26,763 research outputs found

### Searching for the reionization sources

Using a reionization model simultaneously accounting for a number of
experimental data sets, we investigate the nature and properties of
reionization sources. Such model predicts that hydrogen reionization starts at
z \approx 15, is initially driven by metal-free (PopIII) stars, and is 90%
complete by z \approx 8. We find that a fraction f_\gamma >80% of the ionizing
power at z > 7 comes from haloes of mass M<10^9 M_sun predominantly harbouring
PopIII stars; a turnover to a PopII-dominated phase occurs shortly after, with
this population, residing in M>10^9 M_sun haloes, yielding f_\gamma \approx 60%
at z=6. Using Lyman-break broadband dropout techniques, J-band detection of
sources contributing to 50% (90%) of the ionizing power at z \sim 7.5 requires
to reach a magnitude J_{110,AB} = 31.2 (31.7), where about 15 (30) (PopIII)
sources/arcmin^2 are predicted. We conclude that z>7 sources tentatively
identified in broadband surveys are relatively massive (M \approx 10^9 M_sun)
and rare objects which are only marginally (\approx 1%) adding to the
reionization photon budget.Comment: Extended discussions. Accepted to MNRAS Letter

### An improved model of HII bubbles during the epoch of reionization

The size distribution of ionized regions during the epoch of reionization --
a key ingredient in understanding the HI power spectrum observable by 21cm
experiments -- can be modelled analytically using the excursion set formalism
of random walks in the smoothed initial density field. To date, such
calculations have been based on simplifying assumptions carried forward from
the earliest excursion set models of two decades ago. In particular, these
models assume that the random walks have uncorrelated steps and that haloes can
form at arbitrary locations in the initial density field. We extend these
calculations by incorporating recent technical developments that allow us to
(a) include the effect of correlations in the steps of the walks induced by a
realistic smoothing filter and (b) more importantly, account for the fact that
dark matter haloes preferentially form near peaks in the initial density. A
comparison with previous calculations shows that including these features,
particularly the peaks constraint on halo locations, has large effects on the
size distribution of the HII bubbles surrounding these haloes. For example,
when comparing models at the same value of the globally averaged ionized volume
fraction, the typical bubble sizes predicted by our model are more than a
factor 2 larger than earlier calculations. Our results can potentially have a
significant impact on estimates of the observable HI power spectrum.Comment: 13 pages, 6 figures; v2 - added clarifications and fixed typos.
Accepted in MNRA

### A Note on Hypervector Spaces

The main aim of this paper is to generalize the concept of vector space by
the hyperstructure. We generalize some definitions such as hypersubspaces,
linear combination, Hamel basis, linearly dependence and linearly independence.
A few important results like deletion theorem, extension theorem, dimension
theorem have been established in this hypervector space.Comment: 17 page

### Semi analytic approach to understanding the distribution of neutral hydrogen in the universe

Analytic derivations of the correlation function and the column density
distribution for neutral hydrogen in the IGM are presented, assuming that the
non-linear baryonic mass density distribution in the IGM is lognormal. This
ansatz was used earlier by Bi & Davidsen (1997) to perform 1D simulations of
lines-of-sight and analyse the properties of absorption systems. Our approach
is completely analytic, which allows us to explore a wide region of the
parameter space for our model. The analytic results have been compared with
observations, whenever possible. Two kinds of correlation functions are
defined: along the line-of-sight (LOS) and across the transverse direction. We
find that the effects on the LOS correlation due to change in cosmology and the
slope of the equation of state of the IGM, \gamma are of the same order, which
means that we cannot constrain both the parameters simultaneously. However, it
is possible to constrain \gamma and its evolution using the observed LOS
correlation function at different epochs, provided one knows the background
cosmology. We suggest that the constraints on the evolution of \gamma obtained
using the LOS correlation can be used as an independent tool to probe the
reionisation history of the universe. From the transverse correlation function,
we find that the excess probability, over random, of finding two neutral
hydrogen overdense regions separated by an angle \theta, is always less than 1
per cent for redshifts greater than 2. Our models also reproduce the observed
column density distribution for neutral hydrogen and the shape of the
distribution depends on \gamma. Our calculations suggest that one can rule out
\gamma > 1.6 for z \simeq 2.31 using the column density distribution. However,
one cannot rule higher values of \gamma at higher redshifts.Comment: 16 pages, 8 figures. Accepted for publication in MNRAS. Revised
following referee's comment

### Cosmological magnetic fields from nonlinear effects

In the standard cosmological model, magnetic fields and vorticity are generated during the radiation era via second-order density perturbations. In order to clarify the complicated physics of this second-order magnetogenesis, we use a covariant approach and present the electromagneto-dynamical equations in the fully nonlinear regime. We use the tight-coupling approximation to analyze Thomson and Coulomb scattering. At the zero-order limit of exact tight-coupling, we show that the vorticity is zero and no magnetogenesis takes place at any nonlinear order. We show that magnetogenesis also fails at all orders if either protons or electrons have the same velocity as the radiation, and momentum transfer is neglected. At first-order in the tight-coupling approximation, magnetic fields and vorticity still cannot be generated even via nonlinear effects. However, at second-order both of them are generated, and we derive a closed set of nonlinear evolution equations that governs this generation

### Non-Hermitian Quantum Mechanics with Minimal Length Uncertainty

We study non-Hermitian quantum mechanics in the presence of a minimal length.
In particular we obtain exact solutions of a non-Hermitian displaced harmonic
oscillator and the Swanson model with minimal length uncertainty. The spectrum
in both the cases are found to be real. It is also shown that the models are
$\eta$ pseudo-Hermitian and the metric operator is found explicitly in both the
cases

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