Broadening of QSO Lyα forest absorbers

We investigate the dependence of QSO Ly α absorption features on the temperature of the absorbing gas and on the amplitude of the underlying dark-matter fluctuations. We use high-resolution hydrodynamic simulations in cold dark matter dominated cosmological models. In models with a hotter intergalactic medium (IGM), the increased temperature enhances the pressure gradients between low- and high-density regions and this changes the spatial distribution and the velocity field of the gas. Combined with more thermal broadening, this leads to significantly wider absorption features in hotter models. Cosmological models with little small-scale power also have broader absorption features, because fluctuations on the scale of the Jeans length are still in the linear regime. Consequently, both the amplitude of dark-matter fluctuations on small scales and thermal smoothing affect the flux decrement distribution in a similar way. However, the b -parameter distribution of Voigt profile fits, obtained by deblending the absorption features into a sum of thermally broadened lines, is largely independent of the amount of small-scale power, but does depend strongly on the IGM temperature. The same is true for the two-point function of the flux and for the flux power spectrum on small scales. These three flux statistics are thus sensitive probes of the temperature of the IGM. We compare the values computed for our models and obtained from a HIRES spectrum of the quasar Q1422+231 and conclude that the IGM temperature at z ∼3.25 is fairly high, T ₀≳15 000 K. The flux decrement distribution of the observed spectrum is fitted well by that of a ΛCDM model with that temperature.     

The escape of ionizing photons from supernova-dominated primordial galaxies

In order to assess the contribution of Lyman break galaxies (LBGs) and Lyman α emitters (LAEs) at redshifts  3 < z < 7  to the ionization of intergalactic medium (IGM), we investigate the escape fractions of ionizing photons from supernova-dominated primordial galaxies by solving the three-dimensional (3D) radiative transfer. The model galaxy is employed from an ultra-high-resolution chemodynamic simulation of a primordial galaxy by Mori & Umemura, which well reproduces the observed properties of LAEs and LBGs. The total mass of model galaxy is  10¹¹ M_⊙  . We solve not only photoionization but also collisional ionization by shocks. In addition, according to the chemical enrichment, we incorporate the effect of dust extinction, taking the size distributions of dust into account. As a result, we find that dust extinction reduces the escape fractions by a factor of 1.5–8.5 in the LAE phase and by a factor of 2.5–11 in the LBG phase, while the collisional ionization by shocks increases the escape fractions by a factor of  ≈2  . The resultant escape fractions are 0.07–0.47 in the LAE phase and 0.06–0.17 in the LBG phase. These results are well concordant with the recent estimations derived from the flux density ratio at 1500 to 900 Å of LAEs and LBGs. Combining the resultant escape fractions with the luminosity functions of LAEs and LBGs, we find that high- z LAEs and LBGs can ionize the IGM at   z = 3–5  . However, ionizing radiation from LAEs as well as LBGs falls short of ionizing the IGM at   z > 6  . That implies that additional ionization sources may be required at   z > 6  .     

The contribution of the IGM and minihaloes to the 21-cm signal of reionization

We study the statistical properties of the cosmological 21-cm signal from both the intergalactic medium (IGM) and minihaloes, using a reionization simulation that includes a self-consistent treatment of minihalo photoevaporation. We consider two models for minihalo formation and three typical thermal states of the IGM – heating purely by ionization, heating from both ionizing and Lyα photons and a maximal 'strong heating' model. We find that the signal from the IGM is almost always dominant over that from minihaloes. In our calculation, the differential brightness temperature,  δ T _b,  of minihaloes is never larger than 2 mK. Although there are indeed some differences in the signals from the minihaloes and from the IGM, even with the planned generation of radio telescopes it will be unfeasible to detect them. However, minihaloes significantly affect the ionization state of the IGM and the corresponding 21-cm flux.     

Probing the growth of supermassive black holes at z > 6 with LOFAR

H  ii regions surrounding supermassive black holes (SMBHs) in an otherwise still neutral intergalactic medium (IGM) are likely to be the most easily detectable sources by future 21-cm experiments like LOFAR. We have made predictions for the size distribution of such H  ii regions for several physically motivated models for BH growth at high redshift and compared this to the expected LOFAR sensitivity to these sources. The number of potentially detectable H  ii regions does not only depend on the ionization state of the IGM and the decoupling of the spin temperature of the neutral hydrogen from the cosmic microwave background temperature, but is also strongly sensitive to the rate of growth of BHs at high redshift. If the SMBHs at redshift 6 were built up via continuous Eddington-limited accretion from low mass seed BHs at high redshift, then LOFAR is not expected to detect isolated QSO H  ii regions at redshifts much larger than 6, and only if the IGM is still significantly neutral. If the high-redshift growth of BHs starts with massive seed BHs and is driven by short-lived accretion events following the merging of BH hosting galaxies then the detection of H  ii regions surrounding SMBHs may extend to redshifts as large as 8–9 but is still very sensitive to the redshift to which the IGM remains significantly neutral. The most optimistic predictions are for a model where the SMBHs at z > 6 have grown slowly. H  ii regions around SMBHs may then be detected to significantly larger redshifts.     

The effects of photoionization on galaxy formation – I. Model and results at z=0

We develop a coupled model for the evolution of the global properties of the intergalactic medium (IGM) and the formation of galaxies, in the presence of a photoionizing background due to stars and quasars. We use this model to predict the thermodynamic history of the IGM when photoionized by galaxies forming in a cold dark matter (CDM) universe. The evolution of the galaxies is calculated using a semi-analytical model, including a detailed treatment of the effects of tidal stripping and dynamical friction on satellite galaxies orbiting inside larger dark matter haloes. We include in the model the negative feedback on galaxy formation from the photoionizing background. Photoionization inhibits galaxy formation in low-mass dark matter haloes in two ways: (i) heating of the IGM and inhibition of the collapse of gas into dark haloes by the IGM pressure, and (ii) reduction in the rate of radiative cooling of gas within haloes. The result of our method is a self-consistent model of galaxy formation and the IGM. The IGM is reheated twice (during reionization of H  i and He  ii ), and we find that the star formation rate per unit volume is slightly suppressed after each episode of reheating. We find that galaxies brighter than L _★ are mostly unaffected by reionization, while the abundance of faint galaxies is significantly reduced, leading to present-day galaxy luminosity functions with shallow faint-end slopes, in good agreement with recent observational data. Reionization also affects other properties of these faint galaxies, in a readily understandable way.     

Heating of the intergalactic medium by primordial miniquasars

A simple analytical model is used to calculate the X-ray heating of the intergalactic medium (IGM) for a range of black hole masses. This process is efficient enough to decouple the spin temperature of the IGM from the cosmic microwave background (CMB) temperature and produce a differential brightness temperature of the order of ∼ 5–20 mK out to distances as large as a few comoving Mpc, depending on the redshift, black hole mass and lifetime. We explore the influence of two types of black holes, those with and without ionizing ultraviolet radiation. The results of the simple analytical model are compared to those of a full spherically symmetric radiative transfer code. Two simple scenarios are proposed for the formation and evolution of black hole mass density in the Universe. The first considers an intermediate mass black hole that form as an end-product of pop III stars, whereas the second considers supermassive black holes that form directly through the collapse of massive haloes with low spin parameter. These scenarios are shown not to violate any of the observational constraints, yet produce enough X-ray photons to decouple the spin temperature from that of the CMB. This is an important issue for future high-redshift 21-cm observations.     

The correlation between star formation and 21-cm emission during the reionization epoch

Reionization is thought to be dominated by low-mass galaxies, while direct observations of resolved galaxies probe only the most massive, rarest objects. The cross-correlation between fluctuations in the surface brightness of the cumulative Lyα emission (which serves as a proxy for the star formation rate) and the redshifted 21-cm signal from neutral hydrogen in the intergalactic medium (IGM) will directly probe the causal link between the production of ionizing photons in galaxies and the reionization of the IGM. We discuss the prospects for detecting this cross-correlation for unresolved galaxies. We find that on angular scales ≲10 arcmin detection will be practical using wide-field near-infrared (near-IR) imaging from space in combination with the forthcoming Mileura Wide-field Array – Low Frequency Demonstrator. When redshifted 21-cm observations of the neutral IGM are combined with space-based near-IR imaging of Lyα emission, the detection on angular scales ≲3 arcmin will be limited by the sensitivity of the 21-cm signal, even when a small-aperture optical telescope (∼2 m) and a moderate field of view (∼10 deg²) are used. On scales ≳3 arcmin, the measurement of cross-correlation will be limited by the accuracy of the foreground sky subtraction.     

Metal enrichment of the intergalactic medium

I demonstrate by means of high-resolution cosmological simulations, which include modelling of a two-phase interstellar medium, that the dominant mechanism for transporting heavy elements from protogalaxies into the intergalactic medium (IGM) is the merger mechanism as discovered by Gnedin & Ostriker. Direct ejection of the interstellar gas by supernovae plays only a minor role in transporting metals into the IGM: for a realistic cosmological scenario only a small fraction of all metals in the IGM is delivered by the supernova-driven winds, while most of the metals in the IGM are transported by the merger mechanism. As a result, the metallicity distribution in the IGM is highly inhomogeneous, in agreement with studies of the QSO metal absorption systems, and the predicted metallicity distribution of Lyman alpha absorbers as a function of their column density is in excellent agreement with the observational data.     

Radiation from early black holes – I. Effects on the neutral intergalactic medium

In the pre-reionization Universe, the regions of the intergalactic medium (IGM) which are far from luminous sources are the last to undergo reionization. Until then, they should be scarcely affected by stellar radiation; instead, the X-ray emission from an early black hole (BH) population can have much larger influence. We investigate the effects of such emission, looking at a number of BH model populations (differing for the cosmological density evolution of BHs, the BH properties, and the spectral energy distribution of the BH emission). We find that BH radiation can easily heat the IGM to  10³–10⁴ K  , while achieving partial ionization. The most interesting consequence of this heating is that BHs are expected to induce a 21-cm signal (  δ T _b∼ 20–30 mK  at   z ≲ 12  ) which should be observable with forthcoming experiments (e.g. LOFAR). We also find that at   z ≲ 10  BH emission strongly increases the critical mass separating star-forming and non-star-forming haloes.     

H i imaging of galaxies in X-ray bright groups

Environment plays an important role in the evolution of the gas contents of galaxies. Gas deficiency of cluster spirals and the role of the hot intracluster medium in stripping gas from these galaxies is a well-studied subject. Loose groups with diffuse X-ray emission from the intragroup medium (IGM) offer an intermediate environment between clusters and groups without a hot IGM. These X-ray bright groups have smaller velocity dispersion and lower temperature than clusters, but higher IGM density than loose groups without diffuse X-ray emission. A single-dish comparative study of loose groups with and without diffuse X-ray emission from the IGM, showed that the galaxies in X-ray bright groups have lost more gas on average than the galaxies in non X-ray bright groups. In this paper we present GMRT H  i observations of 13 galaxies from four X-ray bright groups: NGC 5044, 720, 1550 and IC1459. The aim of this work is to study the morphology of H  i in these galaxies and to see if the hot IGM has in any way affected their H  i content or distribution. In addition to disturbed H  i morphology, we find that most galaxies have shrunken H  i discs compared to the field spirals. This indicates that IGM-assisted stripping processes like ram pressure may have stripped gas from the outer edges of the galaxies.     

Spatial fluctuations in the spectral shape of the ultraviolet background at 2 < z < 3 and the reionization of helium

The low-density hydrogen and helium in the intergalactic medium (IGM) probed by quasi-stellar object (QSO) absorption lines is sensitive to the amplitude and spectral shape of the metagalactic ultraviolet (UV) background. We use realistic H  i and He  ii Lyα forest spectra, constructed from state-of-the-art hydrodynamical simulations of a Λ cold dark matter (ΛCDM) universe to confirm the reliability of using line profile fitting techniques to infer the ratio of the metagalactic H  i and He  ii ionization rates. We further show that the large spatial variations and the anticorrelation with H  i absorber density observed in the ratio of the measured He  ii to H  i column densities can be explained in a model where the H  i ionization rate is dominated by the combined UV emission from young star-forming galaxies and QSOs and the He  ii ionization rate is dominated by emission from QSOs only. In such a model the large fluctuations in the column density ratio are due to the small number of QSOs expected to contribute at any given point to the He  ii ionization rate. A significant contribution to UV emission at the He  ii photoelectric edge from hot gas in galaxies and galaxy groups would decrease the expected fluctuations in the column density ratio. Consequently, this model appears difficult to reconcile with the large increase in He  ii opacity fluctuations towards higher redshift. Our results further strengthen previous suggestions that observed He  ii Lyα forest spectra at z ∼ 2–3.5 probe the tail end of the reionization of He  ii by QSOs.     

Practical SPH models for major planets

Modelling planets is done for two main reasons – the first to further understanding of their internal structure and the second to provide models to explore astrophysical situations in which planets play a role. For the latter reason, the requirements on accuracy are less severe, although the planet must be realistic in its major features. A numerical model of a layered giant planet is developed with an iron core, a silicate mantle, an ice region and a hydrogen–helium atmosphere. The Tillotson equation of state is used and examples of two model planets are given, one reproducing the mass and radius of Jupiter quite closely and the other with two Jupiter masses. Transferring these results into a smoothed particle hydrodynamics (SPH) model presents two main difficulties. A uniform distribution of SPH points leads to too few points representing the non-atmospheric component. It is shown that using a distorted lattice enables the core + silicate + ice to be represented by several hundred points so that the evolution of these regions can be followed in detail. Another difficulty concerns the density discontinuities attendant on a layered structure. Density estimates of SPH points are either too large or too small near material interfaces leading to unrealistic pressure gradients and, consequently, to large and unphysical local forces. Algorithms are described for avoiding this difficulty both at material interfaces and near the surface of the planet. In some astrophysical situations involving SPH-modelled planets, the main bulk of the planet is so opaque that internal heat transfer can be neglected. However, surface regions should radiate and a convenient way for including radiation from a planetary surface is described.     

Comments on faintness of very small dwarf spheroidal galaxies

Recent observational studies have discovered very small dwarf spheroidal galaxies (dSphs) which are the faintest member of the local group of galaxies. This paper examines their faintness because of the following reason: Comparing their M/L (mass-luminosity ratio) to that of the other normal dSphs, we find very small dSphs are faint for their dark matter mass. This indicates their star formation is suppressed. There are two possibilities for the suppression: (1) ram pressure of IGM (intra-group medium), (2) wind from the Milky Way (MW). Owing to the ram pressure, interstellar medium of very small dSphs is possible to be stripped because of the shallowness of their gravitational potential. That is, star formation can be terminated during their evolution. However, the latter is difficult at the moment since their distance is far from MW. The author suggests star formation was terminated only when very small dSphs were beside MW whose wind was strong.     

Semi-analytic simulations of galactic winds: volume filling factor, ejection of metals and parameter study

We present a semi-analytic treatment of galactic winds within high-resolution, large-scale cosmological N -body simulations of a Λ cold dark matter (ΛCDM) universe. The evolution of winds is investigated by following the expansion of supernova-driven superbubbles around the several hundred thousand galaxies that form in an approximately spherical region of space with diameter 52  h ⁻¹ Mpc and mean density close to the mean density of the universe. We focus our attention on the impact of winds on the diffuse intergalactic medium. Initial conditions for mass loss at the base of winds are taken from Shu, Mo & Mao. Results are presented for the volume filling factor and the mass fraction of the intergalactic medium (IGM) affected by winds, and their dependence on the model parameters is carefully investigated. The mass-loading efficiency of bubbles is a key factor to determine the evolution of winds and their global impact on the IGM: the higher the mass loading, the later the IGM is enriched with metals. Galaxies with 10⁹ < M _★ < 10¹⁰ M_⊙ are responsible for most of the metals ejected into the IGM at   z = 3  , while galaxies with   M _★ < 10⁹ M_⊙   give a non-negligible contribution only at higher redshifts, when larger galaxies have not yet assembled. We find a higher mean IGM metallicity than Lyα forest observations suggest, and we argue that the discrepancy may be explained by the high temperatures of a large fraction of the metals in winds, which may not leave detectable imprints in absorption in the Lyα forest.     

Intergalactic medium metal enrichment through dust sputtering

We study the motion of dust grains into the intergalactic medium (IGM) around redshift   z = 3  , to test the hypothesis that grains can efficiently pollute the gas with metals through sputtering. We use the results available in the literature for radiation-driven dust ejection from galaxies as initial conditions and follow the motion onwards. Via this mechanism, grains are ejected into the IGM with velocities  >100 km s⁻¹  ; as they move supersonically, grains can be efficiently eroded by non-thermal sputtering. However, Coulomb and collisional drag forces effectively reduce the charged grain velocity. Up-to-date sputtering yields for graphite and silicate (olivine) grains have been derived using the code transport of ions in matter ( trim ), for which we provide analytic fits. After training our method on a homogeneous density case, we analyse the grain motion and sputtering in the IGM density field as derived from a Λ cold dark matter (CDM) cosmological simulation at   z = 3.27  . We found that only large  ( a ≳ 0.1μm)  grains can travel up to considerable distances (few  ×100 kpc  physical) before being stopped. Resulting metallicities show a well-defined trend with overdensity δ. The maximum metallicities are reached for  10 < δ < 100  [corresponding to systems, in quasi-stellar object (QSO) absorption spectra, with  14.5 < log N (H  i ) < 16  ]. However the distribution of sputtered metals is very inhomogeneous, with only a small fraction of the IGM volume polluted by dust sputtering (filling factors of 18 per cent for Si and 6 per cent for C). For the adopted size distribution, grains are never completely destroyed; nevertheless, the extinction and gas photoelectric heating effects resulting from this population of intergalactic grains are well below current detection limits.     

Hot and cold gas accretion and feedback in radio-loud active galaxies

We have recently shown that X-ray observations of the population of 'low-excitation' radio galaxies, which includes most low-power, Fanaroff–Riley class I sources as well as some more powerful Fanaroff–Riley class II objects, are consistent with a model in which the active nuclei of these objects are not radiatively efficient at any waveband. In another recent paper, Allen et al. have shown that Bondi accretion of the hot, X-ray emitting phase of the intergalactic medium (IGM) is sufficient to power the jets of several nearby, low-power radio galaxies at the centres of clusters. In this paper, we combine these ideas and suggest that accretion of the hot phase of the IGM is sufficient to power all low-excitation radio sources, while high-excitation sources are powered by accretion of cold gas that is in general unrelated to the hot IGM. This model explains a number of properties of the radio-loud active galaxy population, and has important implications for the energy input of radio-loud active galactic nuclei into the hot phase of the IGM: the energy supply of powerful high-excitation sources does not have a direct connection to the hot phase.     

Redshifted 21-cm observations of high-redshift quasar proximity zones

The introduction of low-frequency radio arrays is expected to revolutionize the study of the reionization epoch. Observation of the contrast in redshifted 21-cm emission between a large H  ii region and the surrounding neutral intergalactic medium (IGM) will be the simplest and most easily interpreted signature. However, the highest redshift quasars known are thought to reside in an ionized IGM. Using a semi-analytic model we describe the redshifted 21-cm signal from the IGM surrounding quasars discovered using the i -drop-out technique (i.e. quasars at   z ∼ 6  ). We argue that while quasars at   z < 6.5  seem to reside in the post-overlap IGM, they will still provide valuable probes of the late stages of the overlap era because the light-travel time across a quasar proximity zone should be comparable to the duration of overlap. For redshifted 21-cm observations within a 32-MHz bandpass, we find that the subtraction of a spectrally smooth foreground will not remove spectral features due to the proximity zone. These features could be used to measure the neutral hydrogen content of the IGM during the late stages of reionization. The density of quasars at   z ∼ 6  is now well constrained. We use the measured quasar luminosity function to estimate the prospects for discovery of high-redshift quasars in fields that will be observed by the Murchison Widefield Array.     

Mixing metals in the early Universe

We investigate the evolution of the metallicity of the intergalactic medium (IGM) with particular emphasis on its spatial distribution. We propose that metal enrichment occurs as a two-step process. First, supernova (SN) explosions eject metals into relatively small regions confined to the surroundings of star-forming galaxies. From a comprehensive treatment of blowout we show that SN by themselves fail by more than one order of magnitude to distribute the products of stellar nucleosynthesis over volumes large enough to pollute the whole IGM to the metallicity levels observed. Thus, an additional (but as yet unknown) physical mechanism must be invoked to mix the metals on scales comparable to the mean distance between the galaxies that are most efficient pollutants. From this simple hypothesis we derive a number of testable predictions for the evolution of the IGM metallicity. Specifically, we find that: (i) the fraction of metals ejected over the star-formation history of the Universe is about 50 per cent at     that is, approximately half of the metals today are found in the IGM; (ii) if the ejected metals were homogeneously mixed with the baryons in the Universe, the average IGM metallicity would be     at     However, due to spatial inhomogeneities, the mean of the distribution of metallicities in the diffusive zones has a wide (more than 2 orders of magnitude) spread around this value; (iii) if metals become more uniformly distributed at     as assumed, at     the metallicity of the IGM is narrowly confined within the range     Finally, we point out that our results can account for the observed metal content of the intracluster medium.     

The effects of primordial non-Gaussianity on the cosmological reionization

We investigate the effects of non-Gaussianity in the primordial density field on the reionization history. We rely on a semi-analytic method to describe the processes acting on the intergalactic medium (IGM), relating the distribution of the ionizing sources to that of dark matter haloes. Extending previous work in the literature, we consider models in which the primordial non-Gaussianity is measured by the dimensionless non-linearity parameter f _NL, using the constraints recently obtained from cosmic microwave background data. We predict the ionized fraction and the optical depth at different cosmological epochs assuming two different kinds of non-Gaussianity characterized by a scale-independent and a scale-dependent f _NL and comparing the results to those for the standard Gaussian scenario. We find that a positive f _NL enhances the formation of high-mass haloes at early epochs when reionization begins, and, as a consequence, the IGM ionized fraction can grow by a factor of up to 5 with respect to the corresponding Gaussian model. The increase of the filling factor has a small impact on the reionization optical depth and is of the order of ∼10 per cent if a scale-dependent non-Gaussianity is assumed. Our predictions for non-Gaussian models are in agreement with the latest Wilkinson Microwave Anisotropy Probe results within the error bars, but a higher precision is required to constrain the scale dependence of non-Gaussianity.