Can 21‐cm observations discriminate between high‐mass and low‐mass galaxies as reionization sources?

Iliev, Ilian T.,Mellema, Garrelt,Shapiro, Paul R.,Pen, Ue‐,Li,Mao, Yi,Koda, Jun,Ahn, Kyungjin Blackwell Publishing Ltd 2012 MONTHLY NOTICES- ROYAL ASTRONOMICAL SOCIETY Vol.423 No.3

<P><B>ABSTRACT</B></P><P>The prospect of detecting the first galaxies by observing their impact on the intergalactic medium (IGM) as they reionized it during the first billion years leads us to ask whether such indirect observations are capable of diagnosing which types of galaxies were most responsible for reionization. We attempt to answer this with new large‐scale radiative transfer simulations of reionization including the entire mass range of atomically cooling haloes (<I>M</I> > 10<SUP>8</SUP> M<SUB>⊙</SUB>). We divide these haloes into two groups, high‐mass, atomically cooling haloes, or HMACHs (<I>M</I> > 10<SUP>9</SUP> M<SUB>⊙</SUB>), and low‐mass, atomically cooling haloes, or LMACHs (10<SUP>8</SUP> < <I>M</I> < 10<SUP>9</SUP> M<SUB>⊙</SUB>), the latter being susceptible to negative feedback due to Jeans mass filtering in ionized regions, which leads to a process we refer to as self‐regulation. We focus here on predictions of the redshifted 21‐cm emission, to see if upcoming observations are capable of distinguishing a universe ionized primarily by HMACHs from one in which both HMACHs and LMACHs are responsible, and to see how these results depend upon the uncertain source efficiencies. We find that 21‐cm fluctuation power spectra observed by the first‐generation Epoch of Reionization 21‐cm radio interferometer arrays should be able to distinguish the case of reionization by HMACHs alone from that by both HMACHs and LMACHs, together. Some reionization scenarios, e.g. one with abundant low‐efficiency sources versus one with self‐regulation, yield very similar power spectra and rms evolution and thus can only be discriminated by their different mean reionization history and 21‐cm probability distribution function (PDF) distributions. We also find that the skewness of the 21‐cm PDF distribution smoothed with Low Frequency Array (LOFAR)‐like resolution shows a clear feature correlated with the rise of the rms due to patchiness. This is independent of the reionization scenario and thus provides a new approach for detecting the rise of large‐scale patchiness. The peak epoch of the 21‐cm rms fluctuations depends significantly on the beam and bandwidth smoothing size as well as on the reionization scenario and can occur for ionized fractions as low as 30 per cent and as high as 70 per cent. Measurements of the mean photoionization rates are sensitive to the average density of the regions being studied and therefore could be strongly skewed in certain cases. Finally, the simulation volume employed has very modest effects on the results during the early and intermediate stages of reionization, but late‐time signatures could be significantly affected.</P>

Redshift‐space distortion of the 21‐cm background from the epoch of reionization – I. Methodology re‐examined

Mao, Yi,Shapiro, Paul R.,Mellema, Garrelt,Iliev, Ilian T.,Koda, Jun,Ahn, Kyungjin Blackwell Publishing Ltd 2012 MONTHLY NOTICES- ROYAL ASTRONOMICAL SOCIETY Vol.422 No.2

<P><B>ABSTRACT</B></P><P>The peculiar velocity of the intergalactic gas responsible for the cosmic 21‐cm background from the epoch of reionization and beyond introduces an anisotropy in the three‐dimensional power spectrum of brightness temperature fluctuations. Measurement of this anisotropy by future 21‐cm surveys is a promising tool for separating cosmology from 21‐cm astrophysics. However, previous attempts to model the signal have often neglected peculiar velocity or only approximated it crudely. This paper re‐examines the effects of peculiar velocity on the 21‐cm signal in detail, improving upon past treatment and addressing several issues for the first time. (1) We show that even the <I>angle‐averaged</I> power spectrum, <I>P</I>(<I>k</I>), is affected significantly by the peculiar velocity. (2) We re‐derive the brightness temperature dependence on atomic hydrogen density, spin temperature, peculiar velocity and its gradient and redshift to clarify the roles of thermal versus velocity broadening and finite optical depth. (3) We show that properly accounting for finite optical depth eliminates the unphysical divergence of the 21‐cm brightness temperature in overdense regions of the intergalactic medium found by previous work that employed the usual optically thin approximation. (4) We find that the approximation made previously to circumvent the diverging brightness temperature problem by capping the velocity gradient can misestimate the power spectrum on all scales. (5) We further show that the observed power spectrum in redshift space remains finite <I>even</I> in the optically thin approximation if one properly accounts for the redshift‐space distortion. However, results that take full account of finite optical depth show that this approximation is only accurate in the limit of high spin temperature. (6) We also show that the linear theory for redshift‐space distortion widely employed to predict the 21‐cm power spectrum results in a ∼30 per cent error in the observationally relevant wavenumber range <I>k</I>∼ 0.1–1 <I>h</I> Mpc<SUP>−1</SUP>, when strong ionization fluctuations exist (e.g. at the 50 per cent ionized epoch). We derive an alternative, quasi‐linear formulation which improves upon the accuracy of the linear theory. (7) We describe and test two numerical schemes to calculate the 21‐cm signal from reionization simulations to incorporate peculiar velocity effects in the optically thin approximation accurately, by real‐ to redshift‐space re‐mapping of the H <SMALL>i</SMALL> density. One is particle based, the other grid based, and while the former is most accurate, we demonstrate that the latter is computationally more efficient and can be optimized so as to achieve sufficient accuracy.</P>

DETECTING THE RISE AND FALL OF THE FIRST STARS BY THEIR IMPACT ON COSMIC REIONIZATION

Ahn, Kyungjin,Iliev, Ilian T.,Shapiro, Paul R.,Mellema, Garrelt,Koda, Jun,Mao, Yi IOP Publishing 2012 ASTROPHYSICAL JOURNAL LETTERS - Vol.756 No.1

<P>The intergalactic medium was reionized before redshift z similar to 6, most likely by starlight which escaped from early galaxies. The very first stars formed when hydrogen molecules (H-2) cooled gas inside the smallest galaxies, minihalos (MHs) of mass between 10(5) and 10(8) M-circle dot. Although the very first stars began forming inside these MHs before redshift z similar to 40, their contribution has, to date, been ignored in large-scale simulations of this cosmic reionization. Here we report results from the first reionization simulations to include these first stars and the radiative feedback that limited their formation, in a volume large enough to follow the crucial spatial variations that influenced the process and its observability. We show that, while MH stars stopped far short of fully ionizing the universe, reionization began much earlier with MH sources than without, and was greatly extended, which boosts the intergalactic electron-scattering optical depth and the large-angle polarization fluctuations of the cosmic microwave background significantly. This boost should be readily detectable by Planck, although within current Wilkinson Microwave Anisotropy Probe uncertainties. If reionization ended as late as z(ov) less than or similar to 7, as suggested by other observations, Planck will thereby see the signature of the first stars at high redshift, currently undetectable by other probes.</P>

THE KINETIC SUNYAEV-ZEL'DOVICH EFFECT AS A PROBE OF THE PHYSICS OF COSMIC REIONIZATION: THE EFFECT OF SELF-REGULATED REIONIZATION

Park, Hyunbae,Shapiro, Paul R.,Komatsu, Eiichiro,Iliev, Ilian T.,Ahn, Kyungjin,Mellema, Garrelt IOP Publishing 2013 The Astrophysical journal Vol.769 No.2

<P>We calculate the angular power spectrum of the cosmic microwave background temperature fluctuations induced by the kinetic Sunyaev-Zel'dovich (kSZ) effect from the epoch of reionization (EOR). We use detailed N-body+radiative-transfer simulations to follow inhomogeneous reionization of the intergalactic medium. For the first time, we take into account the 'self-regulation' of reionization: star formation in low-mass dwarf galaxies (10(8) M-circle dot less than or similar to M less than or similar to 10(9) M-circle dot) or minihalos (10(5) M-circle dot less than or similar to M less than or similar to 10(8) M-circle dot) is suppressed if these halos form in the regions that were already ionized or Lyman-Werner dissociated. Some previous work suggested that the amplitude of the kSZ power spectrum from the EOR can be described by a two-parameter family: the epoch of half-ionization and the duration of reionization. However, we argue that this picture applies only to simple forms of the reionization history which are roughly symmetric about the half-ionization epoch. In self-regulated reionization, the universe begins to be ionized early, maintains a low level of ionization for an extended period, and then finishes reionization as soon as high-mass atomically cooling halos dominate. While inclusion of self-regulation affects the amplitude of the kSZ power spectrum only modestly (similar to 10%), it can change the duration of reionization by a factor of more than two. We conclude that the simple two-parameter family does not capture the effect of a physical, yet complex, reionization history caused by self-regulation. When added to the post-reionization kSZ contribution, our prediction for the total kSZ power spectrum is below the current upper bound from the South Pole Telescope. Therefore, the current upper bound on the kSZ effect from the EOR is consistent with our understanding of the physics of reionization.</P>

2D GENUS TOPOLOGY OF 21-CM DIFFERENTIAL BRIGHTNESS TEMPERATURE DURING COSMIC REIONIZATION

홍성욱,안경진,박창범,김주한,Ilian T. Iliev,Garrelt Mellema 한국천문학회 2014 Journal of The Korean Astronomical Society Vol.47 No.2

A novel method to characterize the topology of the early-universe intergalactic medium during the epoch of cosmic reionization is presented. The 21-cm radiation background from high redshift is analyzed through calculation of the 2-dimensional (2D) genus. The radiative transfer of hydrogen-ionizing photons and ionization-rate equations are calculated in a suite of numerical simulations under various input parameters. The 2D genus is calculated from the mock 21-cm images of high-redshift universe. We construct the 2D genus curve by varying the threshold differential brightness temperature, and compare this to the 2D genus curve of the underlying density field. We find that (1) the 2D genus curve reflects the evolutionary track of cosmic reionization and (2) the 2D genus curve can discriminate between certain reionization scenarios and thus indirectly probe the properties of radiation-sources. Choosing the right beam shape of a radio antenna is found crucial for this analysis. Square Kilometre Array (SKA) is found to be a suitable apparatus for this analysis in terms of sensitivity, even though some deterioration of the data for this purpose is unavoidable under the planned size of the antenna core.

2D genus topology of 21-cm differential brightness temperature during cosmic reionization

Kyungjin Ahn,Sungwook E. Hong,Changbom Park,Juhan Kim,Ilian T. Iliev,Garrelt Mellema 한국천문학회 2010 天文學會報 Vol.35 No.2