577 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
The Luminosity Dependence of Quasar Clustering
We investigate the luminosity dependence of quasar clustering, inspired by
numerical simulations of galaxy mergers that incorporate black hole growth.
These simulations have motivated a new interpretation of the quasar luminosity
function. In this picture, the bright end of the quasar luminosity function
consists of quasars radiating nearly at their peak luminosities, while the
faint end consists mainly of very similar sources, but at dimmer phases in
their evolution. We combine this model with the statistics of dark matter halos
that host quasar activity. We find that, since bright and faint quasars are
mostly similar sources seen in different evolutionary stages, a broad range in
quasar luminosities corresponds to only a narrow range in the masses of quasar
host halos. On average, bright and faint quasars reside in similar host halos.
Consequently, we argue that quasar clustering should depend only weakly on
luminosity. This prediction is in qualitative agreement with recent
measurements of the luminosity dependence of the quasar correlation function
(Croom et al. 2005) and the galaxy-quasar cross-correlation function
(Adelberger & Steidel 2005). Future precision clustering measurements from SDSS
and 2dF, spanning a large range in luminosity, should provide a strong test of
our model.Comment: 9 pages, 4 figures, submitted to Ap
The Line-of-Sight Proximity Effect and the Mass of Quasar Host Halos
We show that the Lyman-alpha optical depth statistics in the proximity
regions of quasar spectra depend on the mass of the dark matter halos hosting
the quasars. This is owing to both the overdensity around the quasars and the
associated infall of gas toward them. For a fiducial quasar host halo mass of
(3.0+/-1.6) h^-1 x 10^12 Msun, as inferred by Croom et al. from clustering in
the 2dF QSO Redshift Survey, we show that estimates of the ionizing background
(Gamma^bkg) from proximity effect measurements could be biased high by a factor
of ~2.5 at z=3 owing to neglecting these effects alone. The clustering of
galaxies and other active galactic nuclei around the proximity effect quasars
enhances the local background, but is not expected to skew measurements by more
than a few percent. Assuming the measurements of Gamma^bkg based on the mean
flux decrement in the Ly-alpha forest to be free of bias, we demonstrate how
the proximity effect analysis can be inverted to measure the mass of the dark
matter halos hosting quasars. In ideal conditions, such a measurement could be
made with a precision comparable to the best clustering constraints to date
from a modest sample of only about 100 spectra. We discuss observational
difficulties, including continuum flux estimation, quasar systematic redshift
determination, and quasar variability, which make accurate proximity effect
measurements challenging in practice. These are also likely to contribute to
the discrepancies between existing proximity effect and flux decrement
measurements of Gamma^bkg.Comment: 25 pages, including 14 figures, accepted by Ap
How neutral is the intergalactic medium surrounding the redshift z=7.085 quasar ULAS J1120+0641?
The quasar ULAS J1120+0641 at redshift z=7.085 has a highly ionised near zone
which is smaller than those around quasars of similar luminosity at z~6. The
spectrum also exhibits evidence for a damping wing extending redward of the
systemic Lya redshift. We use radiative transfer simulations in a cosmological
context to investigate the implications for the ionisation state of the
inhomogeneous IGM surrounding this quasar. Our simulations show that the
transmission profile is consistent with an IGM in the vicinity of the quasar
with a volume averaged HI fraction of f_HI>0.1 and that ULAS J1120+0641 has
been bright for 10^6--10^7 yr. The observed spectrum is also consistent with
smaller IGM neutral fractions, f_HI ~ 10^-3--10-4, if a damped Lya system in an
otherwise highly ionised IGM lies within 5 proper Mpc of the quasar. This is,
however, predicted to occur in only ~5 per cent of our simulated sight-lines
for a bright phase of 10^6--10^7 yr. Unless ULAS J1120+0641 grows during a
previous optically obscured phase, the low age inferred for the quasar adds to
the theoretical challenge of forming a 2x10^9 M_sol black hole at this high
redshift.Comment: 5 pages, 4 figures, accepted to MNRAS letter
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
Angular fluctuations in the CXB: Is Fe 6.4 keV line tomography of the large-scale structure feasible?
AGN are known to account for a major portion, if not all, of the cosmic X-ray
background radiation. The dominant sharp spectral feature in their spectra is
the 6.4 keV fluorescent line of iron, which may contribute to as much as ~ 5-10
% of the CXB spectral intensity at ~ 2-6 keV. Owing to cosmological redshift,
the line photons detected at the energy E carry information about objects
located at the redshift z=6.4/E-1. In particular, imprinted in their angular
fluctuations is the information about the large-scale structure at redshift z.
This opens the possibility of performing the Fe K_alpha line tomography of the
cosmic large-scale structure. We show that detection of the tomographic signal
at ~100 sigma confidence requires an all-sky survey by an instrument with an
effective area of ~10 m^2 and field of view of ~1 deg^2. The signal is
strongest for objects located at the redshift z~1 and at the angular scales
corresponding to l ~ 100-300, therefore an optimal detection can be achieved
with an instrument having a rather modest angular resolution of ~ 0.1-0.5 deg.
For such an instrument, the CCD-type energy resolution of ~ 100-200 eV FWHM is
entirely sufficient for the optimal separation of the signals coming from
different redshifts. The gain in the signal strength that could potentially be
achieved with energy resolution comparable to the line width is nullified by
the photon counting and AGN discreteness noise. Among the currently planned and
proposed missions, these requirements are best satisfied by LOFT, even though
it was proposed for an entirely different purpose. Among others, clear
detection should be achieved by WFXT (~ 20-35 sigma) and ATHENA (~ 10-20
sigma). eROSITA, in the course of its 4 years all-sky survey, will detect the
tomographic signal only marginally.Comment: minor additions, accepted for publication in A&
Reionization Bias in High Redshift Quasar Near-Zones
Absorption spectra of high redshift quasars exhibit an increasingly thick
Ly-alpha forest towards z~6. However, the interpretation of these spectra is
complicated by the fact that the Ly-alpha optical depth is already large for
neutral hydrogen fractions in excess of 10^-4, and also because quasars are
expected to reside in dense regions of the IGM. We present a model for the
evolution of the ionization state of the IGM which is applicable to the dense,
biased regions around high-redshift quasars as well as more typical regions in
the IGM, and combine this with numerical radiative transfer simulations. Our
model is able to simultaneously reproduce the observed Ly-alpha forest opacity
at 4<z<6, the ionizing photon mean-free-path at z~4 and the rapid evolution of
highly ionized near-zone sizes around high-redshift quasars at 5.8<z<6.4. We
find that within 5 physical Mpc of a high redshift quasar, the evolution of the
ionization state of the IGM precedes that in more typical regions by around 0.3
redshift units. More importantly, when combined with the rapid increase in the
ionizing photon mean-free-path expected shortly after overlap, this offset
results in an ionizing background near the quasar which exceeds the value in
the rest of the IGM by a factor of ~2-3. We further find that in the
post-overlap phase of reionization the size of the observed quasar near-zones
is not directly sensitive to the neutral hydrogen fraction of the IGM. Instead,
these sizes probe the level of the background ionization rate and the
temperature of the surrounding IGM. The observed rapid evolution of the quasar
near-zone sizes at 5.8<z<6.4 can thus be explained by the rapid evolution of
the ionizing background, which in our model is caused by the completion of
overlap at the end of reionization by 6<z<7.Comment: 16 Pages, 9 figures. Submitted for publication to MNRA
The effect of neutrinos on the matter distribution as probed by the Intergalactic Medium
We present a suite of full hydrodynamical cosmological simulations that
quantitatively address the impact of neutrinos on the (mildly non-linear)
spatial distribution of matter and in particular on the neutral hydrogen
distribution in the Intergalactic Medium (IGM), which is responsible for the
intervening Lyman-alpha absorption in quasar spectra. The free-streaming of
neutrinos results in a (non-linear) scale-dependent suppression of power
spectrum of the total matter distribution at scales probed by Lyman-alpha
forest data which is larger than the linear theory prediction by about 25% and
strongly redshift dependent. By extracting a set of realistic mock quasar
spectra, we quantify the effect of neutrinos on the flux probability
distribution function and flux power spectrum. The differences in the matter
power spectra translate into a ~2.5% (5%) difference in the flux power spectrum
for neutrino masses with Sigma m_{\nu} = 0.3 eV (0.6 eV). This rather small
effect is difficult to detect from present Lyman-alpha forest data and nearly
perfectly degenerate with the overall amplitude of the matter power spectrum as
characterised by sigma_8. If the results of the numerical simulations are
normalized to have the same sigma_8 in the initial conditions, then neutrinos
produce a smaller suppression in the flux power of about 3% (5%) for Sigma
m_{\nu} = 0.6 eV (2
sigma C.L.), comparable to constraints obtained from the cosmic microwave
background data or other large scale structure probes.Comment: 38 pages, 21 figures. One section and references added. JCAP in pres
Detection of Extended He II Reionization in the Temperature Evolution of the Intergalactic Medium
We present new measurements of the temperature of the intergalactic medium
(IGM) derived from the Lyman-alpha forest over 2.0 < z < 4.8. The small-scale
structure in the forest of 61 high-resolution QSO spectra is quantified using a
new statistic, the curvature, and the conversion to temperature calibrated
using a suite of hydrodynamic simulations. At each redshift we focus on
obtaining the temperature at an optimal overdensity probed by the Lyman-alpha
forest, T(Delta), where the temperature is nearly a one-to-one function of the
curvature regardless of the slope of the temperature-density relation. The
median 2-sigma statistical uncertainty in these measurements is 8 per cent,
though there may be comparable systematic errors due to the unknown amount of
Jeans smoothing in the IGM. We use our T(Delta) results to infer the
temperature at the mean density, T0. Even for a maximally steep
temperature-density relation, T0 must increase from ~8000 K at z ~ 4.4 to
>~12000 K at z ~ 2.8. This increase is not consistent with the monotonic
decline in T0 expected in the absence of He II reionization. We therefore
interpret the observed rise in temperature as evidence of He II reionization
beginning at z >~ 4.4. The evolution of T0 is consistent with an end to He II
reionization at z ~ 3, as suggested by opacity measurements of the He II
Lyman-alpha forest, although the redshift at which T0 peaks will depend
somewhat on the evolution of the temperature-density relation. These new
temperature measurements suggest that the heat input due to the reionization of
He II dominates the thermal balance of the IGM over an extended period with
Delta_z >~ 1.Comment: 19 pages, 15 figures, accepted to MNRA
Intensity Mapping of Lyman-alpha Emission During the Epoch of Reionization
We calculate the absolute intensity and anisotropies of the Lyman-alpha
radiation field present during the epoch of reionization. We consider emission
from both galaxies and the intergalactic medium (IGM) and take into account the
main contributions to the production of Lyman-alpha photons: recombinations,
collisions, continuum emission from galaxies and scattering of Lyman-n photons
in the IGM. We find that the emission from individual galaxies dominates over
the IGM with a total Lyman-alpha intensity (times frequency) of about
(1.43-3.57)x10^{-8} erg s^{-1} cm^{-2} sr^{-1} at a redshift of 7. This
intensity level is low so it is unlikely that the Lyman-\alpha background
during reionization can be established by an experiment aiming at an absolute
background light measurement. Instead we consider Lyman-\alpha intensity
mapping with the aim of measuring the anisotropy power spectrum which has rms
fluctuations at the level of 1 x 10^{-16} [erg s^[-1} cm^{-2} sr^{-1}]^2 at a
few Mpc scales. These anisotropies could be measured with a spectrometer at
near-IR wavelengths from 0.9 to 1.4 \mu m with fields in the order of 0.5 to 1
sq. degrees. We recommend that existing ground-based programs using narrow band
filters also pursue intensity fluctuations to study statistics on the spatial
distribution of faint Lyman-\alpha emitters. We also discuss the
cross-correlation signal with 21 cm experiments that probe HI in the IGM during
reionization. A dedicated sub-orbital or space-based Lyman-\alpha intensity
mapping experiment could provide a viable complimentary approach to probe
reionization, when compared to 21 cm experiments, and is likely within
experimental reach.Comment: 18 pages, 17 figure
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