The reionization of cosmic hydrogen marks a critical juncture in the history of structure formation. Here we present a new formulation of the standard reionization equation for the evolution of the ...volume-averaged H ii fraction that is more consistent with the accepted conceptual model of inhomogeneous intergalactic absorption. The revised equation explicitly accounts for the presence of the optically thick "Lyman-limit systems" that are known to determine the mean-free path of ionizing radiation after overlap. Integration of this equation provides a better characterization of the timing of reionization by smoothly linking the pre-overlap with the post-overlap phases of such a process. We confirm the validity of the quasi-instantaneous approximation as a predictor of reionization completion/maintenance and discuss new insights on the sources of cosmic reionization using the improved formalism. A constant emission rate into the intergalactic medium (IGM) of three Lyman continuum (LyC) photons per atom per gigayear leads to a reionization history that is consistent with a number of observational constraints on the ionization state of the z = 5-9 universe. While star-forming galaxies can dominate the reionization process if the luminosity-weighted fraction of LyC photons that escape into the IGM, , exceeds 15% (for a faint magnitude cut-off of the galaxy UV luminosity function of and a LyC photon yield per unit 1500 luminosity of ), simple models where the product of the two unknowns is not evolving with redshift fail to reproduce the changing neutrality of the IGM observed at these epochs.
Abstract The tentative detection by the EDGES experiment of a global 21-cm absorption trough centred at redshift 17 opens up the opportunity to study the birth of the first luminous sources, the ...intensity of radiation backgrounds at cosmic dawn, the thermal and ionization history of the young intergalactic medium. Here, we focus on the astrophysical implications of the Ly α photon field needed to couple the spin temperature to the kinetic temperature of the gas at these early epochs. Under the basic assumption that the 21-cm signal is activated by extremely metal-poor stellar systems, we show that the EDGES results are consistent with an extrapolation of the declining galaxy UV luminosity density measured at $4\lesssim z\lesssim 9$ by deep Hubble Space Telescope observations. A substantially enhanced star formation rate density or new exotic sources of UV photons are not required at the redshifts of the EDGES signal. The amount of ionizing radiation produced by the same stellar systems that induce Ly α coupling is significant, of order 0.5 LyC photons per H-atom per 100 Myr. To keep hydrogen largely neutral and delay the reionization process consistently with recent Planck cosmic microwave background results, mean escape fractions of $f_{\rm esc}\lesssim 20$ per cent are required at $z$ > 15.
Abstract
We introduce a mathematical framework for statistical exoplanet population and astrobiology studies that may help direct future observational efforts and experiments. The approach is based ...on a set of differential equations and provides a time-dependent mapping between star formation, metal enrichment, and the occurrence of exoplanets and potentially life-harboring worlds over the chemo-population history of the solar neighborhood. Our results are summarized as follows: (1) the formation of exoplanets in the solar vicinity was episodic, starting with the emergence of the thick disk about 11 Gyr ago; (2) within 100 pc from the Sun, there are as many as 11,000(
η
⊕
/0.24) Earth-size planets in the habitable zone (“temperate terrestrial planets” or TTPs) of K-type stars. The solar system is younger than the median TTP, and was created in a star formation surge that peaked 5.5 Gyr ago and was triggered by an external agent; (3) the metallicity modulation of the giant planet occurrence rate results in a later typical formation time, with TTPs outnumbering giant planets at early times; and (4) the closest, life-harboring Earth-like planet would be ≲20 pc away if microbial life arose as soon as it did on Earth in ≳1% of the TTPs around K stars. If simple life is abundant (fast abiogenesis), it is also old, as it would have emerged more than 8 Gyr ago in about one-third of all life-bearing planets today. Older Earth analogs are more likely to have developed sufficiently complex life capable of altering their environment and producing detectable oxygenic biosignatures.
We compute the expected X-ray diffuse background and radiative feedback on the intergalactic medium (IGM) from X-ray binaries prior to and during the epoch of reionization. The cosmic evolution of ...compact binaries is followed using a population synthesis technique that treats separately neutron stars and black hole binaries in different spectral states and is calibrated to reproduce the observed X-ray properties of galaxies at z 4. Together with an updated empirical determination of the cosmic history of star formation, recent modeling of the stellar mass-metallicity relation, and a scheme for absorption by the IGM that accounts for the presence of ionized H ii bubbles during the epoch of reionization, our detailed calculations provide refined predictions of the X-ray volume emissivity and filtered radiation background from "normal" galaxies at z 6. Radiative transfer effects modulate the background spectrum, which shows a characteristic peak between 1 and 2 keV. Because of the energy dependence of photoabsorption, soft X-ray photons are produced by local sources, while more energetic radiation arrives unattenuated from larger cosmological volumes. While the filtering of X-ray radiation through the IGM slightly increases the mean excess energy per photoionization, it also weakens the radiation intensity below 1 keV, lowering the mean photoionization and heating rates. Numerical integration of the rate and energy equations shows that the contribution of X-ray binaries to the ionization of the bulk IGM is negligible, with the electron fraction never exceeding 1%. Direct He i photoionizations are the main source of IGM heating, and the temperature of the largely neutral medium in between H ii cavities increases above the temperature of the cosmic microwave background (CMB) only at z 10, when the volume filling factor of H ii bubbles is already 0.1. Therefore, in this scenario, it is only at relatively late epochs that neutral intergalactic hydrogen may be observable in 21 cm emission against the CMB.
We analyze the formation and evolution of the stellar components in "Eris," a 120 pc resolution cosmological hydrodynamic simulation of a late-type spiral galaxy. The simulation includes the effects ...of a uniform UV background, a delayed-radiative-cooling scheme for supernova feedback, and a star formation recipe based on a high gas density threshold. It allows a detailed study of the relative contributions of "in-situ" (within the main host) and "ex-situ" (within satellite galaxies) star formation to each major Galactic component in a close Milky Way analog. We investigate these two star-formation channels as a function of galactocentric distance, along different lines of sight above and along the disk plane, and as a function of cosmic time. We find that: (1) approximately 70% of today's stars formed in-situ; (2) more than two thirds of the ex-situ stars formed within satellites after infall; (3) the majority of ex-situ stars are found today in the disk and in the bulge; (4) the stellar halo is dominated by ex-situ stars, whereas in-situ stars dominate the mass profile at distances <, ~5 kpc from the center at high latitudes; and (5) approximately 25% of the inner, r <, ~ 20 kpc, halo is composed of in-situ stars that have been displaced from their original birth sites during Eris' early assembly history.
We consider super-critical accretion with angular momentum onto stellar-mass black holes as a possible mechanism for growing billion-solar-mass black holes from light seeds at early times. We use the ...radiatively inefficient "slim disk" solution-advective, optically thick flows that generalize the standard geometrically thin disk model-to show how mildly super-Eddington intermittent accretion may significantly ease the problem of assembling the first massive black holes when the universe was less than 0.8 Gyr old. Because of the low radiative efficiencies of slim disks around non-rotating as well as rapidly rotating black holes, the mass e-folding timescale in this regime is nearly independent of the spin parameter. The conditions that may lead to super-critical growth in the early universe are briefly discussed.
ABSTRACT
While low-mass, star-forming galaxies are often considered as the primary driver of reionization, their actual contribution to the cosmic ultraviolet background is still uncertain, mostly ...because the escape fraction of ionizing photons is only poorly constrained. Theoretical studies have shown that efficient supernova feedback is a necessary condition to create paths through which ionizing radiation can escape into the intergalactic medium. We investigate the possibility that accreting supermassive black holes in early dwarf galaxies may provide additional feedback and enhance the leakage of ionizing radiation. We use a series of high-resolution cosmological radiation hydrodynamics simulations where we isolate the different sources of feedback. We find that supernova feedback prevents the growth of the black hole, thus quenching its associated feedback. Even in cases where the black hole can grow, the structure of the interstellar medium is strongly dominated by supernova feedback. We conclude that, in the dwarf galaxy regime, supermassive black holes do not appear to play a significant role in enhancing the escape fraction and in contributing to the early ultraviolet background.
We analyze the present day structure and assembly history of a high-resolution hydrodynamic simulation of the formation of a Milky-Way-(MW)-like disk galaxy, from the "Eris" simulation suite, ...dissecting it into cohorts of stars formed at different epochs of cosmic history. The younger age cohorts populate disks of progressively longer radial scale lengths and shorter vertical scale heights. At a given radius, the vertical density profiles and velocity dispersions of stars vary smoothly as a function of age, and the superposition of old, vertically extended and young, vertically compact cohorts gives rise to a double-exponential profile like that observed in the MW. Turning to formation history, we find that the trends of spatial structure and kinematics with stellar age are largely imprinted at birth, or immediately thereafter. The predicted correlations of stellar age with spatial and kinematic structure are in good qualitative agreement with the correlations observed for mono-abundance stellar populations in the MW.
We present initial results from "Via Lactea," the highest resolution simulation to date of Galactic CDM substructure. It follows the formation of a Milky Way-sized halo with M sub(halo)= 1.8 x 10 ...super(12) M sub( )in a WMAP three-year cosmology, using 234 million particles. Over 10,000 subhalos can be identified at z = 0: their cumulative mass function is well-fit by N(> M sub(sub)) = 0.0064 (M sub(sub)/M sub(halo)) super(-1) down to M sub(sub) = 4 x 10 super(6) M sub( ). The total mass fraction in subhalos is 5.3%, while the fraction of surface mass density in substructure within a projected distance of 10 kpc from the halo center is 0.3%. Because of the significant contribution from the smallest resolved subhalos, these fractions have not converged yet. Sub-substructure is apparent in all the larger satellites, and a few dark matter lumps are resolved even in the solar vicinity. The number of dark satellites with peak circular velocities above 10 km s super(-1) (5 km s super(-1)) is 124 (812): of these, five (26) are found within 0.1r sub(vir), a region that appeared practically smooth in previous simulations. The neutralino self-annihilation g-ray emission from dark matter clumps is approximately constant per subhalo mass decade. Therefore, while in our run the contribution of substructure to the g-ray luminosity of the Galactic halo amounts to only 40% of the total spherically averaged smooth signal, we expect this fraction to grow significantly as resolution is increased further. An all-sky map of the expected annihilation g-ray flux reaching a fiducial observer at 8 kpc from the Galactic center shows that at the current resolution a small number of subhalos start to be bright enough to be visible against the background from the smooth density field surrounding the observer.
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