3,870 research outputs found
The 21 Centimeter Forest
We examine the prospects for studying the pre-reionization intergalactic
medium (IGM) through the so-called 21 cm forest in spectra of bright
high-redshift radio sources. We first compute the evolution of the mean optical
depth for models that include X-ray heating of the IGM gas, Wouthuysen-Field
coupling, and reionization. Under most circumstances, the spin temperature T_S
grows large well before reionization begins in earnest. As a result, the
optical depth is less than 0.001 throughout most of reionization, and
background sources must sit well beyond the reionization surface in order to
experience measurable absorption. HII regions produce relatively large
"transmission gaps" and may therefore still be observable during the early
stages of reionization. Absorption from sheets and filaments in the cosmic web
fades once T_S becomes large and should be rare during reionization. Minihalos
can produce strong (albeit narrow) absorption features. Measuring their
abundance would yield useful limits on the strength of feedback processes in
the IGM as well as their effect on reionization.Comment: 9 pages, 5 figures, submitted to MNRA
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
Detecting the redshifted 21cm forest during reionization
The 21cm forest -- HI absorption features in the spectra of high-redshift
radio sources -- can potentially provide a unique probe of the largely neutral
intergalactic medium (IGM) during the epoch of reionization. We present
simulations of the 21cm forest due to the large scale structure of the
reionization-era IGM, including a prescription for x-ray heating and the
percolation of photoionization bubbles. We show that, if detected with future
instruments such as the Square Kilometer Array (SKA), the 21cm forest can
provide a significant constraint on the thermal history of the IGM. Detection
will be aided by consideration of the sudden increase in signal variance at the
onset of 21cm absorption. If radio foregrounds and the intrinsic source spectra
are well understood, the flux decrement over wide bandwidths can also improve
detection prospects. Our analysis accounts for the possibility of narrow
absorption lines from intervening dense regions, but, unlike previous studies,
our results do not depend on their properties. Assuming x-ray heating
corresponding to a local stellar population, we estimate that a statistically
significant detection of 21cm absorption could be made by SKA in less than a
year of observing against a Cygnus A-type source at , as opposed to
nearly a decade for a significant detection of the detailed forest features. We
discuss observational challenges due to uncertainties regarding the abundance
of background sources and the strength of the 21cm absorption signal.Comment: Submitted to MNRAS. Revised version includes updated and extended
calculations, some corrections and added reference
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
Contribution of Cross-Correlations to the 21cm Angular Power Spectrum in the Epoch of Reionization
Measurement of the 21cm hyperfine transition of neutral hydrogen provides a
unique probe of the epoch of reionization and the Dark Ages. Three major
mechanisms are believed to dominate the radiation process: emission from
neutral hydrogen surrounding the ionized bubbles of first galaxies and/or
quasars, emission from neutral hydrogen inside minihalos, and absorption of
diffuse neutral hydrogen against the cosmic microwave background. In the
present work, by simply combining the existing analytic models for the three
mechanisms, we investigate the contribution of cross-correlation between these
three components to the total 21cm angular power spectrum, in the sense that
neutral hydrogen associated with different radiation processes traces the
large-scale structures of underlying density perturbations. While the overall
21cm power spectrum remains almost unchanged with the inclusion of the
cross-correlations, the cross-correlation may play a key role in the
determination of the 21cm power spectrum during the transition of 21cm
radiation from emission-dominated phase to absorption-dominated phase at
redshift z~20. A significant suppression in the 21cm angular power spectrum
during this transition is anticipated as the result of negative contribution of
the cross-correlation between the absorption of diffuse neutral hydrogen and
the emission components. Therefore, an accurate prediction of the cosmic 21cm
power spectrum should take the cross-correlation into account especially at the
transition phase.Comment: 10 pages, 4 figures, accepted for publication in 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
Spin Exchange Rates in Electron-Hydrogen Collisions
The spin temperature of neutral hydrogen, which determines the 21 cm optical
depth and brightness temperature, is set by the competition between radiative
and collisional processes. In the high-redshift intergalactic medium, the
dominant collisions are typically those between hydrogen atoms. However,
collisions with electrons couple much more efficiently to the spin state of
hydrogen than do collisions with other hydrogen atoms and thus become important
once the ionized fraction exceeds ~1%. Here we compute the rate at which
electron-hydrogen collisions change the hydrogen spin. Previous calculations
included only S-wave scattering and ignored resonances near the n=2 threshold.
We provide accurate results, including all partial wave terms through the
F-wave, for the de-excitation rate at temperatures T_K < 15,000 K; beyond that
point, excitation to n>=2 hydrogen levels becomes significant. Accurate
electron-hydrogen collision rates at higher temperatures are not necessary,
because collisional excitation in this regime inevitably produces Lyman-alpha
photons, which in turn dominate spin exchange when T_K > 6200 K even in the
absence of radiative sources. Our rates differ from previous calculations by
several percent over the temperature range of interest. We also consider some
simple astrophysical examples where our spin de-excitation rates are useful.Comment: submitted to MNRAS, 9 pages, 5 figure
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
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