5,308 research outputs found

    Searching for the reionization sources

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    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

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    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

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    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

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    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 MM-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

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    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

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    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 (P2s/P0sP^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 xHI≤0.7x_{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 PΔΔ(k)P_{\Delta \Delta}(k), Pxx(k)P_{xx}(k) and PΔx(k)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 xHIx_{HI} entirely in terms of a just two quantities, namely xHIx_{HI} and the ratio Pxx/PΔΔP_{xx}/P_{\Delta \Delta}. The nature of PΔxP_{\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 P2s/P0sP^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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