26,586 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
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
Simulating cosmic metal enrichment by the first galaxies
We study cosmic metal enrichment via AMR hydrodynamical simulations in a (10
Mpc/h) volume following the Pop III-Pop II transition and for different Pop
III IMFs. We have analyzed the joint evolution of metal enrichment on galactic
and intergalactic scales at z=6 and z=4. Galaxies account for <9% of the
baryonic mass; the remaining gas resides in the diffuse phases: (a) voids, i.e.
regions with extremely low density (<1), (b) the true intergalactic
medium (IGM, 1<<10) and (c) the circumgalactic medium (CGM,
10<), the interface between the IGM and galaxies. By z=6 a
galactic mass-metallicity relation is established. At z=4, galaxies with a
stellar mass show log(O/H)+12=8.19, consistent with
observations. The total amount of heavy elements rises from
at z=6 to 8.05 at z=4. Metals in
galaxies make up to ~0.89 of such budget at z=6; this fraction increases to
~0.95 at z=4. At z=6 (z=4) the remaining metals are distributed in
CGM/IGM/voids with the following mass fractions: 0.06/0.04/0.01
(0.03/0.02/0.01). Analogously to galaxies, at z=4 a density-metallicity
(-Z) relation is in place for the diffuse phases: the IGM/voids have a
spatially uniform metallicity, Z~Zsun; in the CGM Z steeply rises
with density up to ~Zsun. In all diffuse phases a considerable
fraction of metals is in a warm/hot (T>K) state. Due to these
physical conditions, CIV absorption line experiments can probe only ~2% of the
total carbon present in the IGM/CGM; however, metal absorption line spectra are
very effective tools to study reionization. Finally, the Pop III star formation
history is almost insensitive to the chosen Pop III IMF. Pop III stars are
preferentially formed in truly pristine (Z=0) gas pockets, well outside
polluted regions created by previous star formation episodes.Comment: 23 pages, 18 figures, 3 tables, Accepted for publication in MNRA
Cosmology at Low Frequencies: The 21 cm Transition and the High-Redshift Universe
Observations of the high-redshift Universe with the 21 cm hyperfine line of
neutral hydrogen promise to open an entirely new window onto the early phases
of cosmic structure formation. Here we review the physics of the 21 cm
transition, focusing on processes relevant at high redshifts, and describe the
insights to be gained from such observations. These include measuring the
matter power spectrum at z~50, observing the formation of the cosmic web and
the first luminous sources, and mapping the reionization of the intergalactic
medium. The epoch of reionization is of particular interest, because large HII
regions will seed substantial fluctuations in the 21 cm background. We also
discuss the experimental challenges involved in detecting this signal, with an
emphasis on the Galactic and extragalactic foregrounds. These increase rapidly
toward low frequencies and are especially severe for the highest redshift
applications. Assuming that these difficulties can be overcome, the redshifted
21 cm line will offer unique insight into the high-redshift Universe,
complementing other probes but providing the only direct, three-dimensional
view of structure formation from z~200 to z~6.Comment: extended review accepted by Physics Reports, 207 pages, 44 figures
(some low resolution); version with high resolution figures available at
http://pantheon.yale.edu/~srf28/21cm/index.htm; minor changes to match
published versio
Financial Fragility, Bubbles and Monetary Policy
The paper models the links between financial fragility, asset markets and monetary policy. It is shown that central bank’s concern about the cost of financial disruption may generate an asymmetric response, thus contributing to the creation of an asset price bubble. In an economy with a highly leveraged financial structure, the central bank has an incentive to prevent a “run” on financial intermediation by injecting liquidity when asset values fall significantly. The inflationary side effect of this policy, reducing the real value of nominal debt, is what gives rise to a “put option” for investors. Leveraged investors, rationally anticipating this liquidity injection, drive asset prices above their fundamental values. The bubble will be equal to the expected value of capital gains on outstanding debt. The paper shows that it is rational for central banks to inject liquidity in a crisis, whenever there is the risk of spillover effects arising from the disruption of financial intermediation
Source Mergers and Bubble Growth During Reionization
The recently introduced models of reionization bubbles based on extended
Press-Schechter theory (Furlanetto, Zaldarriaga & Hernquist 2004) are
generalized to include mergers of ionization sources. Sources with a recent
major merger are taken to have enhanced photon production due to star
formation, and accretion onto a central black hole if a black hole is present.
This produces a scatter in the number of ionized photons corresponding to a
halo of a given mass and a change in photon production over time for any given
halo mass. Photon production histories, bubble distributions, and ionization
histories are computed for several different parameter and recombination
assumptions; the resulting distributions interpolate between previously
calculated limiting cases.Comment: 44 pages, 11 figures, version to appear in MNRAS. Some discussion of
case with WMAP parameters and expanded explanation
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
Spin/3 Magazine: Action Time Vision
Collaboration with London design group Spin, with contributing essays by Russ Bestley and Malcolm Garrett, on the subject of punk graphic design. Published as large format newspaper in plastic slipcase
Time-evolution of ionization and heating around first stars and miniquasars
A one dimensional radiative transfer code is developed to track the
ionization and heating pattern around the first miniquasars and Population III
stars. The code follows the evolution of the ionization of the species of
hydrogen and helium and the intergalactic medium temperature profiles as a
function of redshift. The radiative transfer calculations show that the
ionization signature of the first miniquasars and stars is very similar yet the
heating pattern around the two is very different. Furthermore, the first
massive miniquasars (~>10^5 M_{sun}) do produce large ionized bubbles around
them, which can potentially be imaged directly using future radio telescopes.
It is also shown that the ionized bubbles not only stay ionized for
considerable time after the switching off of the source, but continue to expand
for a short while due to secondary collisions prompted by the X-ray part of
their spectra. Varying spectral shapes also produced sizable variations in
ionized fraction and temperature profile. We also compare the radiative
transfer results with the analytical approximation usually adopted for heating
by miniquasars and find that, because of the inadequate treatment of the He
species, the analytical approach leads to an underestimation of the temperature
in the outer radii by a factor ~5. Population III stars - with masses in the
range of 10 - 1000 M_{sun} and modelled as blackbodies at a temperature of
50000 K - are found to be efficient in ionizing their surroundings.
Observational effects on the 21 cm brightness temperature, the thermal and
kinetic Sunyaev-Ze'ldovich effects, are also studied in the context of the
upcoming radio and microwave telescopes like LOFAR and SPT.Comment: 19 pages, 24 figures, accepted to be published in MNRAS Typos in
formula 1,2 and 21 fixed. Figure 11 caption and Figure 13 change
Detecting the Earliest Galaxies Through Two New Sources of 21cm Fluctuations
The first galaxies that formed at a redshift ~20-30 emitted continuum photons
with energies between the Lyman-alpha and Lyman limit wavelengths of hydrogen,
to which the neutral universe was transparent except at the Lyman-series
resonances. As these photons redshifted or scattered into the Lyman-alpha
resonance they coupled the spin temperature of the 21cm transition of hydrogen
to the gas temperature, allowing it to deviate from the microwave background
temperature. We show that the fluctuations in the radiation emitted by the
first galaxies produced strong fluctuations in the 21cm flux before the
Lyman-alpha coupling became saturated. The fluctuations were caused by biased
inhomogeneities in the density of galaxies, along with Poisson fluctuations in
the number of galaxies. Observing the power-spectra of these two sources would
probe the number density of the earliest galaxies and the typical mass of their
host dark matter halos. The enhanced amplitude of the 21cm fluctuations from
the era of Lyman-alpha coupling improves considerably the practical prospects
for their detection.Comment: 11 pages, 7 figures, ApJ, published. Normalization fixed in top
panels of Figures 4-
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