Completion Of Reionization Research Articles

The 21-cm signal from the cosmic dawn: metallicity dependence of high-mass X-ray binaries

ABSTRACT X-rays from high-mass X-ray binaries (HMXBs) are likely the main source of heating of the intergalactic medium (IGM) during Cosmic Dawn (CD), before the completion of reionization. This Epoch of Heating (EoH; z ∼10–15) should soon be detected via the redshifted 21-cm line from neutral hydrogen, allowing us to indirectly study the properties of HMXBs in the unseen, first galaxies. Low-redshift observations, as well as theoretical models, imply that the integrated X-ray luminosity to star formation rate of HMXBs (LX/SFR) should increase in metal-poor environments, typical of early galaxies. Here, we study the impact of the metallicity (Z) dependence of LX/SFR during the EoH. For our fiducial models, galaxies with star formation rates of order 10−3–$10^{-1}\, M_\odot$ yr−1 and metallicities of order 10−3–$10^{-2}\, Z_\odot$ are the dominant contributors to the X-ray background (XRB) during this period. Different LX/SFR–Z relations result in factors of ∼ 3 differences in these ranges, as well as in the mean IGM temperature and the large-scale 21-cm power, at a given redshift. We compute mock 21-cm observations adopting as a baseline a 1000 h integration with the upcoming Square Kilometer Array (SKA) for two different LX/SFR–Z relations. We perform inference on these mock observations using the common simplification of a constant LX/SFR, finding that constant LX/SFR models can recover the IGM evolution of the more complicated LX/SFR–Z simulations only during the EoH. At z < 10, where the typical galaxies are more polluted, constant LX/SFR models overpredict the XRB and its relative contribution to the early stages of the reionization.

Antisymmetric Cross-correlation between H i and CO Line Intensity Maps as a New Probe of Cosmic Reionization

Abstract Intensity mapping of the H i 21 cm line and the CO 2.61 mm line from the epoch of reionization has emerged as powerful, complementary, probes of the high-redshift universe. However, both maps and their cross-correlation are dominated by foregrounds. We propose a new analysis by which the signal is unbiased by foregrounds, i.e., it can be measured without foreground mitigation. We construct the antisymmetric part of two-point cross-correlation between intensity maps of the H i 21 cm line and the CO 2.61 mm line, arising because the statistical fluctuations of two fields have different evolution in time. We show that the sign of this new signal can distinguish model independently whether inside-out reionization happens during some interval of time. More importantly, within the framework of the excursion-set model of reionization, we demonstrate that the slope of the dipole of H i–CO cross-power spectrum at large scales is linear to the rate of change of global neutral fraction of hydrogen in a manner independent of reionization parameters, until the slope levels out near the end of reionization, but this trend might possibly depend on the framework of reionization modeling. The H i–CO dipole may be a smoking-gun probe for the speed of reionization, or “standard speedometer” for cosmic reionization. Observations of this new signal will unveil the global reionization history from the midpoint to near the completion of reionization.

Studying the Lyman α optical depth fluctuations at z ∼ 5.5 using fast semi-numerical methods

We present a computationally efficient and fast semi-numerical technique for simulating the Lyman-$\alpha$ (Ly$\alpha$) absorption optical depth in presence of neutral hydrogen "islands" left over from reionization at redshifts $5 \lesssim z \lesssim 6$. The main inputs to the analysis are (i) a semi-numerical photon-conserving model of ionized regions during reionization (named SCRIPT) along with a prescription for simulating the shadowing by neutral islands and (ii) the fluctuating Gunn-Peterson approximation to model the Ly$\alpha$ absorption. Our model is then used for simulating the large-scale fluctuations in the effective optical depth as observed along sight lines towards high$-z$ quasars. Our model is fully described by five parameters. By setting two of them to default values and varying the other three, we obtain the constraints on reionization history at $5 \lesssim z \lesssim 6$ as allowed by the data. We confirm that reionization is \emph{not} complete before $z \sim 5.6$ at $\gtrsim 2\sigma$ confidence, with the exact confidence limits depending on how the non-detections of the flux in the data are treated. We also confirm that the completion of reionization can be as late as $z \sim 5.2$. With further improvements in the model and with more sight lines at $z \sim 6$, we can take advantage of the computational efficiency of our analysis to obtain more stringent constraints on the ionization fraction at the tail-end of reionization.

Hydrodynamic Response of the Intergalactic Medium to Reionization

Abstract The intergalactic medium is expected to clump on scales down to M ⊙ before the onset of reionization. The impact of these small-scale structures on reionization is poorly understood despite the modern understanding that gas clumpiness limits the growth of regions. We use a suite of radiation-hydrodynamics simulations that capture the Jeans mass of unheated gas to study density fluctuations during reionization. Our simulations track the complex ionization and hydrodynamical response of gas in the wake of ionization fronts. The clumping factor of ionized gas (proportional to the recombination rate) rises to a peak value of 5–20 approximately Δt = 10 Myr after ionization front passage, depending on the incident intensity, redshift, and degree to which the gas had been preheated by the first X-ray sources. The clumping factor reaches its relaxed value of ≈3 by Δt = 300 Myr. The mean free path of Lyman-limit photons evolves in unison, being up to several times shorter in unrelaxed, recently reionized regions compared to those that were reionized much earlier. Assessing the impact of this response on the global reionization process, we find that unrelaxed gaseous structures boost the total number of recombinations by ≈50% and lead to spatial fluctuations in the mean free path that persist appreciably for several hundred million years after the completion of reionization.

He II Lyβ GUNN-PETERSON ABSORPTION: NEWHSTOBSERVATIONS AND THEORETICAL EXPECTATIONS

Observations of He II Ly{\alpha} Gunn-Peterson troughs have proved to be a valuable probe of the epoch of helium reionization at z~3. Since this optical depth can become unmeasurably large even for modest He II fractions, various alternate techniques have been proposed to push to higher redshift, and among the more promising is looking at higher order Lyman-series troughs. We here report four new observations of the He II Ly{\beta} trough, including new data on the only sightline with a prior Ly{\beta} observation. However, the effective optical depth ratio {\tau}_eff,{\beta}/{\tau}_eff,{\alpha} is not simply predicted by (f_{\beta} {\lambda}_{\beta})/(f_{\alpha} {\lambda}_{\alpha})=0.16, and we analyze cosmological simulations to find that the correct ratio for helium at z~3 is ~0.35. In one case we infer {\tau}_eff,{\alpha} > 8.8, strong evidence that helium was not fully reionized at z=3.2--3.5, in agreement with previous measurements suggesting a later completion of reionization.

The 21-cm power spectrum after reionization

We discuss the 21cm power spectrum (PS) following the completion of reionization. In contrast to the reionization era, this PS is proportional to the PS of mass density fluctuations, with only a small modulation due to fluctuations in the ionization field on scales larger than the mean-free-path of ionizing photons. We derive the form of this modulation, and demonstrate that its effect on the 21cm PS will be smaller than 1% for physically plausible models of damped Ly-alpha systems. In contrast to the 21cm PS observed prior to reionization, in which HII regions dominate the ionization structure, the simplicity of the 21cm PS after reionization will enhance its utility as a cosmological probe by removing the need to separate the PS into physical and astrophysical components. As a demonstration, we consider the Alcock-Paczynski test and show that the next generation of low-frequency arrays could measure the angular distortion of the PS at the percent level for z~3-5.

A method to measure the mass of damped Lyα absorber host galaxies using fluctuations in 21-cm emission

Observations of damped Ly-alpha absorbers (DLA) indicate that the fraction of hydrogen in its neutral form (HI) is significant by mass at all redshifts. This gas represents the reservoir of material that is available for star formation at late times. As a result, observational identification of the systems in which this neutral hydrogen resides is an important missing ingredient in models of galaxy formation. Precise identification of DLA host mass via traditional clustering studies is not practical owing to the small numbers of known systems being spread across sparsely distributed sight lines. However following the completion of reionization, 21cm surface brightness fluctuations will be dominated by neutral hydrogen in DLAs. Observations of these fluctuations will measure the combined clustering signal from all DLAs within a large volume. We show that measurement of the spherically averaged power-spectrum of 21cm intensity fluctuations due to DLAs could be used to measure the galaxy bias for DLA host galaxies when combined with an independent measurement of the cosmological HI mass density from quasar absorption studies. Utilising this technique, the low frequency arrays now under construction could measure the characteristic DLA host mass with a statistical precision as low as 0.3 dex at z~4. In addition, high signal-to-noise observations of the peculiar-motion induced anisotropy of the power-spectrum would facilitate measurement of both the DLA host mass and the cosmic HI density directly from 21cm fluctuations. By exploiting this anisotropy, a second generation of low frequency arrays with an order of magnitude increase in collecting area could measure the values of cosmic HI density and DLA host mass, with uncertainties of a few percent and a few tens of percent respectively.

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 was 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 (~106-107 M?) halos early on. If most of the early generation stars die as pair-instability supernovae with energies up to ~1053 ergs, the volume-averaged mean metallicity will quickly reach Z ~ 10-4 Z? by a redshift of ~15-20, possibly causing a 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 preenrichment by z ~ 15. We conclude that, while the preenrichment may partially account for the metallicity floor in high-redshift Ly? clouds, it does not significantly affect the elemental abundance in the intracluster medium.