Reionization and Cosmic Dawn: theory and simulations

We highlight recent progress in the sophistication and diversification of cosmic dawn and reionization simulations. The application of these modeling tools to current observations has allowed us narrow down the timing of reionization, which we now know to within dz ~ 1 for the bulk of reionization. The strongest constraints come from the optical depth to the CMB measured with the {\it Planck} satellite and the first detection of ongoing reionization from the spectra of the z=7.1 QSOs ULASJ1120+0641. However, we still know virtually nothing about the astrophysical sources during the first billion years. The revolution in our understanding will be led by upcoming interferometric observations of the cosmic 21-cm signal. The properties of the sources and sinks of UV and X-ray photons are encoded in the 3D patterns of the signal. The development of Bayesian parameter recovery techniques, which tap into the wealth of the 21-cm signal, will soon usher in an era of precision astrophysical cosmology.Comment: Invited review for the IAU Symposium 333 "Peering towards Cosmic Dawn", Dubrovnik, October 2-6, 2017; to appear in the proceedings, eds. Vibor Jelic and Thijs van der Hulst [8 pages, 3 figures

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

Simultaneously constraining the astrophysics of reionisation and the epoch of heating with 21CMMC

The cosmic 21 cm signal is set to revolutionise our understanding of the early Universe, allowing us to probe the 3D temperature and ionisation structure of the intergalactic medium (IGM). It will open a window onto the unseen first galaxies, showing us how their UV and X-ray photons drove the cosmic milestones of the epoch of reionisation (EoR) and epoch of heating (EoH). To facilitate parameter inference from the 21 cm signal, we previously developed 21CMMC: a Monte Carlo Markov Chain sampler of 3D EoR simulations. Here we extend 21CMMC to include simultaneous modelling of the EoH, resulting in a complete Bayesian inference framework for the astrophysics dominating the observable epochs of the cosmic 21 cm signal. We demonstrate that second generation interferometers, the Hydrogen Epoch of Reionisation Array (HERA) and Square Kilometre Array (SKA) will be able to constrain ionising and X-ray source properties of the first galaxies with a fractional precision of order $\sim1$-10 per cent (1$\sigma$). The ionisation history of the Universe can be constrained to within a few percent. Using our extended framework, we quantify the bias in EoR parameter recovery incurred by the common simplification of a saturated spin temperature in the IGM. Depending on the extent of overlap between the EoR and EoH, the recovered astrophysical parameters can be biased by $\sim3-10\sigma$.Comment: 20 pages, 10 figures, 4 tables. Accepted to MNRAS (matches online version). Movies showing the imprint of the astrophysical parameters on the 21cm signal can be found at http://homepage.sns.it/mesinger/21CMMC.htm

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

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

Lyman-alpha Emitters During the Early Stages of Reionization

We investigate the potential of exploiting Lya Emitters (LAEs) to constrain the volume-weighted mean neutral hydrogen fraction of the IGM, x_H, at high redshifts (specifically z~9). We use "semi-numerical'' simulations to efficiently generate density, velocity, and halo fields at z=9 in a 250 Mpc box, resolving halos with masses M>2.2e8 solar masses. We construct ionization fields corresponding to various values of x_H. With these, we generate LAE luminosity functions and "counts-in-cell'' statistics. As in previous studies, we find that LAEs begin to disappear rapidly when x_H > 0.5. Constraining x_H(z=9) with luminosity functions is difficult due to the many uncertainties inherent in the host halo mass Lya luminosity mapping. However, using a very conservative mapping, we show that the number densities derived using the six z~9 LAEs recently discovered by Stark et al. (2007) imply x_H < 0.7. On a more fundamental level, these LAE number densities, if genuine, require substantial star formation in halos with M < 10^9 solar masses, making them unique among the current sample of observed high-z objects. Furthermore, reionization increases the apparent clustering of the observed LAEs. We show that a ``counts-in-cell'' statistic is a powerful probe of this effect, especially in the early stages of reionization. Specifically, we show that a field of view (typical of upcoming IR instruments) containing LAEs has >10% higher probability of containing more than one LAE in a x_H>0.5 universe than a x_H=0 universe with the same overall number density. With this statistic, a fully ionized universe can be robustly distinguished from one with x_H > 0.5 using a survey containing only ~ 20--100 galaxies.Comment: 14 pages, 13 figures, moderate changes to match version accepted for publication in the MNRA

The Cosmic 21-cm Revolution Charting the first billion years of our universe

The redshifted 21-cm signal is set to transform astrophysical cosmology, bringing a historically data-starved field into the era of Big Data. Corresponding to the spin-flip transition of neutral hydrogen, the 21-cm line is sensitive to the temperature and ionization state of the cosmic gas, as well as to cosmological parameters. Crucially, with the development of new interferometers it will allow us to map out the first billion years of our universe, enabling us to learn about the properties of the unseen first generations of galaxies. Rapid progress is being made on both the observational and theoretical fronts, and important decisions on techniques and future direction are being made. The Cosmic 21-cm Revolution gathers contributions from current leaders in this fast-moving field, providing both an overview for graduate students and a reference point for current researchers

Early galaxy formation in warm dark matter cosmologies

We present a framework for high-redshift ($z \geq 7$) galaxy formation that traces their dark matter (DM) and baryonic assembly in four cosmologies: Cold Dark Matter (CDM) and Warm Dark Matter (WDM) with particle masses of $m_x =$ 1.5, 3 and 5 ${\rm keV}$. We use the same astrophysical parameters regulating star formation and feedback, chosen to match current observations of the evolving ultra violet luminosity function (UV LF). We find that the assembly of observable (with current and upcoming instruments) galaxies in CDM and $m_x \geq 3 {\rm keV}$ WDM results in similar halo mass to light ratios (M/L), stellar mass densities (SMDs) and UV LFs. However the suppression of small-scale structure leads to a notably delayed and subsequently more rapid stellar assembly in the $1.5 {\rm keV}$ WDM model. Thus galaxy assembly in $m_x \leq 2 {\rm keV}$ WDM cosmologies is characterized by: (i) a dearth of small-mass halos hosting faint galaxies; and (ii) a younger, more UV bright stellar population, for a given stellar mass. The higher M/L ratio (effect ii) partially compensates for the dearth of small-mass halos (effect i), making the resulting UV LFs closer to CDM than expected from simple estimates of halo abundances. We find that the redshift evolution of the SMD is a powerful probe of the nature of DM. Integrating down to a limit of $M_{UV} =-16.5$ for the James Webb Space Telescope (JWST), the SMD evolves as $\log$(SMD)$\propto -0.63 (1+z)$ in $m_x = 1.5 {\rm keV}$ WDM, as compared to $\log$(SMD)$\propto -0.44 (1+z)$ in CDM. Thus high-redshift stellar assembly provides a powerful testbed for WDM models, accessible with the upcoming JWST.Comment: Accepted for publication in Ap