578 research outputs found
Lyman-alpha Damping Wing Constraints on Inhomogeneous Reionization
One well-known way to constrain the hydrogen neutral fraction, x_H, of the
high-redshift intergalactic medium (IGM) is through the shape of the red
damping wing of the Lya absorption line. We examine this method's effectiveness
in light of recent models showing that the IGM neutral fraction is highly
inhomogeneous on large scales during reionization. Using both analytic models
and "semi-numeric" simulations, we show that the "picket-fence" absorption
typical in reionization models introduces both scatter and a systematic bias to
the measurement of x_H. In particular, we show that simple fits to the damping
wing tend to overestimate the true neutral fraction in a partially ionized
universe, with a fractional error of ~ 30% near the middle of reionization.
This bias is generic to any inhomogeneous model. However, the bias is reduced
and can even underestimate x_H if the observational sample only probes a subset
of the entire halo population, such as quasars with large HII regions. We also
find that the damping wing absorption profile is generally steeper than one
would naively expect in a homogeneously ionized universe. The profile steepens
and the sightline-to-sightline scatter increases as reionization progresses. Of
course, the bias and scatter also depend on x_H and so can, at least in
principle, be used to constrain it. Damping wing constraints must therefore be
interpreted by comparison to theoretical models of inhomogeneous reionization.Comment: 11 pages, 10 figures; submitted to MNRA
Radio Recombination Lines at Decametre Wavelengths: Prospects for the Future
This paper considers the suitability of a number of emerging and future
instruments for the study of radio recombination lines (RRLs) at frequencies
below 200 MHz. These lines arise only in low-density regions of the ionized
interstellar medium, and they may represent a frequency-dependent foreground
for next-generation experiments trying to detect H I signals from the Epoch of
Reionization and Dark Ages ("21-cm cosmology"). We summarize existing
decametre-wavelength observations of RRLs, which have detected only carbon
RRLs. We then show that, for an interferometric array, the primary instrumental
factor limiting detection and study of the RRLs is the areal filling factor of
the array. We consider the Long Wavelength Array (LWA-1), the LOw Frequency
ARray (LOFAR), the low-frequency component of the Square Kilometre Array
(SKA-lo), and a future Lunar Radio Array (LRA), all of which will operate at
decametre wavelengths. These arrays offer digital signal processing, which
should produce more stable and better defined spectral bandpasses; larger
frequency tuning ranges; and better angular resolution than that of the
previous generation of instruments that have been used in the past for RRL
observations. Detecting Galactic carbon RRLs, with optical depths at the level
of 10^-3, appears feasible for all of these arrays, with integration times of
no more than 100 hr. The SKA-lo and LRA, and the LWA-1 and LOFAR at the lowest
frequencies, should have a high enough filling factor to detect lines with much
lower optical depths, of order 10^-4 in a few hundred hours. The amount of
RRL-hosting gas present in the Galaxy at the high Galactic latitudes likely to
be targeted in 21-cm cosmology studies is currently unknown. If present,
however, the spectral fluctuations from RRLs could be comparable to or exceed
the anticipated H I signals.Comment: 9 pages; Astron. & Astrophys., in pres
Current models of the observable consequences of cosmic reionization and their detectability
A number of large current experiments aim to detect the signatures of the cosmic reionization at redshifts z > 6. Their success depends crucially on understanding the character of the reionization process and its observable consequences and designing the best strategies to use. We use large-scale simulations of cosmic reionization to evaluate the reionization signatures at redshifted 21-cm and small-scale cosmic microwave background (CMB) anisotropies in the best current model for the background universe, with fundamental cosmological parameters given by Wilkinson Microwave Anisotropy Probe three-year results. We find that the optimal frequency range for observing the `global step of the 21-cm emission is 120150 MHz, while statistical studies should aim at 140160 MHz, observable by GMRT. Some strongly non-Gaussian brightness features should be detectable at frequencies up to ~190 MHz. In terms of sensitivity-signal trade-off relatively low resolutions, corresponding to beams of at least a few arcminutes, are preferable. The CMB anisotropy signal from the kinetic SunyaevZel'dovich effect from reionized patches peaks at tens of K at arcminute scales and has an rms of ~1 K, and should be observable by the Atacama Cosmology Telescope and the South Pole Telescope. We discuss the various observational issues and the uncertainties involved, mostly related to the poorly known reionization parameters and, to a lesser extend, to the uncertainties in the background cosmology
The 21-cm Background from the Cosmic Dark Ages: Minihalos and the Intergalactic Medium before Reionization
The H atoms inside minihalos (i.e. halos with virial temperatures T_vir <
10^4 K, in the mass range roughly from 10^4 M_sun to 10^8 M_sun) during the
cosmic dark ages in a LCDM universe produce a redshifted background of
collisionally-pumped 21-cm line radiation which can be seen in emission
relative to the cosmic microwave background (CMB). Previously, we used
semi-analytical calculations of the 21-cm signal from individual halos of
different mass and redshift and the evolving mass function of minihalos to
predict the mean brightness temperature of this 21-cm background and its
angular fluctuations. Here we use high-resolution cosmological N-body and
hydrodynamic simulations of structure formation at high redshift (z > 8) to
compute the mean brightness temperature of this background from both minihalos
and the intergalactic medium (IGM) prior to the onset of Ly-alpha radiative
pumping. We find that the 21-cm signal from gas in collapsed, virialized
minihalos dominates over that from the diffuse shocked gas in the IGM.Comment: 8 pages, 5 figures. To appear in proceedings of UC Irvine May 2005
workshop on "First Light & Reionization", eds. E. Barton & A. Cooray, New
Astronomy Reviews, in pres
Measuring the History of Cosmic Reionization using the 21-cm PDF from Simulations
The 21-cm PDF (i.e., distribution of pixel brightness temperatures) is
expected to be highly non-Gaussian during reionization and to provide important
information on the distribution of density and ionization. We measure the 21-cm
PDF as a function of redshift in a large simulation of cosmic reionization and
propose a simple empirical fit. Guided by the simulated PDF, we then carry out
a maximum likelihood analysis of the ability of upcoming experiments to measure
the shape of the 21-cm PDF and derive from it the cosmic reionization history.
Under the strongest assumptions, we find that upcoming experiments can measure
the reionization history in the mid to late stages of reionization to 1-10%
accuracy. Under a more flexible approach that allows for four free parameters
at each redshift, a similar accuracy requires the lower noise levels of
second-generation 21-cm experiments.Comment: 13 pages, 16 figures, submitted to MNRA
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
Is Double Reionization Physically Plausible?
Recent observations of z~6 quasars and the cosmic microwave background imply
a complex history to cosmic reionization. Such a history requires some form of
feedback to extend reionization over a long time interval, but the nature of
the feedback and how rapidly it operates remain highly uncertain. Here we focus
on one aspect of this complexity: which physical processes can cause the global
ionized fraction to evolve non-monotonically with cosmic time? We consider a
range of mechanisms and conclude that double reionization is much less likely
than a long, but still monotonic, ionization history. We first examine how
galactic winds affect the transition from metal-free to normal star formation.
Because the transition is actually spatially inhomogeneous and temporally
extended, this mechanism cannot be responsible for double reionization given
plausible parameters for the winds. We next consider photoheating, which causes
the cosmological Jeans mass to increase in ionized regions and hence suppresses
galaxy formation there. In this case, double reionization requires that small
halos form stars efficiently, that the suppression from photoheating is strong
relative to current expectations, and that ionizing photons are preferentially
produced outside of previously ionized regions. Finally, we consider H_2
photodissociation, in which the buildup of a soft ultraviolet background
suppresses star formation in small halos. This can in principle cause the
ionized fraction to temporarily decrease, but only during the earliest stages
of reionization. Finally, we briefly consider the effects of some of these
feedback mechanisms on the topology of reionization.Comment: 13 pages, 5 figures, in press at ApJ (reorganized significantly but
major conclusions unchanged
Possible evidence for an inverted temperature-density relation in the intergalactic medium from the flux distribution of the Lyman-alpha forest
We compare the improved measurement of the Lya forest flux probability
distribution at 1.7<z<3.2 presented by Kim et al. (2007) to a large set of
hydrodynamical simulations of the Lya forest with different cosmological
parameters and thermal histories. The simulations are in good agreement with
the observational data if the temperature-density relation for the low density
intergalactic medium (IGM), T=T_0 Delta^{gamma-1}, is either close to
isothermal or inverted (gamma<1). Our results suggest that the voids in the IGM
may be significantly hotter and the thermal state of the low density IGM may be
substantially more complex than is usually assumed at these redshifts. We
discuss radiative transfer effects which alter the spectral shape of ionising
radiation during the epoch of HeII reionisation as a possible physical
mechanism for achieving an inverted temperature-density relation at z~3.Comment: 16 pages, 6 figures, accepted for publication in MNRAS following
minor revision. The accepted version includes an expanded discussion of the
flux power spectru
Efficient Simulations of Early Structure Formation and Reionization
We present a method to construct semi-numerical ``simulations'', which can
efficiently generate realizations of halo distributions and ionization maps at
high redshifts. Our procedure combines an excursion-set approach with
first-order Lagrangian perturbation theory and operates directly on the linear
density and velocity fields. As such, the achievable dynamic range with our
algorithm surpasses the current practical limit of N-body codes by orders of
magnitude. This is particularly significant in studies of reionization, where
the dynamic range is the principal limiting factor. We test our halo-finding
and HII bubble-finding algorithms independently against N-body simulations with
radiative transfer and obtain excellent agreement. We compute the size
distributions of ionized and neutral regions in our maps. We find even larger
ionized bubbles than do purely analytic models at the same volume-weighted mean
hydrogen neutral fraction. We also generate maps and power spectra of 21-cm
brightness temperature fluctuations, which for the first time include
corrections due to gas bulk velocities. We find that velocities widen the tails
of the temperature distributions and increase small-scale power, though these
effects quickly diminish as reionization progresses. We also include some
preliminary results from a simulation run with the largest dynamic range to
date: a 250 Mpc box that resolves halos with masses M >~ 2.2 x10^8 M_sun. We
show that accurately modeling the late stages of reionization requires such
large scales. The speed and dynamic range provided by our semi-numerical
approach will be extremely useful in the modeling of early structure formation
and reionization.Comment: 13 pages, 10 figures; ApJ submitte
Reionization: Characteristic Scales, Topology and Observability
Recently the numerical simulations of the process of reionization of the
universe at z>6 have made a qualitative leap forward, reaching sufficient sizes
and dynamic range to determine the characteristic scales of this process. This
allowed making the first realistic predictions for a variety of observational
signatures. We discuss recent results from large-scale radiative transfer and
structure formation simulations on the observability of high-redshift Ly-alpha
sources. We also briefly discuss the dependence of the characteristic scales
and topology of the ionized and neutral patches on the reionization parameters.Comment: 4 pages, 5 figures (4 in color), to appear in Astronomy and Space
Science special issue "Space Astronomy: The UV window to the Universe",
proceedings of 1st NUVA Conference ``Space Astronomy: The UV window to the
Universe'' in El Escorial (Spain
- âŠ