371 research outputs found
21-cm signatures of residual HI inside cosmic HII regions during reionization
We investigate the impact of sinks of ionizing radiation on the
reionization-era 21-cm signal, focusing on 1-point statistics. We consider
sinks in both the intergalactic medium and inside galaxies. At a fixed filling
factor of HII regions, sinks will have two main effects on the 21-cm
morphology: (i) as inhomogeneous absorbers of ionizing photons they result in
smaller and more widespread cosmic HII patches; and (ii) as reservoirs of
neutral gas they contribute a non-zero 21-cm signal in otherwise ionized
regions. Both effects damp the contrast between neutral and ionized patches
during reionization, making detection of the epoch of reionization with 21-cm
interferometry more challenging. Here we systematically investigate these
effects using the latest semi-numerical simulations. We find that sinks
dramatically suppress the peak in the redshift evolution of the variance,
corresponding to the midpoint of reionization. As previously predicted,
skewness changes sign at midpoint, but the fluctuations in the residual HI
suppress a late-time rise. Furthermore, large levels of residual HI
dramatically alter the evolution of the variance, skewness and power spectrum
from that seen at lower levels. In general, the evolution of the large-scale
modes provides a better, cleaner, higher signal-to-noise probe of reionization.Comment: Minor edits to agree with MNRAS published versio
The spin-temperature dependence of the 21-cm-LAE cross-correlation
Cross-correlating 21 cm with known cosmic signals will be invaluable proof of the cosmic origin of the first 21-cm detections. As some of the widest fields available, comprising thousands of sources with reasonably known redshifts, narrow-band Lyman-α emitter (LAE) surveys are an obvious choice for such cross-correlation. Here, we revisit the 21-cm-LAE cross-correlation, relaxing the common assumption of reionization occurring in a pre-heated intergalactic medium (IGM). Using specifications from the Square Kilometre Array and the Subaru Hyper Supreme-Cam, we present new forecasts of the 21-cm-LAE cross-correlation function at z ∼7. We sample a broad parameter space of the mean IGM neutral fraction and spin temperature, (arx-H small I,arT-S). The sign of the cross-correlation roughly follows the sign of the 21-cm signal: Ionized regions that surround LAEs correspond to relative hot spots in the 21-cm signal when the neutral IGM is colder than the CMB, and relative cold spots when the neutral IGM is hotter than the CMB. The amplitude of the cross-correlation function generally increases with increasingarx-H small I, following the increasing bias of the cosmic H ii regions. As is the case for 21 cm, the strongest cross signal occurs when the IGM is colder than the CMB, providing a large contrast between the neutral regions and the ionized regions, which host LAEs. We also vary the topology of reionization and the epoch of X-ray heating. The cross-correlation during the first half of reionization is sensitive to these topologies, and could thus be used to constrain them.Cross-correlating 21 cm with known cosmic signals will be invaluable proof of the cosmic origin of the first 21-cm detections. As some of the widest fields available, comprising thousands of sources with reasonably known redshifts, narrow-band Lyman-α emitter (LAE) surveys are an obvious choice for such cross-correlation. Here, we revisit the 21-cm-LAE cross-correlation, relaxing the common assumption of reionization occurring in a pre-heated intergalactic medium (IGM). Using specifications from the Square Kilometre Array and the Subaru Hyper Supreme-Cam, we present new forecasts of the 21-cm-LAE cross-correlation function at z ∼7. We sample a broad parameter space of the mean IGM neutral fraction and spin temperature, (\barx-H small I,\barT-S). The sign of the cross-correlation roughly follows the sign of the 21-cm signal: Ionized regions that surround LAEs correspond to relative hot spots in the 21-cm signal when the neutral IGM is colder than the CMB, and relative cold spots when the neutral IGM is hotter than the CMB. The amplitude of the cross-correlation function generally increases with increasing\barx-H small I, following the increasing bias of the cosmic H ii regions. As is the case for 21 cm, the strongest cross signal occurs when the IGM is colder than the CMB, providing a large contrast between the neutral regions and the ionized regions, which host LAEs. We also vary the topology of reionization and the epoch of X-ray heating. The cross-correlation during the first half of reionization is sensitive to these topologies, and could thus be used to constrain them
The Effect of Absorption Systems on Cosmic Reionization
We use large-scale simulations to investigate the morphology of reionization
during the final, overlap phase. Our method uses an efficient three-dimensional
smoothing technique which takes into account the finite mean free path due to
absorption systems, lambda, by only smoothing over scales R_s<lambda. The large
dynamic range of our calculations is necessary to resolve the neutral patches
left at the end of reionization within a representative volume; we find that
simulation volumes exceeding several hundred Mpc on a side are necessary in
order to properly model reionization when the neutral fraction is ~0.01-0.3.
Our results indicate a strong dependence of percolation morphology on a large
and uncertain region of model parameter space. The single most important
parameter is the mean free path to absorption systems, which serve as opaque
barriers to ionizing radiation. If these absorption systems were as abundant as
some realistic estimates indicate, the spatial structure of the overlap phase
is considerably more complex than previously predicted. In view of the lack of
constraints on the mean free path at the highest redshifts, current theories
that do not include absorption by Lyman-limit systems, and in particular
three-dimensional simulations, may underestimate the abundance of neutral
clouds at the end of reionization. This affects predictions for the 21 cm
signal associated with reionization, interpretation of absorption features in
quasar spectra at z ~5-6, the connection between reionization and the local
universe, and constraints on the patchiness and duration of reionization from
temperature fluctuations measured in the cosmic microwave background arising
from the kinetic Sunyaev-Zel'dovich effect.Comment: Accepted for publication in the Astrophysical Journal. Substantial
revision from previous version. Comments welcom
Constraints on warm dark matter from UV luminosity functions of high-z galaxies with Bayesian model comparison
The number density of small dark matter (DM) haloes hosting faint high-redshift galaxies is sensitive to the DM free-streaming properties. However, constraining these DM properties is complicated by degeneracies with the uncertain baryonic physics governing star formation. In this work, we use a flexible astrophysical model and a Bayesian inference framework to analyse ultraviolet (UV) luminosity functions (LFs) at z = 6-8. We vary the complexity of the astrophysical galaxy model (single versus double power law for the stellar-halo mass relation) as well as the matter power spectrum [cold DM versus thermal relic warm DM (WDM)], comparing their Bayesian evidences. Adopting a conservatively wide prior range for the WDM particle mass, we show that the UV LFs at z = 6-8 only weakly favour cold DM over WDM. We find that particle masses of ≤ 2 keV are rejected at a 95 per cent credible level in all models that have a WDM-like power spectrum cutoff. This bound should increase to ∼2.5 keV with the James Webb Space Telescope (JWST)
Epoch of reionization parameter estimation with the 21-cm bispectrum
We present the first application of the isosceles bispectrum to MCMC
parameter inference from the cosmic 21-cm signal. We extend the MCMC sampler
21cmMC to use the fast bispectrum code, BiFFT, when computing the likelihood.
We create mock 1000h observations with SKA1-low, using PyObs21 to account for
uv-sampling and thermal noise. Assuming the spin temperature is much higher
than that of the CMB, we consider two different reionization histories for our
mock observations: fiducial and late-reionization. For both models we find that
bias on the inferred parameter means and 1- credible intervals can be
substantially reduced by using the isosceles bispectrum (calculated for a wide
range of scales and triangle shapes) together with the power spectrum (as
opposed to just using one of the statistics). We find that making the
simplifying assumption of a Gaussian likelihood with a diagonal covariance
matrix does not notably bias parameter constraints for the three-parameter
reionization model and basic instrumental effects considered here. This is true
even if we use extreme (unlikely) initial conditions which would be expected to
amplify biases. We also find that using the cosmic variance error calculated
with Monte-Carlo simulations using the fiducial model parameters whilst
assuming the late-reionization model for the simulated data also does not
strongly bias the inference. This implies we may be able to sparsely sample and
interpolate the cosmic variance error over the parameter space, substantially
reducing computational costs. All codes used in this work are
publicly-available.Comment: 12 pages, 11 figures (submitted to MNRAS
Interpreting the Transmission Windows of Distant Quasars
We propose the Apparent Shrinking Criterion (ASC) to interpret the spatial
extent, R_w, of transmitted flux windows in the absorption spectra of high-z
quasars. The ASC can discriminate between the two regimes in which R_w
corresponds either to the physical size, R_HII, of the quasar HII region, or to
the distance, R^{max}_w, at which the transmitted flux drops to =0.1 and a
Gunn-Peterson (GP) trough appears. In the first case (HR regime), one can
determine the IGM mean HI fraction, x_HI; in the second (PR regime), the value
of R_w allows to measure the local photoionization rate and the local
enhancement of the photoionization rate, Gamma_G, due to nearby/intervening
galaxies. The ASC has been tested against radiative transfer+SPH numerical
simulations, and applied to 15 high-z (z>5.8) quasars sample from Fan et al.
(2006). All sample quasars are found to be in the PR regime; hence, their
observed spectral properties (inner flux profile, extent of transmission
window) cannot reliably constrain the value of x_HI. Four sample quasars show
evidence for a local enhancement (up to 50%) in the local photoionization rate
possibly produced by a galaxy overdensity. We discuss the possible
interpretations and uncertainties of this result.Comment: 10 pages, 2 figures, accepted for publication in MNRA
Beacons into the Cosmic Dark Ages: Boosted transmission of Ly from UV bright galaxies at
Recent detections of Lyman alpha (Ly) emission from galaxies
were somewhat unexpected given a dearth of previous non-detections in this era
when the intergalactic medium (IGM) is still highly neutral. But these
detections were from UV bright galaxies, which preferentially live in
overdensities which reionize early, and have significantly Doppler-shifted
Ly line profiles emerging from their interstellar media (ISM), making
them less affected by the global IGM state. Using a combination of reionization
simulations and empirical ISM models we show, as a result of these two effects,
UV bright galaxies in overdensities have higher transmission through
the IGM than typical field galaxies, and this boosted transmission is
enhanced as the neutral fraction increases. The boosted transmission is not
sufficient to explain the observed high Ly fraction of galaxies (Stark et al. 2017), suggesting Ly emitted by
these galaxies must be stronger than expected due to enhanced production and/or
selection effects. Despite the bias of UV bright galaxies to reside in
overdensities we show Ly observations of such galaxies can accurately
measure the global neutral hydrogen fraction, particularly when Ly from
UV faint galaxies is extinguished, making them ideal candidates for
spectroscopic follow-up into the cosmic Dark Ages.Comment: 6 pages, 5 figures. Accepted for publication in ApJ
Reionization and galaxy inference from the high-redshift Ly α forest
The transmission of Lyman α (Ly α) in the spectra of distant quasars depends on the density, temperature, and ionization state of the intergalactic medium. Therefore, high-redshift (z > 5) Ly α forests could be invaluable in studying the late stages of the epoch of reionization (EoR), as well as properties of the sources that drive it. Indeed, high-quality quasar spectra have now firmly established the existence of large-scale opacity fluctuations at z > 5, whose physical origins are still debated. Here, we introduce a Bayesian framework capable of constraining the EoR and galaxy properties by forward-modelling the high-z Ly α forest. Using priors from galaxy and cosmic microwave background observations, we demonstrate that the final overlap stages of the EoR (when >95 per cent of the volume was ionized) should occur at z < 5.6, in order to reproduce the large-scale opacity fluctuations seen in forest spectra. However, it is the combination of patchy reionization and the inhomogeneous ultraviolet background that produces the longest Gunn-Peterson troughs. Ly α forest observations tighten existing constraints on the characteristic ionizing escape fraction of galaxies, with the combined observations suggesting fesc ≈ 7+4-3} per cent, and disfavouring a strong evolution with the galaxy's halo (or stellar) mass
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