600 research outputs found
Detecting the Rise and Fall of 21 cm Fluctuations with the Murchison Widefield Array
We forecast the sensitivity with which the Murchison Widefield Array (MWA)
can measure the 21 cm power spectrum of cosmic hydrogen, using radiative
transfer simulations to model reionization and the 21 cm signal. The MWA is
sensitive to roughly a decade in scale (wavenumbers of k ~ 0.1 - 1 h Mpc^{-1}),
with foreground contamination precluding measurements on larger scales, and
thermal detector noise limiting the small scale sensitivity. This amounts
primarily to constraints on two numbers: the amplitude and slope of the 21 cm
power spectrum on the scales probed. We find, however, that the redshift
evolution in these quantities can yield important information about
reionization. Although the power spectrum differs substantially across
plausible models, a generic prediction is that the amplitude of the 21 cm power
spectrum on MWA scales peaks near the epoch when the intergalactic medium (IGM)
is ~ 50% ionized. Moreover, the slope of the 21 cm power spectrum on MWA scales
flattens as the ionization fraction increases and the sizes of the HII regions
grow. Considering detection sensitivity, we show that the optimal MWA antenna
configuration for power spectrum measurements would pack all 500 antenna tiles
as close as possible in a compact core. The MWA is sensitive enough in its
optimal configuration to measure redshift evolution in the slope and amplitude
of the 21 cm power spectrum. Detecting the characteristic redshift evolution of
our models will confirm that observed 21 cm fluctuations originate from the
IGM, and not from foregrounds, and provide an indirect constraint on the
volume-filling factor of HII regions during reionization. After two years of
observations under favorable conditions, the MWA can constrain the filling
factor at an epoch when ~ 0.5 to within roughly +/- 0.1 at 2-sigma.Comment: 14 pages, 9 figures, submitted to Ap
A Direct Precision Measurement of the Intergalactic Lyman-alpha Opacity at 2<z<4.2
We directly measure the evolution of the intergalactic Lya effective optical
depth, tau_eff, over the redshift range 2<z<4.2 from a sample of 86
high-resolution, high-signal-to-noise quasar spectra obtained with the ESI and
HIRES spectrographs on Keck, and with the MIKE spectrograph on Magellan. This
represents an improvement over previous analyses of the Lya forest from
high-resolution spectra in this redshift interval of a factor of two in the
size of the data set alone. We pay particular attention to robust error
estimation and extensively test for systematic effects. We find that our
estimates of the quasar continuum levels in the Lya forest obtained by spline
fitting are systematically biased low, with the magnitude of the bias
increasing with redshift, but that this bias can be accounted for using mock
spectra. The mean fractional error is <1% at z=2, 4% at z=3, and 12% at z=4.
Previous measurements of tau_eff at z>~3 based on directly fitting the quasar
continua in the Lya forest, which have generally neglected this effect, are
therefore likely biased low. We provide estimates of the level of absorption
arising from metals in the Lya forest based on both direct and statistical
metal removal results in the literature, finding that this contribution is
~6-9% at z=3 and decreases monotonically with redshift. The high precision of
our measurement, attaining 3% in redshift bins of width Delta z=0.2 around z=3,
indicates significant departures from the best-fit power-law redshift evolution
(tau_eff=0.0018(1+z)^3.92, when metals are left in), particularly near z=3.2.
The observed downward departure is statistically consistent with a similar
feature detected in a precision statistical measurement using Sloan Digital Sky
Survey spectra by Bernardi and coworkers using an independent approach.Comment: 27 pages, including 18 figures, published in Ap
Gravitational Lensing as Signal and Noise in Lyman-alpha Forest Measurements
In Lyman-alpha forest measurements it is generally assumed that quasars are
mere background light sources which are uncorrelated with the forest.
Gravitational lensing of the quasars violates this assumption. This effect
leads to a measurement bias, but more interestingly it provides a valuable
signal. The lensing signal can be extracted by correlating quasar magnitudes
with the flux power spectrum and with the flux decrement. These correlations
will be challenging to measure but their detection provides a direct measure of
how features in the Lyman-alpha forest trace the underlying mass density field.
Observing them will test the fundamental hypothesis that fluctuations in the
forest are predominantly driven by fluctuations in mass, rather than in the
ionizing background, helium reionization or winds. We discuss ways to
disentangle the lensing signal from other sources of such correlations,
including dust, continuum and background residuals. The lensing-induced
measurement bias arises from sample selection: one preferentially collects
spectra of magnified quasars which are behind overdense regions. This
measurement bias is ~0.1-1% for the flux power spectrum, optical depth and the
flux probability distribution. Since the effect is systematic, quantities such
as the amplitude of the flux power spectrum averaged across scales should be
interpreted with care.Comment: 22 pages, 8 figures; v2: references added, discussion expanded,
matches PRD accepted versio
The Impact of Temperature Fluctuations on the Lyman-alpha Forest Power Spectrum
We explore the impact of spatial fluctuations in the intergalactic medium
temperature on the Lyman-alpha forest flux power spectrum near z ~ 3. We
develop a semianalytic model to examine temperature fluctuations resulting from
inhomogeneous HI and incomplete HeII reionizations. Detection of these
fluctuations might provide insight into the reionization histories of hydrogen
and helium. Furthermore, these fluctuations, neglected in previous analyses,
could bias constraints on cosmological parameters from the Lyman-alpha forest.
We find that the temperature fluctuations resulting from inhomogeneous HI
reionization are likely to be very small, with an rms amplitude of < 5%,
. More important are the temperature fluctuations
that arise from incomplete HeII reionization, which might plausibly be as large
as 50%, . In practice, however, these temperature
fluctuations have only a small effect on flux power spectrum predictions. The
smallness of the effect is possibly due to density fluctuations dominating over
temperature fluctuations on the scales probed by current measurements. On the
largest scales currently probed, k ~ 0.001 s/km (~0.1 h/Mpc), the effect on the
flux power spectrum may be as large as ~10% in extreme models. The effect is
larger on small scales, up to ~20% at k = 0.1 s/km, due to thermal broadening.
Our results suggest that the omission of temperature fluctuations effects from
previous analyses does not significantly bias constraints on cosmological
parameters.Comment: 11 pages, 5 figures, ApJ accepte
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
The intergalactic medium thermal history at redshift z=1.7--3.2 from the Lyman alpha forest: a comparison of measurements using wavelets and the flux distribution
We investigate the thermal history of the intergalactic medium (IGM) in the
redshift interval z=1.7--3.2 by studying the small-scale fluctuations in the
Lyman alpha forest transmitted flux. We apply a wavelet filtering technique to
eighteen high resolution quasar spectra obtained with the Ultraviolet and
Visual Echelle Spectrograph (UVES), and compare these data to synthetic spectra
drawn from a suite of hydrodynamical simulations in which the IGM thermal state
and cosmological parameters are varied. From the wavelet analysis we obtain
estimates of the IGM thermal state that are in good agreement with other
recent, independent wavelet-based measurements. We also perform a reanalysis of
the same data set using the Lyman alpha forest flux probability distribution
function (PDF), which has previously been used to measure the IGM
temperature-density relation. This provides an important consistency test for
measurements of the IGM thermal state, as it enables a direct comparison of the
constraints obtained using these two different methodologies. We find the
constraints obtained from wavelets and the flux PDF are formally consistent
with each other, although in agreement with previous studies, the flux PDF
constraints favour an isothermal or inverted IGM temperature-density relation.
We also perform a joint analysis by combining our wavelet and flux PDF
measurements, constraining the IGM thermal state at z=2.1 to have a temperature
at mean density of T0/[10^3 K]=17.3 +/- 1.9 and a power-law temperature-density
relation exponent gamma=1.1 +/- 0.1 (1 sigma). Our results are consistent with
previous observations that indicate there may be additional sources of heating
in the IGM at z<4.Comment: 15 pages, 14 figures, matches version accepted for publication on
MNRA
Glimpsing through the high redshift neutral hydrogen fog
We analyze the transmitted flux in a sample of 17 QSOs spectra at
5.74<zem<6.42 to obtain tighter constraints on the volume-averaged neutral
hydrogen fraction, xHI, at z~6. We study separately the narrow transmission
windows (peaks) and the wide dark portions (gaps) in the observed absorption
spectra. By comparing the statistics of these spectral features with Lyalpha
forest simulations, we conclude that xHI evolves smoothly from 10^{-4.4} at
z=5.3 to 10^{-4.2} at z=5.6, with a robust upper limit xHI<0.36 at z=6.3. The
frequency and physical sizes of the peaks imply an origin in cosmic underdense
regions and/or in HII regions around faint quasars or galaxies. In one case
(the intervening HII region of the faint quasar RD J1148+5253 at z=5.70 along
the LOS of SDSS J1148+5251 at z=6.42) the increase of the peak spectral density
is explained by the first-ever detected transverse proximity effect in the HI
Lyalpha forest; this indicates that at least some peaks result from a locally
enhanced radiation field. We then obtain a strong lower limit on the foreground
QSO lifetime of tQ>11 Myr. The observed widths of the peaks are found to be
systematically larger than the simulated ones. Reasons for such discrepancy
might reside either in the photoionization equilibrium assumption or in
radiative transfer effects.Comment: 12 pages, 9 figures, revised to match the accepted version including
a detailed analysis of the foreground QSO redshift and of the relativistic
effects on the HII region shape; MNRAS in pres
Dwarf Galaxy Formation Was Suppressed By Cosmic Reionization
A large number of faint galaxies, born less than a billion years after the
big bang, have recently been discovered. The fluctuations in the distribution
of these galaxies contributed to a scatter in the ionization fraction of cosmic
hydrogen on scales of tens of Mpc, as observed along the lines of sight to the
earliest known quasars. Theoretical simulations predict that the formation of
dwarf galaxies should have been suppressed after cosmic hydrogen was reionized,
leading to a drop in the cosmic star formation rate. Here we present evidence
for this suppression. We show that the post-reionization galaxies which
produced most of the ionizing radiation at a redshift z~5.5, must have had a
mass in excess of ~10^{10.6+/-0.4} solar masses or else the aforementioned
scatter would have been smaller than observed. This limiting mass is two orders
of magnitude larger than the galaxy mass that is thought to have dominated the
reionization of cosmic hydrogen (~10^8 solar masses). We predict that future
surveys with space-based infrared telescopes will detect a population of
smaller galaxies that reionized the Universe at an earlier time, prior to the
epoch of dwarf galaxy suppression.Comment: 19 pages, 3 figures. Accepted for publication in Nature; press
embargo until publishe
Redshift Space Distortion of the 21cm Background from the Epoch of Reionization I: Methodology Re-examined
The peculiar velocity of the intergalactic gas responsible for the cosmic 21cm background from the epoch of reionization and beyond introduces an anisotropy in the three-dimensional power spectrum of brightness temperature fluctuations. Measurement of this anisotropy by future 21cm surveys is a promising tool for separating cosmology from 21cm astrophysics. However, previous attempts to model the signal have often neglected peculiar velocity or only approximated it crudely. This paper presents a detailed treatment of the effects of peculiar velocity on the 21cm signal. (1) We show that properly accounting for finite optical depth eliminates the unphysical divergence of 21cm brightness temperature in the IGM overdense regions found in previous work that employed the usual optically-thin approximation. (2) We show that previous attempts to circumvent this divergence by capping the velocity gradient result in significant errors in the power spectrum on all scales. (3) We further show that the observed power spectrum in redshift-space remains finite even in the optically-thin approximation if one properly accounts for the redshift-space distortion. However, results that take full account of finite optical depth show that this approximation is only accurate in the limit of high spin temperature. (4) We also show that the linear theory for redshift-space distortion results in a ~30% error in the power spectrum at the observationally relevant wavenumber range, at the 50% ionized epoch. (5) We describe and test two numerical schemes to calculate the 21cm signal from reionization simulations which accurately incorporate peculiar velocity in the optically-thin approximation. One is particle-based, the other grid-based, and while the former is most accurate, we demonstrate that the latter is computationally more efficient and can achieve sufficient accuracy. [Abridged
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