Photoevaporation of Cosmological Minihalos during Reionization

We present the first gas dynamical simulations of the photoevaporation of cosmological minihalos overtaken by the ionization fronts which swept through the IGM during reionization in a LCDM universe, including the effects of radiative transfer. We demonstrate the phenomenon of I-front trapping inside minihalos, in which the weak, R-type fronts which traveled supersonically across the IGM decelerated when they encountered the dense, neutral gas inside minihalos, becoming D-type I-fronts, preceded by shock waves. For a minihalo with virial temperature T_vir < 10^4 K, the I-front gradually burned its way through the minihalo which trapped it, removing all of its baryonic gas by causing a supersonic, evaporative wind to blow backwards into the IGM, away from the exposed layers of minihalo gas just behind the advancing I-front. Such hitherto neglected feedback effects were widespread during reionization. N-body simulations and analytical estimates of halo formation suggest that sub-kpc minihalos such as these, with T_vir < 10^4 K, were so common as to cover the sky around larger-mass source halos and possibly dominate the absorption of ionizing photons. This means that previous estimates of the number of ionizing photons per H atom required to complete reionization which neglected this effect may be too low. Regardless of their effect on the progress of reionization, however, the minihalos were so abundant that random lines of sight thru the high-z universe should encounter many of them, which suggests that it may be possible to observe the processes described here in the absorption spectra of distant sources.Comment: 34 pages, 34 figures, submitted to MNRAS. Computer animations at http://galileo.as.utexas.ed

Photon Consumption in Minihalos during Cosmological Reionization

At the earliest epochs of structure formation in cold dark matter (CDM) cosmologies, the smallest nonlinear objects are the numerous small halos that condense with virial temperatures below 10,000 K. Such ``minihalos'' are not yet resolved in large-scale three-dimensional cosmological simulations. Here we employ a semi-analytic method, combined with three-dimensional simulations of individual minihalos, to examine their importance during cosmological reionization. We show that, depending on when reionization takes place, they potentially play an important role as sinks of ionizing radiation. If reionization occurs at sufficiently high redshifts (z_r > 20), the intergalactic medium is heated to 10,000 K and most minihalos never form. On the other hand, if z_r 10 percent) of all baryons have already collapsed into minihalos, and are subsequently removed from the halos by photoevaporation as the ionizing background flux builds up. We show that this process can require a significant budget of ionizing photons; exceeding the production by a straightforward extrapolation back in time of known quasar and galaxy populations by a factor of up to 10 and 3, respectively.Comment: ApJ accepted version, with clarifications added in tex

The Impact of Small-Scale Structure on Cosmological Ionization Fronts and Reionization

The propagation of cosmological ionization fronts during the reionization of the universe is strongly influenced by small-scale gas inhomogeneities due to structure formation. These inhomogeneities include both collapsed minihalos, which are generally self-shielding, and lower-density structures, which are not. The minihalos are dense and sufficiently optically-thick to trap intergalactic ionization fronts, blocking their path and robbing them of ionizing photons until the minihalo gas is expelled as an evaporative wind. The lower-density structures do not trap these fronts, but they can slow them down by increasing the overall recombination rate in the intergalactic medium. In this paper we study the effects of both types of inhomogeneities, including nonlinear clustering effects, and we find that both IGM clumping and collapsed minihalos have significant yet qualitatively different impacts on reionization. While the number density of minihalos on average increases strongly with time, the density of minihalos inside H II regions around ionizing sources is largely constant. Thus the impact of minihalos is essentially to decrease the number of ionizing photons available to the IGM at all epochs, which is equivalent to a reduction in the luminosity of each source. On the other hand, the effect of IGM clumping increases strongly with time, slowing down reionization and extending it. Thus while the impact of minihalos is largely degenerate with the unknown source efficiency, IGM clumping can help significantly in reconciling the recent observations of cosmic microwave background polarization with quasar absorption spectra at z~6, which together point to an early but extended reionization epoch.Comment: 15 pages, 9 figures, minor revisions to respond to referee comments, accepted for publication in The Astrophysical Journa

The Effect of Absorption Systems on Cosmic Reionization

We use large-scale simulations to investigate the morphology of reionization during the final, overlap phase. Our method uses an efficient three-dimensional smoothing technique which takes into account the finite mean free path due to absorption systems, lambda, by only smoothing over scales R_s<lambda. The large dynamic range of our calculations is necessary to resolve the neutral patches left at the end of reionization within a representative volume; we find that simulation volumes exceeding several hundred Mpc on a side are necessary in order to properly model reionization when the neutral fraction is ~0.01-0.3. Our results indicate a strong dependence of percolation morphology on a large and uncertain region of model parameter space. The single most important parameter is the mean free path to absorption systems, which serve as opaque barriers to ionizing radiation. If these absorption systems were as abundant as some realistic estimates indicate, the spatial structure of the overlap phase is considerably more complex than previously predicted. In view of the lack of constraints on the mean free path at the highest redshifts, current theories that do not include absorption by Lyman-limit systems, and in particular three-dimensional simulations, may underestimate the abundance of neutral clouds at the end of reionization. This affects predictions for the 21 cm signal associated with reionization, interpretation of absorption features in quasar spectra at z ~5-6, the connection between reionization and the local universe, and constraints on the patchiness and duration of reionization from temperature fluctuations measured in the cosmic microwave background arising from the kinetic Sunyaev-Zel'dovich effect.Comment: Accepted for publication in the Astrophysical Journal. Substantial revision from previous version. Comments welcom

Implications of WMAP 3 Year Data for the Sources of Reionization

New results on the anisotropy of the cosmic microwave background (CMB) and its polarization based on the first 3 years of data from the Wilkinson Microwave Anisotropy Probe (WMAP) have revised the electron scattering optical depth downward from tau_es=0.17+0.08-0.07 to tau_es=0.09+/-0.03. This implies a shift of the effective reionization redshift from z_r~17 to z_r~11. Previous attempts to explain the high redshift of reionization inferred from the WMAP 1 year data have led to widespread speculation that the sources of reionization must have been much more efficient than those associated with the star formation observed at low redshift. This is consistent, for example, with the suggestion that early star formation involved massive, Population III stars that early on produced most of the ionizing radiation escaping from halos. It is therefore tempting to interpret the new WMAP results as implying that we can now relax those previous high demands on the efficiency of the sources of reionization and perhaps even turn the argument around as evidence against such high efficiency. We show that this is not the case, however. The new WMAP results also find that the primordial density fluctuation power spectrum has a lower amplitude, sigma_8, and departs substantially from the scale-invariant spectrum. We show that these effects combine to cancel the impact of the later reionization implied by the new value of tau_es on the required ionizing efficiency per collapsed baryon. The delay of reionization is surprisingly well matched by a comparable delay (by a factor of ~1.4 in scale factor) in the formation of the halos responsible for reionization.Comment: 4 pages, 3 figures, Published in ApJ Letters, revised to match published versio

The First Galaxies: Assembly under Radiative Feedback from the First Stars

We investigate how radiative feedback from the first stars affects the assembly of the first dwarf galaxies. We perform cosmological zoomed SPH simulations of a dwarf galaxy assembling inside a halo of virial mass 10^9 solar at z = 10. The simulations follow the non-equilibrium chemistry/cooling of primordial gas and the conversion of the gas into metal-free stars. To quantify the radiative feedback, we compare a simulation in which stars emit both molecular hydrogen dissociating and hydrogen/helium ionizing radiation with a simulation in which stars emit only dissociating radiation, and with a simulation in which stars remain dark. Photodissociation and -ionization exert a strong negative feedback on the assembly of the simulated galaxy. Gas condensation is strongly impeded, and star formation is strongly suppressed in comparison with the simulation in which stars remain dark. The feedback on the gas implies a suppression of the central dark matter densities in the minihalo progenitor by factors of up to a few, which is a significant deviation from the singular isothermal density profile characterizing the dark matter distribution in the absence of radiative feedback. The evolution of gas densities, star formation rates, and the distribution of dark matter becomes insensitive to the inclusion of dissociating radiation in the late stages of the minihalo assembly, and it becomes insensitive to the inclusion of ionizing radiation once the minihalo turns into an atomically cooling galaxy. The formation of an extended disk inside the dwarf galaxy is a robust outcome not affected by the inclusion of radiation. We estimate that dwarf galaxies such as simulated here will be among the faintest galaxies the upcoming James Webb Space Telescope will detect. Our conclusions are subject to our neglect of feedback from supernovae and chemical enrichment as well as to cosmic variance. [abridged]Comment: 25 pages (including 5 pages appendix), 13 figures. Accepted for publication in Ap

The Opacity of the Intergalactic Medium During Reionization: Resolving Small-Scale Structure

Early in the reionization process, the intergalactic medium (IGM) would have been quite inhomogeneous on small scales, due to the low Jeans mass in the neutral IGM and the hierarchical growth of structure in a cold dark matter Universe. This small-scale structure acted as an important sink during the epoch of reionization, impeding the progress of the ionization fronts that swept out from the first sources of ionizing radiation. Here we present results of high-resolution cosmological hydrodynamics simulations that resolve the cosmological Jeans mass of the neutral IGM in representative volumes several Mpc across. The adiabatic hydrodynamics we follow are appropriate in an unheated IGM, before the gas has had a chance to respond to the photoionization heating. Our focus is determination of the resolution required in cosmological simulations in order to sufficiently sample and resolve small-scale structure regulating the opacity of an unheated IGM. We find that a dark matter particle mass of m_dm 1 Mpc are required. With our converged results we show how the mean free path of ionizing radiation and clumping factor of ionized hydrogen depends upon the ultraviolet background (UVB) flux and redshift. We find, for example at z = 10, clumping factors typically of 10 to 20 for an ionization rate of Gamma ~ 0.3 - 3 x 1e-12 s^-1, with corresponding mean free paths of ~ 3 - 15 Mpc, extending previous work on the evolving mean free path to considerably smaller scales and earlier times.Comment: Accepted for publication in the Astrophysical Journa

Relativistic Ionization Fronts

We derive the equations for the propagation of relativistic ionization fronts in both static and moving gases. We focus on the supersonic R-type phase that occurs right after a source turns on, and we compare the nonrelativistic and relativistic solutions for several important cases. Relativistic corrections can be significant up until the light-crossing time of the equilibrium Stromgren sphere. For a static medium, we obtain exact analytical solutions and apply them to the illustrative problems of an O star in a molecular cloud and a starburst in a high-redshift cosmological halo. Relativistic corrections can be important at early times when the H II regions are small, as well as at later times, if a density gradient causes the I-front to accelerate. For the cosmologically-expanding IGM, we derive an analytical solution in the case of a steady source and a constant clumping factor. Here relativistic corrections are significant for short-lived, highly-luminous sources like QSOs at the end of reionization, but negligible for weaker or higher-redshift sources. Finally, we numerically calculate the evolution of relativistic I-fronts in the presence of small-scale structure and infall, for a large galaxy undergoing a starburst and a luminous, high-redshift QSO. For such strong and short-lived sources, the relativistic corrections are quite significant, and small-scale structure can decrease the size of the H II region by up to an additional ~25%. (abridged)Comment: 18 pages, 7 figures, ApJ in print, resubmitted to match the accepted version, comments welcom

The Era of Massive Population III Stars: Cosmological Implications and Self-Termination

The birth and death of the first generation of stars have important implications for the thermal state and chemical properties of the intergalactic medium (IGM) in the early universe. Sometime after recombination, the neutral, chemically pristine gas was reionized by ultraviolet photons emitted from the first stars, but also enriched with heavy elements when these stars ended their lives as energetic supernovae. Using the results from previous high-resolution cosmological simulations of early structure formation that include radiative transfer, we show that a significant volume fraction of the IGM can be metal-polluted, as well as ionized, by massive Population III stars formed in small-mass (10^6-10^7 Msun) halos early on. If most of the early generation stars die as pair-instability supernovae with energies up to 10^{53} ergs, the volume-averaged mean metallicity will quickly reach Z ~ 10^{-4}Zsun by a redshift of 15-20, possibly causing a prompt transition to the formation of a stellar population that is dominated by low-mass stars. In this scenario, the early chemical enrichment history should closely trace the reionization history of the IGM, and the end of the Population III era is marked by the completion of reionization and pre-enrichment by z=15. We conclude that, while the pre-enrichment may partially account for the ``metallicity-floor'' in high-redshift Lyman-alpha clouds, it does not significantly affect the elemental abundance in the intracluster medium.Comment: Version accepted by ApJ. Minor revisions and a few citations adde