108 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
Large Fluctuations in the High-Redshift Metagalactic Ionizing Background
Recent observations have shown that the scatter in opacities among coeval
segments of the Lyman-alpha forest increases rapidly at z > 5. In this paper,
we assess whether the large scatter can be explained by fluctuations in the
ionizing background in the post-reionization intergalactic medium. We find that
matching the observed scatter at z ~ 5.5 requires a short spatially averaged
mean free path of 3 shorter than direct
measurements at z ~ 5.2. We argue that such rapid evolution in the mean free
path is difficult to reconcile with our measurements of the global H I
photoionization rate, which stay approximately constant over the interval z ~
4.8 - 5.5. However, we also show that measurements of the mean free path at z >
5 are likely biased towards higher values by the quasar proximity effect. This
bias can reconcile the short values of the mean free path that are required to
explain the large scatter in opacities. We discuss the implications of this
scenario for cosmological reionization. Finally, we investigate whether other
statistics applied to the z > 5 Lyman-alpha forest can shed light on the origin
of the scatter. Compared to a model with a uniform ionizing background, models
that successfully account for the scatter lead to enhanced power in the
line-of-sight flux power spectrum on scales k < 0.1 h/Mpc. We find tentative
evidence for this enhancement in observations of the high-redshift Lyman-alpha
forest.Comment: Matches version published by MNRAS with clarifications and expanded
discussio
The halo mass function through the cosmic ages
In this paper we investigate how the halo mass function evolves with
redshift, based on a suite of very large (with N_p = 3072^3 - 6000^3 particles)
cosmological N-body simulations. Our halo catalogue data spans a redshift range
of z = 0-30, allowing us to probe the mass function from the dark ages to the
present. We utilise both the Friends-of-Friends (FOF) and Spherical Overdensity
(SO) halofinding methods to directly compare the mass function derived using
these commonly used halo definitions. The mass function from SO haloes exhibits
a clear evolution with redshift, especially during the recent era of dark
energy dominance (z < 1). We provide a redshift-parameterised fit for the SO
mass function valid for the entire redshift range to within ~20% as well as a
scheme to calculate the mass function for haloes with arbitrary overdensities.
The FOF mass function displays a weaker evolution with redshift. We provide a
`universal' fit for the FOF mass function, fitted to data across the entire
redshift range simultaneously, and observe redshift evolution in our data
versus this fit. The relative evolution of the mass functions derived via the
two methods is compared and we find that the mass functions most closely match
at z=0. The disparity at z=0 between the FOF and SO mass functions resides in
their high mass tails where the collapsed fraction of mass in SO haloes is ~80%
of that in FOF haloes. This difference grows with redshift so that, by z>20,
the SO algorithm finds a ~50-80% lower collapsed fraction in high mass haloes
than does the FOF algorithm, due in part to the significant over-linking
effects known to affect the FOF method.Comment: v4, 16 pages, 16 colour figures. Changed to match MNRAS print
version. NOTE: v1 of this paper has a typo in the fitting function. Please
ensure you use the latest versio
The scale-dependent signature of primordial non-Gaussianity in the large-scale structure of cosmic reionization
(ABRIDGED)The rise of cosmic structure depends upon the statistical
distribution of initial density fluctuations generated by inflation. While the
simplest models predict an almost perfectly Gaussian distribution, more-general
models predict a level of primordial non-Gaussianity (PNG) that observations
might yet be sensitive enough to detect. Recent Planck Collaboration
measurements of the CMB temperature anisotropy bispectrum significantly tighten
the observational limits, but they are still far from the PNG level predicted
by the simplest models of inflation. Probing levels below CMB sensitivities
will require other methods, such as searching for the statistical imprint of
PNG on galactic halo clustering. During the epoch of reionization (EoR), the
first stars and galaxies released radiation into the intergalactic medium (IGM)
that created ionized patches whose large-scale geometry and evolution reflected
the underlying abundance and large-scale clustering of the star-forming
galaxies. This statistical connection between ionized patches in the IGM and
galactic halos suggests that observing reionization may be another way to
constrain PNG. We employ the linear perturbation theory of reionization and
semi-analytic models based on the excursion-set formalism to model the effects
of PNG on the EoR. We quantify the effects of PNG on the large-scale structure
of reionization by deriving the ionized density bias, i.e. ratio of ionized
atomic to total matter overdensities in Fourier space, at small wavenumber.
Just as previous studies found that PNG creates a scale-dependent signature in
the halo bias, so, too, we find a scale-dependent signature in the ionized
density bias. Our results, which differ significantly from previous attempts in
the literature to characterize this PNG signature, will be applied elsewhere to
predict its observable consequences, e.g. in the cosmic 21cm background.Comment: Accepted by MNRAS with minor changes. Minor typos corrected and
references adde
Primordial Non-Gaussianity Estimation using 21 cm Tomography from the Epoch of Reionization
Measuring the small primordial nonGaussianity (PNG) predicted by cosmic
inflation theories may help diagnose them. The detectability of PNG by its
imprint on the 21cm power spectrum from the epoch of reionization is reassessed
here in terms of , the local nonlinearity parameter. We find that an
optimum, multi-frequency observation by SKA can achieve
(comparable to recent Planck CMB limits), while a cosmic-variance-limited array
of this size like Omniscope can even detect . This
substantially revises the methods and results of previous work.Comment: Accepted by PRD with minor changes. References added and update
The Linear Perturbation Theory of Reionization in Position-Space: Cosmological Radiative Transfer Along the Light-Cone
The linear perturbation theory of inhomogeneous reionization (LPTR) has been
developed as an analytical tool for predicting the global ionized fraction and
large-scale power spectrum of ionized density fluctuations during reionization.
In the original formulation of the LPTR, the ionization balance and radiative
transfer equations are linearized and solved in Fourier space. However, the
LPTR's approximation to the full solution of the radiative transfer equation is
not straightforward to interpret, since the latter is most intuitively
conceptualized in position space. To bridge the gap between the LPTR and the
language of numerical radiative transfer, we present a new, equivalent,
position-space formulation of the LPTR that clarifies the approximations it
makes and facilitates its interpretation. We offer a comparison between the
LPTR and the excursion-set model of reionization (ESMR), and demonstrate the
built-in capability of the LPTR to explore a wide range of reionization
scenarios, and to go beyond the ESMR in exploring scenarios involving X-rays.Comment: 10 pages, 1 figure, 1 table. Accepted by PRD with minor change
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