Inhomogeneous recombinations during cosmic reionization

By depleting the ionizing photon budget available to expand cosmic HII regions, recombining systems (or Lyman limit systems) can have a large impact during (and following) cosmic reionization. Unfortunately, directly resolving such structures in large-scale reionization simulations is computationally impractical. Instead, here we implement a sub-grid prescription for tracking inhomogeneous recombinations in the intergalactic medium. Building on previous work parameterizing photo-heating feedback on star-formation, we present large-scale, semi-numeric reionization simulations which self-consistently track the local (sub-grid) evolution of both sources and sinks of ionizing photons. Our simple, single-parameter model naturally results in both an extended reionization and a modest, slowly-evolving emissivity, consistent with observations. Recombinations are instrumental in slowing the growth of large HII regions, and damping the rapid rise of the ionizing background in the late stages of (and following) reionization. As a result, typical HII regions are smaller by factors of $\sim 2-3$ throughout reionization. The large-scale (k\lesssim 0.2\text{ Mpc^{-1}}) ionization power spectrum is suppressed by factors of $\gtrsim 2-3$ in the second half of reionization. Therefore properly modeling recombinations is important in interpreting virtually all reionization observables, including upcoming interferometry with the redshifted 21 cm line. Consistent with previous works, we find the clumping factor of ionized gas to be $C_{\rm HII}\sim 4$ at the end of reionization.Comment: 12 pages, 12 figures, submitted to MNRA

The effect of rotation on the thermal instability of stratified galactic atmospheres - I. Local analysis

Observations show that (i) multiple gas phases can coexist in the atmospheres of galaxies and clusters; (ii) these atmospheres may be significantly rotating in the inner parts, with typical velocities that approach or even exceed the local sound speed. The thermal instability is a natural candidate to explain the formation of cold structures via condensation of a hotter gas phase. Here we systematically study the effect of rotation on the thermal stability of stratified plane-parallel atmospheres, using both analytical arguments and numerical simulations. We find that the formation of cold structures starting from small isobaric perturbations is enhanced in the regions where the rotation of the system is dynamically important (i.e. when the rotational velocity becomes comparable to the sound speed). In particular, the threshold value of the ratio between the cooling and dynamical time $t_{\rm cool}/t_{\rm dyn}$ below which condensations can form is increased by a factor up to $\sim 10$ in the presence of significant rotation. We briefly discuss the implications of our results for galaxies and clusters.Comment: Accepted for publication in MNRAS. Animations of all the simulations in the paper can be downloaded at the following url: https://www.ita.uni-heidelberg.de/~mattia/download/therm/paper_I.zi

The depletion of gas in high-redshift dwarf galaxies from an inhomogeneous reionization

The reionization of the intergalactic medium (IGM) was likely inhomogeneous and extended. By heating the IGM and photo-evaporating gas from the outskirts of galaxies, this process can have a dramatic impact on the growth of structures. Using a suite of spherically-symmetric collapse simulations spanning a large parameter space, we study the impact of an ionizing ultraviolet background (UVB) on the condensation of baryons onto dark matter halos. We present an expression for the halo baryon fraction, which is an explicit function of: (i) halo mass; (ii) UVB intensity; (iii) redshift; (iv) redshift at which the halo was exposed to a UVB. We also present a corresponding expression for the characteristic or critical mass, defined as the halo mass which retains half of its baryons compared to the global value. Since our results are general and physically-motivated, they can be broadly applied to inhomogeneous reionization models.Comment: 5 pages, 3 figure

How does radiative feedback from a UV background impact reionization?

An ionizing UV background (UVB) inhibits gas accretion and photo-evaporates gas from the shallow potential wells of small, dwarf galaxies. During cosmological reionization, this effect can result in negative feedback: suppressing star-formation inside HII regions, thus impeding their continued growth. It is difficult to model this process, given the enormous range of scales involved. We tackle this problem using a tiered approach: combining parameterized results from single-halo collapse simulations with large-scale models of reionization. In the resulting reionization models, the ionizing emissivity of galaxies depends on the local values of the reionization redshift and the UVB intensity. We present a physically-motivated analytic expression for the average minimum mass of star-forming galaxies, which can be readily used in modeling galaxy formation. We find that UVB feedback: (i) delays the end stages of reionization by less than 0.5 in redshift; (ii) results in a more uniform distribution of HII regions, peaked on smaller-scales (with large-scale ionization power suppressed by tens of percent); and (iii) suppresses the global photoionization rate per baryon by a factor of < 2 towards the end of reionization. However, the impact is modest, since the hydrodynamic response of the gas to the UVB occurs on a time-scale comparable to reionization. In particular, the popular approach of modeling UVB feedback with an instantaneous transition in the minimum mass of star-forming galaxies, dramatically overestimates its importance. UVB feedback does not significantly affect reionization unless: (i) molecularly-cooled galaxies contribute significantly to reionization; or (ii) internal feedback processes strongly couple with UVB feedback in the early Universe. Since both are considered unlikely, we conclude that there is no significant self-regulation of reionization by UVB feedback.Comment: 9 pages, 9 figure

The Evolution of 21-cm Structure (EOS): public, large-scale simulations of Cosmic Dawn and Reionization

We introduce the Evolution of 21-cm Structure (EOS) project: providing periodic, public releases of the latest cosmological 21-cm simulations. 21-cm interferometry is set to revolutionize studies of the Cosmic Dawn (CD) and epoch of reionization (EoR), eventually resulting in 3D maps of the first billion years of our Universe. Progress will depend on sophisticated data analysis pipelines, which are in turn tested on large-scale mock observations. Here we present the 2016 EOS data release, consisting of the largest (1.6 Gpc on side with a 1024^3 grid), public 21-cm simulations of the CD and EoR. We include calibrated, sub-grid prescriptions for inhomogeneous recombinations and photo-heating suppression of star formation in small mass galaxies. We present two simulation runs that approximately bracket the contribution from faint unseen galaxies. From these two extremes, we predict that the duration of reionization (defined as a change in the mean neutral fraction from 0.9 to 0.1) should be between 2.7 < Delta z < 5.7. The large-scale 21-cm power during the advanced EoR stages can be different by up to a factor of ~10, depending on the model. This difference has a comparable contribution from: (i) the typical bias of sources; and (ii) a more efficient negative feedback in models with an extended EoR driven by faint galaxies. We also make detectability forecasts. With a 1000h integration, HERA and SKA1-low should achieve a signal-to-noise of ~few-hundreds throughout the EoR/CD, while in the maximally optimistic scenario of perfect foreground cleaning, all instruments should make a statistical detection of the cosmic signal. We also caution that our ability to clean foregrounds determines the relative performance of narrow/deep vs. wide/shallow surveys expected with SKA1. Our 21-cm power spectra, simulation outputs and visualizations are publicly available.Comment: 12 pages, 9 figures, MNRAS submitted; data and visualizations are available at http://homepage.sns.it/mesinger/EOS.htm