278 research outputs found
Polarization Signals of the 21 cm Background from the Era of Reionization
While emission and absorption lines of the 21 cm spin-flip transition of
neutral Hydrogen are intrinsically unpolarized, a magnetic field creates left-
and right-handed polarized components through the Zeeman effect. Here we
consider the resulting polarization of the redshifted 21 cm background from the
intergalactic medium before reionization. The polarization is detectable in
regions with a strong gradient in the mean brightness temperature. In
principle, this can open a new window on the evolution of intergalactic
magnetic fields. One possible approach is an extended integration of an
individual target during this era, such as the Mpc-scale HII regions inferred
to surround quasars at z ~ 6.5. The differential intensity between the two
polarization states can be used as a probe of the magnetic field at the edge of
the HII region. We estimate that the SKA could (ignoring systematics) detect B
\~ 200 (10) \muG coherent over several kiloparsecs with an observational
bandwidth of 100 (2) kHz. Beyond individual sources, the statistical properties
of wide-field 21 cm polarization maps, such as the angular power spectrum, can
be used to constrain the large-scale magnetic field. In this case, the SKA can
detect G fields coherent over many megaparsecs. The magnetic
field can be measured in any epoch over which the 21 cm background changes
rapidly (for example because the ionized fraction or spin temperature change).
Although the resulting constraints with SKA are relatively weak compared to
theoretical expectations, they nevertheless offer a unique direct probe of
magnetic fields in the high-redshift universe.Comment: 6 pages, 2 figures, submitted to MNRA
Free-Free Emission at Low Radio Frequencies
We discuss free-free radio emission from ionized gas in the intergalactic
medium. Because the emissivity is proportional to the square of the electron
density, the mean background is strongly sensitive to the spatial clumping of
free electrons. Using several existing models for the clumping of ionized gas,
we find that the expected free-free distortion to the cosmic microwave
background (CMB) blackbody spectrum is at a level detectable with upcoming
experiments such as the Absolute Radiometer for Cosmology, Astrophysics, and
Diffuse Emission (ARCADE). However, the dominant contribution to the distortion
comes from clumpy gas at z < 3, and the integrated signal does not strongly
constrain the epoch of reionization. In addition to the mean emission, we
consider spatial fluctuations in the free-free background and the extent to
which these anisotropies confuse the search for fluctuations in 21 cm line
emission from neutral hydrogen during and prior to reionization. This
background is smooth in frequency space and hence can be removed through
frequency differencing, but only so long as the 21 cm signal and the free-free
emission are uncorrelated. We show that, because the free-free background is
generated primarily at low redshifts, the cross-correlation between the two
fields is smaller than a few percent. Thus, multifrequency cleaning should be
an effective way to eliminate the free-free confusion.Comment: 4 pages, 2 figure
Lyman-alpha Emitters During the Early Stages of Reionization
We investigate the potential of exploiting Lya Emitters (LAEs) to constrain
the volume-weighted mean neutral hydrogen fraction of the IGM, x_H, at high
redshifts (specifically z~9). We use "semi-numerical'' simulations to
efficiently generate density, velocity, and halo fields at z=9 in a 250 Mpc
box, resolving halos with masses M>2.2e8 solar masses. We construct ionization
fields corresponding to various values of x_H. With these, we generate LAE
luminosity functions and "counts-in-cell'' statistics. As in previous studies,
we find that LAEs begin to disappear rapidly when x_H > 0.5. Constraining
x_H(z=9) with luminosity functions is difficult due to the many uncertainties
inherent in the host halo mass Lya luminosity mapping. However, using a
very conservative mapping, we show that the number densities derived using the
six z~9 LAEs recently discovered by Stark et al. (2007) imply x_H < 0.7. On a
more fundamental level, these LAE number densities, if genuine, require
substantial star formation in halos with M < 10^9 solar masses, making them
unique among the current sample of observed high-z objects. Furthermore,
reionization increases the apparent clustering of the observed LAEs. We show
that a ``counts-in-cell'' statistic is a powerful probe of this effect,
especially in the early stages of reionization. Specifically, we show that a
field of view (typical of upcoming IR instruments) containing LAEs has >10%
higher probability of containing more than one LAE in a x_H>0.5 universe than a
x_H=0 universe with the same overall number density. With this statistic, a
fully ionized universe can be robustly distinguished from one with x_H > 0.5
using a survey containing only ~ 20--100 galaxies.Comment: 14 pages, 13 figures, moderate changes to match version accepted for
publication in the MNRA
High-redshift voids in the excursion set formalism
Voids are a dominant feature of the low-redshift galaxy distribution. Several
recent surveys have found evidence for the existence of large-scale structure
at high redshifts as well. We present analytic estimates of galaxy void sizes
at redshifts z ~ 5 - 10 using the excursion set formalism. We find that recent
narrow-band surveys at z ~ 5 - 6.5 should find voids with characteristic scales
of roughly 20 comoving Mpc and maximum diameters approaching 40 Mpc. This is
consistent with existing surveys, but a precise comparison is difficult because
of the relatively small volumes probed so far. At z ~ 7 - 10, we expect
characteristic void scales of ~ 14 - 20 comoving Mpc assuming that all galaxies
within dark matter haloes more massive than 10^10 M_sun are observable. We find
that these characteristic scales are similar to the sizes of empty regions
resulting from purely random fluctuations in the galaxy counts. As a result,
true large-scale structure will be difficult to observe at z ~ 7 - 10, unless
galaxies in haloes with masses less than ~ 10^9 M_sun are visible. Galaxy
surveys must be deep and only the largest voids will provide meaningful
information. Our model provides a convenient picture for estimating the
"worst-case" effects of cosmic variance on high-redshift galaxy surveys with
limited volumes.Comment: 12 pages, 9 figures, 1 table, accepted by MNRA
Constraints on the Star Formation Efficiency of Galaxies During the Epoch of Reionization
Reionization is thought to have occurred in the redshift range of , which is now being probed by both deep galaxy surveys and CMB observations.
Using halo abundance matching over the redshift range and assuming
smooth, continuous gas accretion, we develop a model for the star formation
efficiency of dark matter halos at that matches the measured
galaxy luminosity functions at these redshifts. We find that peaks
at at halo masses --~M, in
qualitative agreement with its behavior at lower redshifts. We then investigate
the cosmic star formation histories and the corresponding models of
reionization for a range of extrapolations to small halo masses. We use a
variety of observations to further constrain the characteristics of the galaxy
populations, including the escape fraction of UV photons. Our approach provides
an empirically-calibrated, physically-motivated model for the properties of
star-forming galaxies sourcing the epoch of reionization. In the case where
star formation in low-mass halos is maximally efficient, an average escape
fraction can reproduce the optical depth reported by Planck, whereas
inefficient star formation in these halos requires either about twice as many
UV photons to escape, or an escape fraction that increases towards higher
redshifts. Our models also predict how future observations with JWST can
improve our understanding of these galaxy populations.Comment: 19 pages, 12 figures, accepted for publication in MNRAS, minor
modification
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
Extreme Galaxies During Reionization: Testing ISM and Disk Models
We test the ability of equilibrium galactic disk and one-zone interstellar
medium models to describe the physical and emission properties of quasar hosts,
submillimeter galaxies, and Lyman-alpha emitters at z>~6. The size, line
widths, star formation rates, black hole accretion rates, gas masses and
temperatures, and the relationships between these properties are all
well-described by our model, and we provide approximate fitting formulae for
comparison with future observations. However, comparing our carbon line
predictions to observations reveals differences between the ISM at low and high
redshifts. Our underestimate of the [CII] line emission indicates either higher
star formation efficiencies in high-redshift molecular clouds or less depletion
of metals into dust at fixed metallicity. Further, our over-prediction of the
CO(6-5)/CO(1-0) ratio suggests that molecular clouds in real high-redshift
galaxies have a lower turbulent Mach number and more subthermal CO(6-5)
emission than expected owing either to sizes smaller than the local Jeans mass
or to a pressure support mechanism other than turbulence.Comment: Accepted in MNRAS; 19 pages; 10 figures; 4 table
The Effect of Fluctuations on the Helium-Ionizing Background
Interpretation of He II Ly{\alpha} absorption spectra after the epoch of He
II reionization requires knowledge of the He II ionizing background. While past
work has modelled the evolution of the average background, the standard
cosmological radiative transfer technique assumes a uniform radiation field
despite the discrete nature of the (rare) bright quasars that dominate the
background. We implement a cosmological radiative transfer model that includes
the most recent constraints on the ionizing spectra and luminosity function of
quasars and the distribution of IGM absorbers. We also estimate, for the first
time, the effects of fluctuations on the evolving continuum opacity in two
ways: by incorporating the complete distribution of ionizing background
amplitudes into the standard approach, and by explicitly treating the quasars
as discrete -- but isolated -- sources. Our model results in a He II ionization
rate that evolves steeply with redshift, increasing by a factor ~2 from z=3.0
to z=2.5. This causes rapid evolution in the mean He II Ly{\alpha} optical
depth -- as recently observed -- without appealing to the reionization of He
II. The observed behaviour could instead result from rapid evolution in the
mean free path of ionizing photons as the helium in higher H I column density
absorbers becomes fully ionized.Comment: 14 pages, 9 figures. Accepted by MNRAS; significantly modified from
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