Andrei Mesinger

Evidence of a Cosmological Strömgren Surface and of Significant Neutral Hydrogen Surrounding the Quasar SDSS J1030+0524

A bright quasar residing in a dense and largely neutral intergalactic medium at high redshifts (z 6) will be surrounded by a large cosmological Stromgren sphere. The quasars spectrum will then show a sharp increase in resonant Lyman line absorption at wavelengths approaching and shorter than that corresponding to the Stromgren spheres boundary along the line of sight. We show here that simultaneously considering the measured absorption in two or more hydrogen Lyman lines can provide the dynamical range required to detect this feature. We model broad and robust features of the Lyα and Lyβ regions of the spectrum of the z = 6.28 quasar SDSS J1030+0524, using a hydrodynamical simulation. From the steep wavelength dependence of the inferred absorption opacity, we detect the boundary of the Stromgren sphere at a proper distance of 6.0 ± 0.2 Mpc away from the source redshift. From the spectrum alone, we also find that beyond this distance, cosmic hydrogen turns nearly neutral, with a neutral fraction of x 0.2, and that the ionizing luminosity of this quasar is in the range of (5.2 ± 2.5) × 1056 photons s-1. The method presented here, when applied to future quasars, can probe the complex topology of overlapping ionized regions and can be used to study the details of the reionization process.

Ultraviolet radiative feedback on high-redshift protogalaxies

We use three-dimensional hydrodynamic simulations to investigate the effects of a transient photoionizing UV flux on the collapse and cooling of pregalactic clouds. These clouds have masses in the range 105-107 M?, form at high redshifts (z 18), and are assumed to lie within the short-lived cosmological H II regions around the first generation of stars. In addition, we study the combined effects of this transient UV flux and a persistent Lyman-Werner (LW) background (at photon energies below 13.6 eV) from distant sources. In the absence of a LW background, we find that a critical specific intensity of JUV ~ 0.1 ? 10-21 ergs s-1 cm-2 Hz-1 sr-1 demarcates a transition from net negative to positive feedback for the halo population. A weaker UV flux stimulates subsequent star formation inside the fossil H II regions, by enhancing the H2 molecule abundance. A stronger UV flux significantly delays star formation by reducing the gas density, and increasing the cooling time, at the centers of collapsing halos. At a fixed JUV, the sign of the feedback also depends strongly on the density of the gas at the time of UV illumination. Regardless of whether the feedback is positive or negative, we find that once the UV flux is turned off, its impact starts to diminish after ~30% of the Hubble time. In the more realistic case when a LW background is present, with JLW 0.01 ? 10-21 ergs s-1 cm-2 Hz-1 sr-1, strong suppression persists down to the lowest redshift (z = 18) in our simulations. Finally, we find evidence that heating and photoevaporation by the transient UV flux render the ~106 M? halos inside fossil H II regions more vulnerable to subsequent H2 photodissociation by a LW background.

Constraints on Warm Dark Matter models from high-redshift long gamma-ray bursts

Structures in Warm Dark Matter (WDM) models are exponentially suppressed below a certain scale, characterized by the dark matter particle mass, mx. Since structures form hierarchically, the presence of collapsed objects at high-redshifts can set strong lower limits on mx. We place robust constraints on mx using recent results from the Swift database of high-redshift gamma-ray bursts (GRBs). We parameterize the redshift evolution of the ratio between the cosmic GRB rate and star formation rate (SFR) as / (1+z) � , thereby allowing astrophysical uncertainties to partially mimic the cosmological suppression of structures in WDM models. Using a maximum likelihood estimator on two different z > 4 GRB subsamples (including two bursts at z > 8), we constrain mx & 1.6-1.8 keV at 95% CL, when marginalized over a flat prior in α. We further estimate that 5 years of a SVOM-like mission would tighten these constraints to mx & 2.3 keV. Our results show that GRBs are a powerful probe of high-redshift structures, providing robust and competitive constraints on mx.

Probing the Reionization History Using the Spectra of High-Redshift Sources

We quantify and discuss the footprints of neutral hydrogen in the intergalactic medium (IGM) on the spectra of high-redshift (z ~ 6) sources, using mock spectra generated from hydrodynamical simulations of the IGM. We show that it should be possible to extract relevant parameters, including the mean neutral fraction in the IGM and the radius of the local cosmological Stromgren region, from the flux distribution in the observed spectra of distant sources. We focus on quasars, but a similar analysis is applicable to galaxies and gamma-ray burst (GRB) afterglows. We explicitly include uncertainties in the spectral shape of the assumed source template near the Lyα line. Our results suggest that a mean neutral hydrogen fraction, xH, of unity can be statistically distinguished from xH ≈ 10-2 by combining the spectra of tens of bright (M ≈ -27) quasars. Alternatively, the same distinction can be achieved using the spectra of several hundred sources that are ~100 times fainter. Furthermore, if the radius of the Stromgren sphere can be independently constrained to within ~10%, this distinction can be achieved using a single source. The information derived from such spectra will help in settling the current debate as to what extent the universe was reionized at redshifts near z ~ 6.

Constraints on the Small-Scale Power Spectrum of Density Fluctuations from High-Redshift Gamma-Ray Bursts

Cosmological models that include suppression of the power spectrum of density fluctuations on small scales exhibit an exponential reduction of high-redshift, nonlinear structures, including a reduction in the rate of gamma-ray bursts (GRBs). Here we quantify the constraints that the detection of distant GRBs would place on structure formation models with reduced small-scale power. We compute the number of GRBs that could be detectable by the Swift satellite at high redshifts (z 6), assuming that the GRBs trace the cosmic star formation history, which itself traces the formation of nonlinear structures. We calibrate simple models of the intrinsic luminosity function of the bursts to the number and flux distribution of GRBs observed by the Burst and Transient Source Experiment. We find that a discovery of high-z GRBs would imply strong constraints on models with reduced small-scale power. For example, a single GRB at z 10 or 10 GRBs at z 5 discovered by Swift during its scheduled 2 year mission would rule out an exponential suppression of the power spectrum on scales below Rc = 0.09 Mpc (exemplified by warm dark matter models with a particle mass of mx = 2 keV). Models with a less sharp suppression of small-scale power, such as those with a red tilt or a running scalar index, ns, are more difficult to constrain, because they are more degenerate with an increase in the power-spectrum normalization, σ8, and with models in which star formation is allowed in low-mass minihalos. We find that a tilt of δns ≈ 0.1 is difficult to detect; however, an observed rate of one GRB yr-1 at z 12 would yield an upper limit on the running of the spectral index, α ≡ dns/d ln k > -0.05.

Detectability of the first cosmic explosions

We present a fully self-consistent simulation of a syntheti c survey of the furthermost cosmic explosions. The appearance of the first generation of sta rs (Population III) in the Universe represents a critical point during cosmic evolution, signa ling the end of the dark ages, a period of absence of light sources. Despite their importance, ther e is no confirmed detection of Population III stars so far. A fraction of these primordial stars are expected to die as pair-instability supernovae (PISNe), and should be bright enough to be observed up to a few hundred million years after the big bang. While the quest for Population III stars continues, detailed theoretical models and computer simulations serve as a testbed for their observability. With the upcoming near-infrared missions, estimates of the feasibility of de tecting PISNe are not only timely but imperative. To address this problem, we combine state-of-the-art cosmological and radiative simulations into a complete and self-consistent framework, which includes detailed features of the observational process. We show that a dedicated observational strategy using . 8 per cent of total allocation time of the James Webb Space Telescopemission can provide us up to � 9 15 detectable PISNe per year.

The redshift distribution of distant supernovae and its use in probing reionization

We model the number of detectable supernovae (SNe) as a function of redshift at different flux thresholds, making use of the observed properties of local SNe, such as their light curves, fiducial spectra, and peak magnitude distributions. We assume that the star formation rate (SFR) at high redshift traces the formation rate of dark matter halos. We obtain a rate of 0.4-2.3 SNe arcmin-2 yr-1 at z 5 at the near-infrared (4.5 ?m) flux density threshold of 3 nJy (achievable with the James Webb Space Telescope [JWST] in a 105 s integration). In a hypothetical 1 yr survey, it should be possible to detect up to several thousand SNe per unit redshift at z ~ 6. We discuss the possible application of such a large sample of distant SNe as a probe of the epoch of reionization. By heating the intergalactic medium (IGM) and raising the cosmological Jeans mass, the process of reionization can suppress star formation in low-mass galaxies. This could have produced a relatively sharp drop in the SN rate around the redshift of reionization (zre). We quantify the detectability of this feature in future surveys of distant SNe by varying the redshift and duration of reionization, as well as its impact on the SFR in low-mass halos, which results in different redshifts, widths, and sizes of the drop in the expected SFR. We find that the drop can be detected out to zre ~ 13, as long as (1) the reionization history contains a relatively rapid feature that is synchronized over different regions to within ?z 1-3, (2) the star formation efficiency in halos that dominate reionization is * ~ 10%, and (3) reionization significantly suppresses the star formation in low-mass halos. Depending on the details of (1)-(3), this could be achieved with a survey lasting less than 2 weeks. Detecting this signature would also help elucidate the feedback mechanism that regulates reionization.

Implications of the Lyα Emission Line from a Candidate z = 10 Galaxy

The z = 10 galaxy recently discovered by Pello and coworkers has a strong Lyα emission line that does not appear to have the expected asymmetry with more transmission on the red side. The blue wing of a Lyα line originating at high redshift should be strongly suppressed by resonant hydrogen absorption along the line of sight, an expectation borne out by the observed asymmetric shapes of the existing sample of Lyα-emitting sources at lower redshifts (3 < z < 6.7). The observed line is inconsistent with the galaxy being embedded in a fully neutral intergalactic medium (IGM) and having no receding peculiar velocity relative to the surrounding absorbing gas at the 95.0%-98.8% confidence level. Absorption on the blue side of the line of the Pello et al. source could be reduced if the IGM in the vicinity of the galaxy is highly ionized, but we show that this requires an unrealistically high ionizing emissivity. We suggest instead that the Lyα-emitting gas is receding relative to the surrounding gas with a velocity of 35 km s-1, which reduces the inconsistency confidence level to less than 76.0%-94.5%. We find that with this velocity shift, the observed strength and shape of the line is more consistent with the galaxy being surrounded by its own Stromgren sphere embedded in a fully neutral IGM. More generally, we predict that at any given redshift, the bright Lyα emitters with broader lines would exhibit stronger asymmetry than fainter ones. Bright galaxies with symmetric Lyα lines may be signposts for groups and clusters of galaxies, within which they can acquire random velocities comparable to or larger than their line widths.

The Cosmic Dawn and Epoch of Reionisation with SKA

Concerted effort is currently ongoing to open up the Epoch of Reionization (z ∼15-6) for studies with IR and radio telescopes. Whereas IR detections have been made of sources (Lyman-α emitters, quasars and drop-outs) in this redshift regime in relatively small fields of view, no direct detection of neutral hydrogen, via the redshifted 21-cm line, has yet been established. Such a direct detection is expected in the coming years, with ongoing surveys, and could open up the entire universe from z ∼6-200 for astrophysical and cosmological studies, opening not only the Epoch of Reionization, but also its preceding Cosmic Dawn (z ∼30-15) and possibly even the later phases of the Dark Ages (z ∼200-30). All currently ongoing experiments attempt statistical detections of the 21-cm signal during the Epoch of Reionization, with limited signal-to-noise. Direct imaging, except maybe on the largest (degree) scales at lower redshifts, as well as higher redshifts will remain out of reach. The Square Kilometre Array (SKA) will revolutionize the field, allowing direct imaging of neutral hydrogen from scales of arc-minutes to degrees over most of the redshift range z ∼6-28 with SKA1-LOW, and possibly even higher redshifts with the SKA2-LOW. In this SKA will be unique, and in parallel provide enormous potential of synergy with other upcoming facilities (e.g. JWST). In this chapter we summarize the physics of 21-cm emission, the different phases the universe is thought to go through, and the observables that the SKA can probe, referring where needed to detailed chapters in this volume. This is done within the framework of the current SKA1 baseline design and a nominal CD/EoR straw-man survey, consisting of a shallow, medium-deep and deep survey, the latter probing down to ∼1 mK brightness temperature on arc-minute scales at the end of reionization. Possible minor modifications to the design of SKA1 and the upgrade to SKA2 are discussed, in addition to science that could be done already during roll-out when SKA1 still has limited capabilities and/or core collecting area.

UV radiative feedback on high-redshift proto-galaxies

We use three-dimensional hydrodynamic simulations to investigate the effects of a transient photoionizing ultraviolet (UV) flux on the collapse and cooling of pregalactic clouds. These clouds have masses in the range 10 – 10 M⊙, form at high redshifts (z ∼ > 18), and are assumed to lie within the short–lived cosmological HII regions around the first generation of stars. In addition, we study the combined effects of this transient UV flux and a persistent Lyman–Werner (LW) background (at photon energies below 13.6eV) from distant sources. In the absence of a LW background, we find that a critical specific intensity of JUV ∼ 0.1×10 ergs s cm Hz sr demarcates a transition from net negative to positive feedback for the halo population. A weaker UV flux stimulates subsequent star formation inside the fossil HII regions, by enhancing the H2 molecule abundance. A stronger UV flux significantly delays star–formation by reducing the gas density, and increasing the cooling time, at the centers of collapsing halos. At a fixed JUV, the sign of the feedback also depends strongly on the density of the gas at the time of UV illumination. Regardless of the whether the feedback is positive or negative, we find that once the UV flux is turned off, its impact stars to diminish after ∼ 30% of the Hubble time. In the more realistic case when a LW background is present, with JLW ∼ > 0.01× 10 ergs s cm Hz sr, strong suppression persists down to the lowest redshift (z = 18) in our simulations. Finally, we find evidence that heating and photoevaporation by the transient UV flux renders the ∼ 10 M⊙ halos inside fossil HII regions more vulnerable to subsequent H2 photo–dissociation by a LW background. Subject headings: cosmology: theory – early Universe – galaxies: high-redshift – evolution