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Diffuse, extended radio emission in galaxy clusters, commonly referred to as radio haloes, indicate the presence of high energy cosmic ray (CR) electrons and cluster-wide magnetic fields. We can predict from theory the expected surface brightness of a radio halo, given magnetic field and CR density profiles. Previous studies have shown that the nature of CR transport can radically effect the expected radio halo emission from clusters (Wiener, Oh & Guo 2013). Reasonable levels of magnetohydrodynamic (MHD) wave damping can lead to significant CR streaming speeds. But a careful treatment of MHD waves in a high β plasma, as expected in cluster environments, reveals damping rates may be enhanced by a factor of β1/2. This leads to faster CR streaming and lower surface brightnesses than without this effect. In this work, we re-examine the simplified, 1D Coma cluster simulations (with radial magnetic fields) of Wiener et al. (2013) and discuss observable consequences of this high β damping. Future work is required to study this effect in more realistic simulations. [ABSTRACT FROM AUTHOR]

A key physical quantity during reionization is the size of H II regions. Previous studies found a characteristic bubble size which increases rapidly during reionization, with apparent agreement between simulations and analytic excursion set theory. Using four different methods, we critically examine this claim. In particular, we introduce the use of the watershed algorithm - widely used for void finding in galaxy surveys - which we show to be an unbiased method with the lowest dispersion and best performance on Monte Carlo realizations of a known bubble size probability density function (PDF).We find that a friends-of-friends algorithm declares most of the ionized volume to be occupied by a network of volume-filling regions connected by narrow tunnels. For methods tuned to detect the volume-filling regions, previous apparent agreement between simulations and theory is spurious, and due to a failure to correctly account for the window function of measurement schemes. The discrepancy is already obvious from visual inspection. Instead, H II regions in simulations are significantly larger (by factors of 10-1000 in volume) than analytic predictions. The size PDF is narrower, and evolves more slowly with time, than predicted. It becomes more sharply peaked as reionization progresses. These effects are likely caused by bubble mergers, which are inadequately modelled by analytic theory. Our results have important consequences for high-redshift 21 cm observations, the mean free path of ionizing photons, and the visibility of Lyα emitters, and point to a fundamental failure in our understanding of the characteristic scales of the reionization process. [ABSTRACT FROM AUTHOR]

The ionizing background of cosmic hydrogen is an important probe of the sources and absorbers of ionizing radiation in the post-reionization universe. Previous studies showthat the ionization rate should be very sensitive to changes in the source population: as the emissivity rises, absorbers shrink in size, increasing the ionizing mean free path and, hence, the ionizing background. By contrast, observations of the ionizing background find a very flat evolution from z ~ 2-5, before falling precipitously at z ~ 6. We resolve this puzzling discrepancy by pointing out that, at z ~ 2-5, optically thick absorbers are associated with the same collapsed haloes that host ionizing sources. Thus, an increasing abundance of galaxies is compensated for by a corresponding increase in the absorber population, which moderates the instability in the ionizing background. However, by z~5-6, gas outside of haloes dominates the absorption, the coupling between sources and absorbers is lost, and the ionizing background evolves rapidly. Our halo-based model reproduces observations of the ionizing background, its flatness and sudden decline, as well as the redshift evolution of the ionizing mean free path. Our work suggests that, through much of their history, both star formation and photoelectric opacity in the universe track halo growth. [ABSTRACT FROM AUTHOR]

We assess the impact of Galactic synchrotron foreground removal on the observation of high-redshift quasar H ii regions in redshifted 21-cm emission. We consider the case where a quasar is observed in an intergalactic medium (IGM) whose ionization structure evolves slowly relative to the light crossing time of the H ii region, as well as the case where the evolution is rapid. The latter case is expected towards the end of the reionization era where the highest redshift luminous quasars will be observed. In the absence of foregrounds, the fraction of neutral hydrogen in the IGM could be measured directly from the contrast between the H ii region and surrounding IGM. However, we find that foreground removal lowers the observed contrast between the H ii region and the IGM. This indicates that measurement of the neutral fraction would require modelling to correct for this systematic effect. On the other hand, foreground removal does not modify the most prominent features of the 21-cm maps. Using a simple algorithm, we demonstrate that measurements of the size and shape of observed H ii regions will not be affected by continuum foreground removal. Moreover, measurements of these quantities will not be adversely affected by the presence of a rapidly evolving IGM. [ABSTRACT FROM AUTHOR]

Hydrogen Lyman α (Lyα) is our primary emission-line window into high-redshift galaxies. Despite an extensive literature, Lyα radiative transfer in the most realistic case of a dusty, multiphase medium has received surprisingly little detailed theoretical attention. We investigate Lyα resonant scattering through an ensemble of dusty, moving, optically thick gas clumps. We treat each clump as a scattering particle and use Monte Carlo simulations of surface scattering to quantify continuum and Lyα surface scattering angles, absorption probabilities, and frequency redistribution, as a function of the gas dust content. This atomistic approach speeds up the simulations by many orders of magnitude, making possible calculations which are otherwise intractable. Our fitting formulae can be readily adapted for fast radiative transfer in numerical simulations. With these surface scattering results, we develop an analytic framework for estimating escape fractions and line widths as a function of gas geometry, motion, and dust content. Our simple analytic model shows good agreement with full Monte Carlo simulations. We show that the key geometric parameter is the average number of surface scatters for escape in the absence of absorption, , and we provide fitting formulae for several geometries of astrophysical interest. We consider the following two interesting applications. (i) Equivalent widths ( EWs). Lyα can preferentially escape from a dusty multiphase interstellar medium if most of the dust lies in cold neutral clouds, which Lyα photons cannot penetrate. This might explain the anomalously high EWs sometimes seen in high-redshift/submillimetre sources. (ii) Multiphase galactic outflows. We show the characteristic profile is asymmetric with a broad red tail, and relate the profile features to the outflow speed and gas geometry. Many future applications are envisaged. [ABSTRACT FROM AUTHOR]

ABSTRACT Both pre-heating of the intergalactic medium and radiative cooling of low entropy gas have been proposed to explain the deviation from self-similarity in the cluster L[sub X]–T[sub X] relation and the observed entropy floor in these systems. However, severe overcooling of gas in groups is necessary for radiative cooling alone to explain the observations. Non-gravitational entropy injection must therefore still be important in these systems. We point out that, on scales of groups and below, gas heated to the required entropy floor cannot cool in a Hubble time, regardless of its subsequent adiabatic compression. Pre-heating therefore shuts off the gas supply to galaxies, and should be an important global feedback mechanism for galaxy formation. Constraints on global gas cooling can be placed from the joint evolution of the comoving star formation rate and neutral gas density. Pre-heating at high redshift can be ruled out; however, the data do not rule out passive gas consumption without inflow as z∼ 2 . Because for pre-heated gas t[sub cool] > t[sub dyn] , we speculate that pre-heating could play a role in determining the Hubble sequence; at a given mass scale, high σ peaks in the density field collapse early to form ellipticals, while low σ peaks collapse late and quiescently accrete pre-heated gas to form spirals. The entropy produced by large-scale shock-heating of the intergalatic medium is significant only at late times, z < 1 , and cannot produce these effects. [ABSTRACT FROM AUTHOR]

ABSTRACT WMAP's detection of high electron-scattering optical depth τ[sub e] suggests substantial star formation at high redshift z∼ 17 ± 5 . On the other hand, the recovered σ[sub 8]∼ 0.84 ± 0.04 argues against a cluster Sunyaev–Zeldovich (SZ) origin for the observed small-scale cosmic microwave background (CMB) fluctuation excess, which generally requires σ[sub 8]∼ 1.1 . Here we consider the effects of high-redshift star formation on the CMB. We derive a fairly model-independent relation between τ[sub e] and the number of ionizing photons emitted per baryon N[sub γ] , and use this to calibrate the amount of high-redshift supernova activity. The resulting supernova remnants Compton cool against the CMB creating a Compton y distortion y∼ few × 10[sup -6] within observational bounds. However they also create small-scale SZ fluctuations, which could be comparable with SZ fluctuations from unresolved galaxy clusters. This raises the exciting possibility that we have already detected signatures of the first stars not just once, but twice, in the CMB. [ABSTRACT FROM AUTHOR]

Abstract We show that gas in a large fraction of low-mass dark matter haloes may form Toomre-stable discs, if angular momentum is conserved when the gas contracts. Such haloes would be stable to star formation and therefore remain dark. This may potentially explain deviation between the predicted and the observed faint-end slope of the luminosity function and the discrepancy between the predicted and observed number of dwarf satellites in the Local Group. The above mechanism does not require a strong variation of the baryon fraction with the virial mass of the dark halo. We show that model fits to rotation curves are also consistent with this hypothesis: none of the observed galaxies lies in the region of parameter space forbidden by the Toomre stability criterion. [ABSTRACT FROM AUTHOR]