Detecting Early Galaxies Through Their 21-cm Signature

New observations over the next few years of the emission of distant objects will help unfold the chapter in cosmic history around the era of the first galaxies. These observations will use the neutral hydrogen emission or absorption at a wavelength of 21-cm as a detector of the hydrogen abundance. We predict the signature on the 21-cm signal of the early generations of galaxies. We calculate the 21-cm power spectrum including two physical effects that were neglected in previous calculations. The first is the redistribution of the UV photons from the first galaxies due to their scattering off of the neutral hydrogen, which results in an enhancement of the 21-cm signal. The second is the presence of an ionized hydrogen bubble near each source, which produces a cutoff at observable scales. We show that the resulting clear signature in the 21-cm power spectrum can be used to detect and study the population of galaxies that formed just 200 million years after the Big Bang.Comment: 5 pages, 3 figures, submitted to MNRAS Let

Energy transfer by the scattering of resonant photons

A formal derivation is presented of the energy transfer rate between radiation and matter due to the scattering of an isotropic distribution of resonant photons. The derivation is developed in the context of the two-level atom in the absence of collisions and radiative transitions to and from the continuum, but includes the full angle-averaged redistribution function for photon scattering. The result is compared with previous derivations, all of which have been based on the Fokker-Planck approximation to the radiative transfer equation. A new Fokker-Planck approximation, including an extension to higher (post-diffusive) orders, is derived to solve the radiative transfer equation, and time-dependent numerical solutions are found. The relaxation of the colour temperature to the matter temperature is computed as the radiation field approaches statistical equilibrium through scattering. The results are discussed in the context of the Wouthuysen-Field mechanism for coupling the 21cm spin temperature of neutral hydrogen to the kinetic temperature of the gas through LyA scattering. The evolution of the heating rate is also computed, and shown to diminish as the gas approaches statistical equilibrium.Comment: 13 pages, 4 figures. Submitted to MNRAS. RT eq. simplified to generalise results including stimulated emissio

Contribution of Cross-Correlations to the 21cm Angular Power Spectrum in the Epoch of Reionization

Measurement of the 21cm hyperfine transition of neutral hydrogen provides a unique probe of the epoch of reionization and the Dark Ages. Three major mechanisms are believed to dominate the radiation process: emission from neutral hydrogen surrounding the ionized bubbles of first galaxies and/or quasars, emission from neutral hydrogen inside minihalos, and absorption of diffuse neutral hydrogen against the cosmic microwave background. In the present work, by simply combining the existing analytic models for the three mechanisms, we investigate the contribution of cross-correlation between these three components to the total 21cm angular power spectrum, in the sense that neutral hydrogen associated with different radiation processes traces the large-scale structures of underlying density perturbations. While the overall 21cm power spectrum remains almost unchanged with the inclusion of the cross-correlations, the cross-correlation may play a key role in the determination of the 21cm power spectrum during the transition of 21cm radiation from emission-dominated phase to absorption-dominated phase at redshift z~20. A significant suppression in the 21cm angular power spectrum during this transition is anticipated as the result of negative contribution of the cross-correlation between the absorption of diffuse neutral hydrogen and the emission components. Therefore, an accurate prediction of the cosmic 21cm power spectrum should take the cross-correlation into account especially at the transition phase.Comment: 10 pages, 4 figures, accepted for publication in MNRA

The scattering of LyA radiation in the intergalactic medium: numerical methods and solutions

Two methods are developed for solving the steady-state spherically symmetric radiative transfer equation for resonance line radiation emitted by a point source in the Intergalactic Medium. One method is based on solving the ray and moment equations using finite differences. The second uses a Monte Carlo approach incorporating methods that greatly improve the accuracy compared with previous approaches in this context. Several applications are presented serving as test problems for both a static medium and an expanding medium, including inhomogeneities in the density and velocity fields. Solutions are obtained in the coherent scattering limit and for Doppler RII redistribution with and without recoils. We find generally that the radiation intensity is linear in the cosine of the azimuthal angle with respect to radius to high accuracy over a broad frequency region across the line centre for both linear and perturbed velocity fields, yielding the Eddington factors f(nu) = 1/3 and g(nu) = 3/5. We show the radiation field produced by a point source divides into three spatial regimes for a uniformly expanding homogeneous medium: at radii r small compared with a characteristic radius r*, the mean intensity near line centre varies as 1/ r^(7/3), while at r > r* it approaches 1/ r^2; for r << r* it is modified by frequency redistribution. Before the reionization epoch, r* takes on the universal value 1.1 Mpc, independent of redshift. The mean intensity and scattering rate are found to be very sensitive to the gradient of the velocity field, growing exponentially with the amplitude of the perturbation as the limit of a vanishing velocity gradient is approached near the source. We expect the 21cm signal from the Epoch of Reionization to thus be a sensitive probe of both the density and the peculiar velocity fields.Comment: 27 pages, 26 figures, 10 supplementary tables; submitted to MNRA

Lyalpha heating and its impact on early structure formation

In this paper we have calculated the effect of Lyalpha photons emitted by the first stars on the evolution of the IGM temperature. We have considered both a standard Salpeter IMF and a delta-function IMF for very massive stars with mass 300 M_sun. We find that the Lyalpha photons produced by the stellar populations considered here are able to heat the IGM at z<25, although never above ~100 K. Stars with a Salpeter IMF are more effective as, due to the contribution from small-mass long-living stars, they produce a higher Lyalpha background. Lyalpha heating can affect the subsequent formation of small mass objects by producing an entropy floor that may limit the amount of gas able to collapse and reduce the gas clumping.We find that the gas fraction in halos of mass below ~ 5 x 10^6 M_sun is less than 50% (for the smallest masses this fraction drops to 1% or less) compared to a case without Lyalpha heating. Finally, Lyalpha photons heat the IGM temperature above the CMB temperature and render the 21cm line from neutral hydrogen visible in emission at z<15.Comment: 7 pages, 5 figures, to be printed in MNRA

HI as a Probe of the Large Scale Structure in the Post-Reionization Universe

We model the distribution of neutral Hydrogen (HI hereafter) in the post-reionization universe. This model uses gravity only N-Body simulations and an ansatz to assign HI to dark matter haloes that is consistent with observational constraints and theoretical models. We resolve the smallest haloes that are likely to host HI in the simulations, care is also taken to ensure that any errors due to the finite size of the simulation box are small. We then compute the smoothed one point probability distribution function and the power spectrum of fluctuations in HI. This is compared with other predictions that have been made using different techniques. We highlight the significantly high bias for the HI distribution at small scales. This aspect has not been discussed before. We then discuss the prospects for detection with the MWA, GMRT and the hypothetical MWA5000. The MWA5000 can detect visibility correlations at large angular scales at all redshifts in the post-reionization era. The GMRT can detect visibility correlations at lower redshifts, specifically there is a strong case for a survey at z=1.3. We also discuss prospects for direct detection of rare peaks in the HI distribution using the GMRT. We show that direct detection should be possible with an integration time that is comparable to, or even less than, the time required for a statistical detection. Specifically, it is possible to make a statistical detection of the HI distribution by measuring the visibility correlation, and, direct detection of rare peaks in the HI distribution at z = 1.3 with the GMRT in less than 1000 hours of observations.Comment: 15 pages, 11 figures. Accepted for publication in the MNRAS. This is a merged manuscript also containing material covered in 0908.385

The Spin Temperature of Warm Interstellar H I

Collisional excitation of the 21cm HI hyperfine transition is not strong enough to thermalize it in warm neutral (``intercloud'') interstellar gas, which we show by simultaneously solving the equations of ionization and collisional equilibrium under typical conditions. Coupling of the 21cm excitation temperature and local gas motions may be established by the Ly-alpha radiation field, but only if strong Galactic Ly-alpha radiation permeates the gas in question. The Ly-alpha radiation tends to impart to the gas its own characteristic temperature, which is determined by the range of gas motions that occur on the spatial scale of the Ly-alpha scattering. In general, the calculation of H I spin temperatures is a more difficult and interesting problem than might have been expected, as is any interpretation of H I spin temperature measurements.Comment: 11 pages, 8 figures, accepted for A&

Lensing of 21cm Absorption "Halos" of z∼z\sim20-30 First Galaxies

Extended 21cm absorption regions (dubbed ``21cm absorption halos'') around first galaxies at $z\sim 30$ are likely the first distinctive structures accessible to radio observations. Though the radio array capable of detecting and resolving them must have $\sim 200$ km$^2$ total collecting area, given the great impact of such detections to the understanding of the reionization process and cosmology, such radio survey would be extremely profitable. As an example, we point out a potentially useful byproduct of such survey. The resolved 21cm absorption ``halos'', likely close to spherical, can serve as (almost) ideal sources for measuring the {\it cosmic shear} and mapping the matter distribution to $z\sim 30$. We investigate the expected lensing signal and consider a variety of noise contributions on the shear measurement. We find that S/N $\sim 1$ can be achieved for individual ``halos''. Given millions of 21cm absorption ``halos'' across the sky, the total S/N will be comparable to traditional shear measurement of $\sim$$10^9$ galaxies at $z\sim 1$.Comment: Minor revisions and expanded discussions. Accepted to MNRA

21 cm Tomography of the High-Redshift Universe with the Square Kilometer Array

We discuss the prospects for ``tomography'' of the intergalactic medium (IGM) at high redshifts using the 21 cm transition of neutral hydrogen. Existing observational constraints on the epoch of reionization imply a complex ionization history that may require multiple generations of sources. The 21 cm transition provides a unique tool to probe this era in detail, because it does not suffer from saturation effects, retains full redshift information, and directly probes the IGM gas. Observations in the redshifted 21cm line will allow one to study the history and morphology of reionization in detail. Depending on the characteristics of the first sources, they may also allow us to probe the era before reionization, when the first structures and luminous sources were forming. The construction of high signal-to-noise ratio maps on arcminute scales will require approximately one square kilometer of collecting area.Comment: 15 pages, 8 figures, to appear in "Science with the Square Kilometer Array," eds. C. Carilli and S. Rawlings, New Astronomy Reviews (Elsevier: Amsterdam), corrected Fig.

Distinctive rings in the 21 cm signal of the epoch of reionization

It is predicted that sources emitting UV radiation in the Lyman band during the epoch of reionization (EoR) showed a series of discontinuities in their Ly-alpha flux radial profile as a consequence of the thickness of the Lyman line series in the primeval intergalactic medium. Through unsaturated Wouthuysen-Field coupling, these spherical discontinuities are also present in the 21 cm emission of the neutral IGM. In this article, we study the effects these discontinuities have on the differential brightness temperature of the 21 cm signal of neutral hydrogen in a realistic setting including all other sources of fluctuations. We focus on the early phases of the EoR, and we address the question of the detectability by the planned Square Kilometre Array. Such a detection would be of great interest, because these structures could provide an unambiguous diagnostic for the cosmological origin of the signal remaining after the foreground cleaning procedure. Also, they could be used as a new type of standard rulers. We determine the differential brightness temperature of the 21 cm signal in the presence of inhomogeneous Wouthuysen-Field effect using simulations which include (hydro)dynamics and both ionizing and Lyman lines 3D radiative transfer with the code LICORICE. We find that the Lyman horizons are clearly visible on the maps and radial profiles around the first sources of our simulations, but for a limited time interval, typically \Delta z \approx 2 at z \sim 13. Stacking the profiles of the different sources of the simulation at a given redshift results in extending this interval to \Delta z \approx 4. When we take into account the implementation and design planned for the SKA (collecting area, sensitivity, resolution), we find that detection will be challenging. It may be possible with a 10 km diameter for the core, but will be difficult with the currently favored design of a 5 km core.Comment: 10 pages, 10 figures; v2: Section 5.5 rewritten; some new references added; accepted for publication in Astronomy and Astrophysic