542 research outputs found
Reionization Through the Lens of Percolation Theory
The reionization of intergalactic hydrogen has received intense theoretical
scrutiny over the past two decades. Here, we approach the process formally as a
percolation process and phase transition. Using semi-numeric simulations, we
demonstrate that an infinitely-large ionized region abruptly appears at an
ionized fraction of ~0.1 and quickly grows to encompass most of the ionized
gas: by an ionized fraction of 0.3, nearly ninety percent of the ionized
material is part of this region. Throughout most of reionization, nearly all of
the intergalactic medium is divided into just two regions, one ionized and one
neutral, and both infinite in extent. We also show that the discrete ionized
regions that exist before and near this transition point follow a near-power
law distribution in volume, with equal contributions to the total filling
factor per logarithmic interval in size up to a sharp cutoff in volume. These
qualities are generic to percolation processes, with the detailed behavior a
result of long-range correlations in the underlying density field. These
insights will be crucial to understanding the distribution of ionized and
neutral gas during reionization and provide precise meaning to the intuitive
description of reionization as an "overlap" process.Comment: 16 pages, version accepted by MNRAS (conclusions unchanged from
original
Photo-heating and the fate of hard photons during the reionisation of HeII by quasars
We use a combination of analytic and numerical arguments to consider the
impact of quasar photo-heating during HeII reionisation on the thermal
evolution of the intergalactic medium (IGM). We demonstrate that rapid (\Delta
z 10^4 K) photo-heating is difficult to achieve
across the entire IGM unless quasar spectra are significantly harder than
implied by current observational constraints. Although filtering of intrinsic
quasar radiation through dense regions in the IGM does increase the mean excess
energy per HeII photo-ionisation, it also weakens the radiation intensity and
lowers the photo-ionisation rate, preventing rapid heating over time intervals
shorter than the local photo-ionisation timescale. Moreover, the hard photons
responsible for the strongest heating are more likely to deposit their energy
inside dense clumps. The abundance of such clumps is, however, uncertain and
model-dependent, leading to a fairly large uncertainty in the photo-heating
rates. Nevertheless, although some of the IGM may be exposed to a hardened and
weakened ionising background for long periods, most of the IGM must instead be
reionised by the more abundant, softer photons and with accordingly modest
heating rates (\Delta T < 10^4 K). The repeated ionisation of fossil quasar
HeIII regions does not increase the net heating because the recombination times
in these regions typically exceed the IGM cooling times and the average time
lag between successive rounds of quasar activity. Detailed line-of-sight
radiative transfer simulations confirm these expectations and predict a rich
thermal structure in the IGM during HeII reionisation. [Abridged]Comment: 20 pages, 6 figures, accepted by MNRA
The Effects of Dark Matter Decay and Annihilation on the High-Redshift 21 cm Background
The radiation background produced by the 21 cm spin-flip transition of
neutral hydrogen at high redshifts can be a pristine probe of fundamental
physics and cosmology. At z~30-300, the intergalactic medium (IGM) is visible
in 21 cm absorption against the cosmic microwave background (CMB), with a
strength that depends on the thermal (and ionization) history of the IGM. Here
we examine the constraints this background can place on dark matter decay and
annihilation, which could heat and ionize the IGM through the production of
high-energy particles. Using a simple model for dark matter decay, we show
that, if the decay energy is immediately injected into the IGM, the 21 cm
background can detect energy injection rates >10^{-24} eV cm^{-3} sec^{-1}. If
all the dark matter is subject to decay, this allows us to constrain dark
matter lifetimes <10^{27} sec. Such energy injection rates are much smaller
than those typically probed by the CMB power spectra. The expected brightness
temperature fluctuations at z~50 are a fraction of a mK and can vary from the
standard calculation by up to an order of magnitude, although the difference
can be significantly smaller if some of the decay products free stream to lower
redshifts. For self-annihilating dark matter, the fluctuation amplitude can
differ by a factor <2 from the standard calculation at z~50. Note also that, in
contrast to the CMB, the 21 cm probe is sensitive to both the ionization
fraction and the IGM temperature, in principle allowing better constraints on
the decay process and heating history. We also show that strong IGM heating and
ionization can lead to an enhanced H_2 abundance, which may affect the earliest
generations of stars and galaxies.Comment: submitted to Phys Rev D, 14 pages, 8 figure
Intensity Mapping with Carbon Monoxide Emission Lines and the Redshifted 21 cm Line
We quantify the prospects for using emission lines from rotational
transitions of the CO molecule to perform an `intensity mapping' observation at
high redshift during the Epoch of Reionization (EoR). The aim of CO intensity
mapping is to observe the combined CO emission from many unresolved galaxies,
to measure the spatial fluctuations in this emission, and use this as a tracer
of large scale structure at very early times in the history of our Universe.
This measurement would help determine the properties of molecular clouds -- the
sites of star formation -- in the very galaxies that reionize the Universe. We
further consider the possibility of cross-correlating CO intensity maps with
future observations of the redshifted 21 cm line. The cross spectrum is less
sensitive to foreground contamination than the auto power spectra, and can
therefore help confirm the high redshift origin of each signal. Furthermore,
the cross spectrum measurement would help extract key information about the
EoR, especially regarding the size distribution of ionized regions. We discuss
uncertainties in predicting the CO signal at high redshift, and discuss
strategies for improving these predictions. Under favorable assumptions, and
feasible specifications for a CO survey mapping the CO(2-1) and CO(1-0) lines,
the power spectrum of CO emission fluctuations and its cross spectrum with
future 21 cm measurements from the MWA are detectable at high significance.Comment: 19 pages, 8 figures, submitted to Ap
Searching for the earliest galaxies in the 21 cm forest
We use a model developed by Xu et al. (2010) to compute the 21 cm line
absorption signatures imprinted by star-forming dwarf galaxies (DGs) and
starless minihalos (MHs). The method, based on a statistical comparison of the
equivalent width (W_\nu) distribution and flux correlation function, allows us
to derive a simple selection criteria for candidate DGs at very high (z >= 8)
redshift. We find that ~ 18% of the total number of DGs along a line of sight
to a target radio source (GRB or quasar) can be identified by the condition
W_\nu < 0; these objects correspond to the high-mass tail of the DG
distribution at high redshift, and are embedded in large HII regions. The
criterion W_\nu > 0.37 kHz instead selects ~ 11% of MHs. Selected candidate DGs
could later be re-observed in the near-IR by the JWST with high efficiency,
thus providing a direct probe of the most likely reionization sources.Comment: 8 pages, 3 figures. Accepted for publication in Science in China
Series
How Universal is the Gunn-Peterson Trough at z~6?: A Closer Look at the Quasar SDSS J1148+5251
Detectable flux is visible in the Ly-alpha and Ly-beta troughs of the highest
redshift () quasar found to date, SDSS J1148+5251. This has previously
been interpreted as continuum contamination from an interloper galaxy at
z=4.94. We examine the Ly-gamma trough of SDSS J1148+5251 and show that this
interpretation is untenable: the spectrum does not show the continuum break in
a z=4.94 galaxy expected from absorption by the intervening Ly-alpha forest.
Therefore, flux must be leaking through at least one of the troughs from the
quasar itself. Contrary to previous claims, the flux ratios in the Ly-alpha and
Ly-beta troughs are consistent with pure transmission. From the Ly-gamma
trough, we place an upper bound on the effective Ly-alpha optical depth at
z~6.2 of tau < 14.3 (2 sigma). This implies a highly ionized IGM along this
line of sight and significant cosmic variance in the transition toward complete
Gunn-Peterson absorption. Detailed study of the observed transmission features
will shed light on this era.Comment: Submitted to ApJ Letter
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