5,308 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

### 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

### Smooth and non-smooth traveling wave solutions of some generalized Camassa-Holm equations

In this paper we employ two recent analytical approaches to investigate the
possible classes of traveling wave solutions of some members of a
recently-derived integrable family of generalized Camassa-Holm (GCH) equations.
A recent, novel application of phase-plane analysis is employed to analyze the
singular traveling wave equations of three of the GCH NLPDEs, i.e. the possible
non-smooth peakon and cuspon solutions. One of the considered GCH equations
supports both solitary (peakon) and periodic (cuspon) cusp waves in different
parameter regimes. The second equation does not support singular traveling
waves and the last one supports four-segmented, non-smooth $M$-wave solutions.
Moreover, smooth traveling waves of the three GCH equations are considered.
Here, we use a recent technique to derive convergent multi-infinite series
solutions for the homoclinic orbits of their traveling-wave equations,
corresponding to pulse (kink or shock) solutions respectively of the original
PDEs. We perform many numerical tests in different parameter regime to pinpoint
real saddle equilibrium points of the corresponding GCH equations, as well as
ensure simultaneous convergence and continuity of the multi-infinite series
solutions for the homoclinic orbits anchored by these saddle points. Unlike the
majority of unaccelerated convergent series, high accuracy is attained with
relatively few terms. We also show the traveling wave nature of these pulse and
front solutions to the GCH NLPDEs

### Reionization constraints using Principal Component Analysis

Using a semi-analytical model developed by Choudhury & Ferrara (2005) we
study the observational constraints on reionization via a principal component
analysis (PCA). Assuming that reionization at z>6 is primarily driven by
stellar sources, we decompose the unknown function N_{ion}(z), representing the
number of photons in the IGM per baryon in collapsed objects, into its
principal components and constrain the latter using the photoionization rate
obtained from Ly-alpha forest Gunn-Peterson optical depth, the WMAP7 electron
scattering optical depth and the redshift distribution of Lyman-limit systems
at z \sim 3.5. The main findings of our analysis are: (i) It is sufficient to
model N_{ion}(z) over the redshift range 2<z<14 using 5 parameters to extract
the maximum information contained within the data. (ii) All quantities related
to reionization can be severely constrained for z<6 because of a large number
of data points whereas constraints at z>6 are relatively loose. (iii) The weak
constraints on N_{ion}(z) at z>6 do not allow to disentangle different feedback
models with present data. There is a clear indication that N_{ion}(z) must
increase at z>6, thus ruling out reionization by a single stellar population
with non-evolving IMF, and/or star-forming efficiency, and/or photon escape
fraction. The data allows for non-monotonic N_{ion}(z) which may contain sharp
features around z \sim 7. (iv) The PCA implies that reionization must be 99%
completed between 5.8<z<10.3 (95% confidence level) and is expected to be 50%
complete at z \approx 9.5-12. With future data sets, like those obtained by
Planck, the z>6 constraints will be significantly improved.Comment: Accepted in MNRAS. Revised to match the accepted versio

### The effect of peculiar velocities on the epoch of reionization (EoR) 21-cm signal

We have used semi-numerical simulations of reionization to study the
behaviour of the power spectrum of the EoR 21-cm signal in redshift space. We
have considered two models of reionization, one which has homogeneous
recombination (HR) and the other incorporating inhomogeneous recombination
(IR). We have estimated the observable quantities --- quadrupole and monopole
moments of HI power spectrum at redshift space from our simulated data. We find
that the magnitude and nature of the ratio between the quadrupole and monopole
moments of the power spectrum ($P^s_2 /P^s_0$) can be a possible probe for the
epoch of reionization. We observe that this ratio becomes negative at large
scales for $x_{HI} \leq 0.7$ irrespective of the reionization model, which is a
direct signature of an inside-out reionization at large scales. It is possible
to qualitatively interpret the results of the simulations in terms of the
fluctuations in the matter distribution and the fluctuations in the neutral
fraction which have power spectra and cross-correlation $P_{\Delta \Delta}(k)$,
$P_{xx}(k)$ and $P_{\Delta x}(k)$ respectively. We find that at large scales
the fluctuations in matter density and neutral fraction is exactly
anti-correlated through all stages of reionization. This provides a simple
picture where we are able to qualitatively interpret the behaviour of the
redshift space power spectra at large scales with varying $x_{HI}$ entirely in
terms of a just two quantities, namely $x_{HI}$ and the ratio $P_{xx}/P_{\Delta
\Delta}$. The nature of $P_{\Delta x}$ becomes different for HR and IR
scenarios at intermediate and small scales. We further find that it is possible
to distinguish between an inside-out and an outside-in reionization scenario
from the nature of the ratio $P^s_2 /P^s_0$ at intermediate length scales.Comment: 11 pages, 6 figures. Accepted for publication in MNRAS. Replaced to
match the accepted version. Added one appendix to quantify the possible
uncertainties in the estimation of the multipole moments of redshift space
power spectru

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