211 research outputs found
High-redshift voids in the excursion set formalism
Voids are a dominant feature of the low-redshift galaxy distribution. Several
recent surveys have found evidence for the existence of large-scale structure
at high redshifts as well. We present analytic estimates of galaxy void sizes
at redshifts z ~ 5 - 10 using the excursion set formalism. We find that recent
narrow-band surveys at z ~ 5 - 6.5 should find voids with characteristic scales
of roughly 20 comoving Mpc and maximum diameters approaching 40 Mpc. This is
consistent with existing surveys, but a precise comparison is difficult because
of the relatively small volumes probed so far. At z ~ 7 - 10, we expect
characteristic void scales of ~ 14 - 20 comoving Mpc assuming that all galaxies
within dark matter haloes more massive than 10^10 M_sun are observable. We find
that these characteristic scales are similar to the sizes of empty regions
resulting from purely random fluctuations in the galaxy counts. As a result,
true large-scale structure will be difficult to observe at z ~ 7 - 10, unless
galaxies in haloes with masses less than ~ 10^9 M_sun are visible. Galaxy
surveys must be deep and only the largest voids will provide meaningful
information. Our model provides a convenient picture for estimating the
"worst-case" effects of cosmic variance on high-redshift galaxy surveys with
limited volumes.Comment: 12 pages, 9 figures, 1 table, accepted by MNRA
Constraints on the Star Formation Efficiency of Galaxies During the Epoch of Reionization
Reionization is thought to have occurred in the redshift range of , which is now being probed by both deep galaxy surveys and CMB observations.
Using halo abundance matching over the redshift range and assuming
smooth, continuous gas accretion, we develop a model for the star formation
efficiency of dark matter halos at that matches the measured
galaxy luminosity functions at these redshifts. We find that peaks
at at halo masses --~M, in
qualitative agreement with its behavior at lower redshifts. We then investigate
the cosmic star formation histories and the corresponding models of
reionization for a range of extrapolations to small halo masses. We use a
variety of observations to further constrain the characteristics of the galaxy
populations, including the escape fraction of UV photons. Our approach provides
an empirically-calibrated, physically-motivated model for the properties of
star-forming galaxies sourcing the epoch of reionization. In the case where
star formation in low-mass halos is maximally efficient, an average escape
fraction can reproduce the optical depth reported by Planck, whereas
inefficient star formation in these halos requires either about twice as many
UV photons to escape, or an escape fraction that increases towards higher
redshifts. Our models also predict how future observations with JWST can
improve our understanding of these galaxy populations.Comment: 19 pages, 12 figures, accepted for publication in MNRAS, minor
modification
Reionization Through the Lens of Percolation Theory
The reionization of intergalactic hydrogen has received intense theoretical
scrutiny over the past two decades. Here, we approach the process formally as a
percolation process and phase transition. Using semi-numeric simulations, we
demonstrate that an infinitely-large ionized region abruptly appears at an
ionized fraction of ~0.1 and quickly grows to encompass most of the ionized
gas: by an ionized fraction of 0.3, nearly ninety percent of the ionized
material is part of this region. Throughout most of reionization, nearly all of
the intergalactic medium is divided into just two regions, one ionized and one
neutral, and both infinite in extent. We also show that the discrete ionized
regions that exist before and near this transition point follow a near-power
law distribution in volume, with equal contributions to the total filling
factor per logarithmic interval in size up to a sharp cutoff in volume. These
qualities are generic to percolation processes, with the detailed behavior a
result of long-range correlations in the underlying density field. These
insights will be crucial to understanding the distribution of ionized and
neutral gas during reionization and provide precise meaning to the intuitive
description of reionization as an "overlap" process.Comment: 16 pages, version accepted by MNRAS (conclusions unchanged from
original
Extreme Galaxies During Reionization: Testing ISM and Disk Models
We test the ability of equilibrium galactic disk and one-zone interstellar
medium models to describe the physical and emission properties of quasar hosts,
submillimeter galaxies, and Lyman-alpha emitters at z>~6. The size, line
widths, star formation rates, black hole accretion rates, gas masses and
temperatures, and the relationships between these properties are all
well-described by our model, and we provide approximate fitting formulae for
comparison with future observations. However, comparing our carbon line
predictions to observations reveals differences between the ISM at low and high
redshifts. Our underestimate of the [CII] line emission indicates either higher
star formation efficiencies in high-redshift molecular clouds or less depletion
of metals into dust at fixed metallicity. Further, our over-prediction of the
CO(6-5)/CO(1-0) ratio suggests that molecular clouds in real high-redshift
galaxies have a lower turbulent Mach number and more subthermal CO(6-5)
emission than expected owing either to sizes smaller than the local Jeans mass
or to a pressure support mechanism other than turbulence.Comment: Accepted in MNRAS; 19 pages; 10 figures; 4 table
The Effect of Fluctuations on the Helium-Ionizing Background
Interpretation of He II Ly{\alpha} absorption spectra after the epoch of He
II reionization requires knowledge of the He II ionizing background. While past
work has modelled the evolution of the average background, the standard
cosmological radiative transfer technique assumes a uniform radiation field
despite the discrete nature of the (rare) bright quasars that dominate the
background. We implement a cosmological radiative transfer model that includes
the most recent constraints on the ionizing spectra and luminosity function of
quasars and the distribution of IGM absorbers. We also estimate, for the first
time, the effects of fluctuations on the evolving continuum opacity in two
ways: by incorporating the complete distribution of ionizing background
amplitudes into the standard approach, and by explicitly treating the quasars
as discrete -- but isolated -- sources. Our model results in a He II ionization
rate that evolves steeply with redshift, increasing by a factor ~2 from z=3.0
to z=2.5. This causes rapid evolution in the mean He II Ly{\alpha} optical
depth -- as recently observed -- without appealing to the reionization of He
II. The observed behaviour could instead result from rapid evolution in the
mean free path of ionizing photons as the helium in higher H I column density
absorbers becomes fully ionized.Comment: 14 pages, 9 figures. Accepted by MNRAS; significantly modified from
previous versio
The Global 21-cm Signal in the Context of the High-z Galaxy Luminosity Function
Motivated by recent progress in studies of the high- Universe, we build a
new model for the global 21-cm signal that is explicitly calibrated to
measurements of the galaxy luminosity function (LF) and further tuned to match
the Thomson scattering optical depth of the cosmic microwave background,
. Assuming that the galaxy population can be smoothly
extrapolated to higher redshifts, the recent decline in best-fit values of
and the inefficient heating induced by X-ray binaries (HMXBs; the
presumptive sources of the X-ray background at high-) imply that the
entirety of cosmic reionization and reheating occurs at redshifts . In contrast to past global 21-cm models, whose (
MHz) absorption features and strong mK emission features were driven
largely by the assumption of efficient early star-formation and X-ray heating,
our new fiducial model peaks in absorption at MHz at a depth of
mK and has a negligible emission component. As a result, a strong
emission signal would provide convincing evidence that HMXBs are not the only
drivers of cosmic reheating. Shallow absorption troughs should accompany strong
heating scenarios, but could also be caused by a low escape fraction of
Lyman-Werner photons. Generating signals with troughs at MHz
requires a floor in the star-formation efficiency in halos below , which is equivalent to steepening the faint-end of the galaxy LF.
These findings demonstrate that the global 21-cm signal is a powerful
complement to current and future galaxy surveys and efforts to better
understand the interstellar medium in high- galaxies.Comment: 17 pages, 9 figures, in pres
The Fundamentals of the 21-cm Line
We review some of the fundamental physics necessary for computing the
highly-redshifted spin-flip background. We first discuss the radiative transfer
of the 21-cm line and define the crucial quantities of interest. We then review
the processes that set the spin temperature of the transition, with a
particular focus on Wouthuysen-Field coupling, which is likely to be the most
important process during and after the Cosmic Dawn. Finally, we discuss
processes that heat the intergalactic medium during the Cosmic Dawn, including
the scattering of Lyman-alpha, cosmic microwave background, and X-ray photons.Comment: To appear as a book chapter in "The Cosmic 21-cm Revolution: Charting
the first billion years of our Universe," Ed Andrei Mesinger (Bristol: IOP
Publishing Ltd) AAS-IOP ebooks http://www.iopscience.org/books/aas. arXiv
admin note: text overlap with arXiv:astro-ph/060803
Quasi-equilibrium models of high-redshift disc galaxy evolution
In recent years, simple models of galaxy formation have been shown to provide
reasonably good matches to available data on high-redshift luminosity
functions. However, these prescriptions are primarily phenomenological, with
only crude connections to the physics of galaxy evolution. Here we introduce a
set of galaxy models that are based on a simple physical framework but
incorporate more sophisticated models of feedback, star formation, and other
processes. We apply these models to the high-redshift regime, showing that most
of the generic predictions of the simplest models remain valid. In particular,
the stellar mass--halo mass relation depends almost entirely on the physics of
feedback (and is thus independent of the details of small-scale star formation)
and the specific star formation rate is a simple multiple of the cosmological
accretion rate. We also show that, in contrast, the galaxy's gas mass is
sensitive to the physics of star formation, although the inclusion of
feedback-driven star formation laws significantly changes the naive
expectations. While these models are far from detailed enough to describe every
aspect of galaxy formation, they inform our understanding of galaxy formation
by illustrating several generic aspects of that process, and they provide a
physically-grounded basis for extrapolating predictions to faint galaxies and
high redshifts currently out of reach of observations. If observations show
violations from these simple trends, they would indicate new physics occurring
inside the earliest generations of galaxies.Comment: 20 pages, 13 figures, in press at MNRA
The Persistence of Population III Star Formation
We present a semi-analytic model of star formation in the early universe,
beginning with the first metal-free stars. By employing a completely
feedback-limited star formation prescription, stars form at maximum efficiency
until the self-consistently calculated feedback processes halt formation. We
account for a number of feedback processes including a meta-galactic
Lyman-Werner background, supernovae, photoionization, and chemical feedback.
Halos are evolved combining mass accretion rates found through abundance
matching with our feedback-limited star formation prescription, allowing for a
variety of Population III (Pop III) initial mass functions (IMFs). We find
that, for a number of models, massive Pop III star formation can continue on
until at least and potentially past at rates of around
to M yr Mpc, assuming these stars
form in isolation. At this point Lyman-Werner feedback pushes the minimum halo
mass for star formation above the atomic cooling threshold, cutting off the
formation of massive Pop III stars. We find that, in most models, Pop II and
Pop III star formation co-exist over cosmological time-scales, with the total
star formation rate density and resulting radiation background strongly
dominated by the former before Pop III star formation finally ends. These halos
form at most M of massive Pop III stars during this phase
and typically have absolute magnitudes in the range of to . We also briefly discuss how future observations from telescopes such as
JWST or WFIRST and 21-cm experiments may be able to constrain unknown
parameters in our model such as the IMF, star formation prescription, or the
physics of massive Pop III stars.Comment: 16 pages, 13 figures, submitted to MNRA
- β¦