scale structure in the Ly-a forest from 40 high resolution, high
signal-to-noise, VLT spectra at z=2.2-4.2. We convolve each Ly-a forest
spectrum with a suitably chosen wavelet filter, which allows us to extract the
amount of small-scale structure in the forest as a function of position across
each spectrum. We compare these measurements with high resolution hydrodynamic
simulations of the Ly-a forest which track more than 2 billion particles. This
comparison suggests that the IGM temperature close to the cosmic mean density
(T_0) peaks near z=3.4, at which point it is greater than 20,000 K at 2-sigma
confidence. The temperature at lower redshift is consistent with the fall-off
expected from adiabatic cooling (), after the peak
temperature is reached near z=3.4. At z=4.2 our results favor a temperature of
T_0 = 15-20,000 K. However, owing mostly to uncertainties in the mean
transmitted flux at this redshift, a cooler IGM model with T_0 = 10,000 K is
only disfavored at the 2-sigma level here, although such cool IGM models are
strongly discrepant with the z ~ 3-3.4 measurement. We do not detect large
spatial fluctuations in the IGM temperature at any redshift covered by our data
set. The simplest interpretation of our measurements is that HeII reionization
completes sometime near z ~ 3.4, although statistical uncertainties are still
large [Abridged].Comment: Submitted to ApJ. Best printed in colo
HeII Reionization and its Effect on the IGM
Observations of the intergalactic medium (IGM) suggest that quasars reionize
HeII in the IGM at z ~ 3. We have run a set of 190 and 430 comoving Mpc
simulations of HeII being reionized by quasars to develop an understanding of
the nature of HeII reionization and its potential impact on observables. We
find that HeII reionization heats regions in the IGM by as much as 25,000 K
above the temperature that is expected otherwise, with the volume-averaged
temperature increasing by ~ 12,000 K and with large temperature fluctuations on
~ 50 Mpc scales. Much of the heating occurs far from QSOs by hard photons. We
find a temperature-density equation of state of gamma -1 ~ 0.3 during HeII
reionization, but with a wide dispersion in this relation having sigma ~ 10^4
K. HeII reionization by the observed population of quasars cannot produce an
inverted relation (gamma - 1 < 0). Our simulations are consistent with the
observed evolution in the mean transmission of the HeII Ly-alpha forest. We
argue that the heat input due to HeII reionization is unable to cause the
observed depression at z = 3.2 in the HI Ly-alpha forest opacity as has been
suggested. We investigate how uncertainties in the properties of QSOs and of
HeII Lyman-limit systems influence our predictions.Comment: 19 pages, 15 figures, plus 9 pages of Appendix. accepted by Ap
Studying Reionization with Ly-alpha Emitters
We show that observations of high-redshift Ly-alpha emitters (LAEs) have the
potential to provide definitive evidence for reionization in the near future.
Using 200 Mpc radiative transfer simulations, we calculate the effect that
patchy reionization has on the line profile, on the luminosity function, and,
most interestingly, on the clustering of emitters for several realistic models
of reionization. Reionization increases the measured clustering of emitters,
and we show that this enhancement would be essentially impossible to attribute
to anything other than reionization. Our results motivate looking for the
signature of reionization in existing LAE data. We find that for stellar
reionization scenarios the angular correlation function of the 58 LAEs in the
Subaru Deep Field z = 6.6 photometric sample is more consistent with a fully
ionized universe (mean volume ionized fraction x_i = 1) than a universe with
x_i 2-sigma confidence level. Measurements in the next year on Subaru
will increase their z = 6.6 LAE sample by a factor of five and tighten these
limits. If the clustering signature of reionization is detected in a LAE
survey, a comparison with a Lyman-break or a H-alpha survey in the same field
would confirm the reionization hypothesis. We discuss the optimal LAE survey
specifications for detecting reionization, with reference to upcoming programs.Comment: 24 pages, 17 figures, accepted by MNRA