Cosmic Reionization Research Articles

Early Results from GLASS-JWST. I: Confirmation of Lensed z ≥ 7 Lyman-break Galaxies behind the Abell 2744 Cluster with NIRISS

We present the first search for z ≥ 7, continuum-confirmed Lyman break sources with NIRISS/WFS spectroscopy over the Abell 2744 Frontier Fields cluster, as part of the GLASS-JWST-ERS survey. With ∼15 hr of preimaging and multiangle grism exposures in the F115W, F150W, and F200W filters, we describe the general data handling (i.e., reduction, cleaning, modeling, and extraction processes) and analysis for the GLASS-JWST survey. We showcase the power of JWST to peer deep into reionization, when most intergalactic hydrogen is neutral, by confirming two galaxies at z = 8.04 ± 0.15 and z = 7.90 ± 0.13 by means of their Lyman breaks. Fainter continuum spectra are observed in both the F150W and F200W bands, indicative of blue (−1.69 and −1.33) UV slopes and moderately bright absolute magnitudes (−20.37 and −19.68 mag). We do not detect strong Lyα in either galaxy, but do observe tentative (∼2.7–3.8σ) He ii λ1640 Å, O iii]λλ1661,1666 Å, and N iii]λλ1747,1749 Å line emission in one, suggestive of low-metallicity, star-forming systems with possible nonthermal contributions. These novel observations provide a first look at the extraordinary potential of JWST/NIRISS for confirming representative samples of bright z ≥ 7 sources in the absence of strong emission lines, and gain unprecedented insight into their contributions toward cosmic reionization.

Impact of Population III homogeneous stellar evolution on early cosmic reionisation

Context. Population III (Pop III) stars may be fast rotating. An expected consequence of fast rotation is strong internal mixing that deeply affects their evolutionary tracks in the Hertzsprung-Russell diagram and hence their ionising power. Aims. We investigate the impact on the ionising power of Pop III stars in an extreme case of internal mixing, the one leading to chemically homogeneous evolution (CHE). In that situation, during the main sequence phase, the star keeps the same chemical composition from its centre to its surface. Homogeneous stars have larger effective temperatures and luminosities than stars evolving non-homogeneously and thus are stronger ionising sources. Methods. The stellar evolution models are based on n = 3 polytropes with a time-varying mass fraction of hydrogen. The ionisation model employs the self-similar champagne flow solution from Shu et al. (2002, ApJ, 580, 969) and numerical simulations for the stochastic treatment of star clusters over a grid of redshifts and halo masses. Results. We find that haloes containing chemically homogeneous stars have an escape fraction of ionising photons up to twice that of haloes containing classical Pop III stars. By extrapolating the high-z ionisation history powered by Pop III stars (at z ≳ 15) to the post-reionisation epoch, we derived the Thomson scattering optical depth τ, which is compared with the value measured by Planck. We find that τ is overproduced by ∼1.5 − 5σ when all Pop III stars evolve homogeneously. This indicates that CHE is unlikely to be realised in the majority of Pop III stars, although the present study cannot exclude that a fraction of them undergo CHE. Conclusions. Fast rotation might have a significant impact on the ionising budget of Pop III stars and thus on early cosmic reionisation. The impact is stronger for less top-heavy initial mass functions of Pop III stars.

Lyα Escape from Low-mass, Compact, High-redshift Galaxies

We investigate the effects of stellar populations and sizes on Lyα escape in 27 spectroscopically confirmed and 35 photometric Lyα emitters (LAEs) at z ≈ 2.65 in seven fields of the Boötes region of the NOAO Deep Wide-Field Survey. We use deep HST/WFC3 imaging to supplement ground-based observations and infer key galaxy properties. Compared to typical star-forming galaxies (SFGs) at similar redshifts, the LAEs are less massive (M ⋆ ≈ 107–109 M ⊙), younger (ages ≲1 Gyr), smaller (r e < 1 kpc), and less dust-attenuated (E(B−V) ≤ 0.26 mag) but have comparable star formation rates (SFRs ≈ 1–100 M ⊙ yr−1). Some of the LAEs in the sample may be very young galaxies having low nebular metallicities (Z neb ≲ 0.2 Z ⊙) and/or high ionization parameters (). Motivated by previous studies, we examine the effects of the concentration of star formation and gravitational potential on Lyα escape by computing SFR surface density, ΣSFR, and specific SFR surface density, ΣsSFR. For a given ΣSFR, the Lyα escape fraction is higher for LAEs with lower stellar masses. The LAEs have a higher ΣsSFR, on average, compared to SFGs. Our results suggest that compact star formation in a low gravitational potential yields conditions amenable to the escape of Lyα photons. These results have important implications for the physics of Lyα radiative transfer and for the type of galaxies that may contribute significantly to cosmic reionization.

Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQs) – XV. Constraining the cosmic reionization at 5.5 &amp;lt; z &amp;lt; 7

ABSTRACT Revealing the cosmic hydrogen reionization history is one of the main goals of the modern cosmology. z &amp;gt; 5 quasars (QSOs) have been used as back-lights to investigate the evolution of the intervening intergalactic medium (IGM) during the cosmic reionization since their first discovery. However, due to the small population of luminous QSOs (∼130 QSOs known to date), a tight constraint on the reionization history has not yet been placed. In this work, we aim to tighten the constraint using the 93 QSOs (5.5 &amp;lt; z &amp;lt; 7.1) recently discovered in the Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQS) project. This is the largest QSO sample used to constrain the epoch of reionization. We measure the mean IGM Ly α transmission and the QSO near-zone size using the UV spectra of these QSOs. The mean IGM Ly α transmission rises above zero at z ≲ 6, indicating the end of the reionization. The near-zone sizes of the SHELLQs QSOs are consistent with sizes spanned by QSOs of lifetime tq ∼ 1–100 Myr in simulations. Due to the scatter created by the low signal-to-noise spectra and large Ly α redshift uncertainty, we cannot conclude whether the redshift evolution of the near-zone size is affected by the reionization effect.

Astroparticle Constraints from Cosmic Reionization and Primordial Galaxy Formation

We derived astroparticle constraints in different dark matter scenarios that are alternatives to cold dark matter (CDM): thermal relic warm dark matter, WDM; fuzzy dark matter, ψDM; self-interacting dark matter, SIDM; sterile neutrino dark matter, νDM. Our framework is based on updated determinations of the high-redshift UV luminosity functions for primordial galaxies to redshift z∼10, on redshift-dependent halo mass functions in the above DM scenarios from numerical simulations, and on robust constraints on the reionization history of the Universe from recent astrophysical and cosmological datasets. First, we built an empirical model of cosmic reionization characterized by two parameters, namely the escape fraction fesc of ionizing photons from primordial galaxies, and the limiting UV magnitude MUVlim down to which the extrapolated UV luminosity functions steeply increased. Second, we performed standard abundance matching of the UV luminosity function and the halo mass function, obtaining a relationship between UV luminosity and the halo mass, whose shape depends on an astroparticle quantity X specific to each DM scenario (e.g., WDM particle mass); we exploited such a relationship to introduce (in the analysis) a constraint from primordial galaxy formation, in terms of the threshold halo mass above which primordial galaxies can efficiently form stars. Third, we performed Bayesian inference on the three parameters fesc, MUVlim, and X via a standard MCMC technique, and compared the outcomes of different DM scenarios on the reionization history. We also investigated the robustness of our findings against educated variations of still uncertain astrophysical quantities. Finally, we highlight the relevance of our astroparticle estimates in predicting the behavior of the high-redshift UV luminosity function at faint, yet unexplored magnitudes, which may be tested with the advent of the James Webb Space Telescope.

Unresolved z ∼ 8 Point Sources and Their Impact on the Bright End of the Galaxy Luminosity Function

The distribution and properties of the first galaxies and quasars are critical pieces of the puzzle in understanding galaxy evolution and cosmic reionization. Previous studies have often excluded unresolved sources as potential low-redshift interlopers. We combine broadband color and photometric redshift analysis with morphological selections to identify a robust sample of candidates consistent with unresolved point sources at redshifts z ∼ 8 using deep Hubble Space Telescope images. We also examine G141 grism spectroscopic data to identify and eliminate dwarf star contaminants. From these analyses, we identify three bright (M UV ≲ −22 AB mag) dropout point sources at 7.5 < z < 8.1. Spectral energy distribution analyses suggest that these sources are either quasars or compact star-forming galaxies. The flux captured by the IRAC 4.5 μm channel suggests that they have moderate Hβ + [O III] equivalent widths. We calculate the number density of point sources at z ∼ 7–8, and find that a double-power-law model well describes the point-source distribution. We then extend our analysis to estimate the combined point-source + galaxy luminosity function and find that the point sources have a nonnegligible contribution to the bright-end excess. The fact that the point sources dominate only at M UV ≲ −22 suggests that their contribution to cosmic reionization is likely limited. While spectroscopic follow-up is needed to confirm the nature of these point sources, this work demonstrates that the inclusion of Lyman-dropout point sources is necessary for a complete census of early galaxies at the epoch of cosmic reionization.

Cosmic Reionization on Computers: Physical Origin of Long Dark Gaps in Quasar Absorption Spectra

I explore the properties of “dark gaps”—regions in quasar absorption spectra without significant transmission—with several simulations from the Cosmic Reionization On Computers (CROC) project. The CROC simulations in the largest available boxes (120 cMpc) come close to matching both the distribution of mean opacities and the frequency of dark gaps, but alas not in the same model: the run that matches the mean opacities fails to contain enough dark gaps and vice versa. Nevertheless, the run that matches the dark gap distributions serves as a counterexample to claims in the literature that the dark gap statistics requires a late end to reionization—in that run reionization ends at z = 6.7 (likely too early). While multiple factors contribute to the frequency of large dark gaps in the simulations, the primary factor that controls the overall shape of the dark gap distribution is the ionization level in voids—the lowest-density regions produce the highest transmission spikes that terminate long gaps. As the result, the dark gap distribution correlates strongly with the fraction of the spectrum above the gap detection threshold and the observed distribution is matched by the simulation in which this fraction is 2%. Hence, the gap distribution by itself does not constrain the timing of reionization, although it may do so in combination with the distribution of mean opacities.

Discovery of the most distant luminous quasar to date: J0313-1806

<p indent="0mm">Distant quasars are unique media for studying topics such as the formation mechanisms of first-generation seed black holes, the interaction of active galactic nuclei and their host galaxies, and the history of cosmic reionization. So far, there have been found only six quasars with <italic>z</italic> &gt; 7, and two of them (ULAS J1342 + 0928, <italic>z</italic> = 7.54; J100758.264+211529.207, <italic>z </italic>= 7.52) have a redshift larger than 7.5. On January 20, 2021, in <italic>the Astrophysical Journal Letters</italic>, an international collaborative team of astronomers led by the University of Arizona reported a bright quasar, named J0313-1806, with a bolometric luminosity of 1.4 × <sc>10⁴⁷ erg s^–1.</sc> The process of discovering the quasar, the technique, and the scientific significance of this discovery are explained in this paper. Their work is based on a data set obtained mainly from the Pan-STARRS1, the DESI Legacy Imaging Surveys, the UKIRT Hemisphere Survey, the VISTA Hemisphere Survey, and the Wide-field Infrared Survey Explorer Survey. To measure the redshift of this quasar more accurately, the dust continuum and the CII emission from the quasar’s host galaxy were observed using the Atacama Large Millimeter/Submillimeter Array in Chile. Via spectral fitting, they calculated the quasar’s redshift of 7.642, that is more than 13 billion light-years from Earth and dating back to only 670 million years after the Big Bang, which makes the quasar the most distant and earliest one. Based on the spectral fitting and the MgII virial estimator, the mass of the supermassive black hole was calculated to be 1.6×10⁹<italic>M</italic>_⊙. The formation of such a massive supermassive black hole in a short period time imposes strict constraints on the mass and the formation model of the seed black hole. Suppose the black hole accreted at the Eddington rate, if it started its growth at redshift <italic>z</italic> = 15 – 30 (i.e., 400–570 Ma growth time), to what we observe today, it requires a 10⁴–10⁵<italic>M</italic>_⊙ seed black hole, that might be more supportive of direct-collapse black holes forming in pre-galactic dark matter halos. The quasar’s spectra also exhibit evident broad absorption line features in CIV and SiIV, together with a strongly blueshifted CIV emission line, indicating the relativistic outflows with the maximum speed of up to 20% of the speed of light. In addition, the ALMA observations imply a strong star formation of 200 <italic>M</italic>_⊙ a⁻¹ taking place in the host galaxy of J0313-1806. The continuum observations indicate that substantial dust (7 × 10⁷<italic>M</italic>_⊙) was already built up in the quasar host galaxy. Those makes J0313-1806 an ideal target for probing the effects of active galactic nuclei on their host galaxies. The number of high-redshift quasars is still small, making it difficult for constraining the formation model of the seed black holes, understanding the relation of the supermassive black hole and its host galaxy and so on. We look forward to those ongoing and upcoming optical/infrared sky surveys, such as the James Webb Space Telescope (JWST) and the Large Synoptic Survey Telescope (LSST), which will bring breakthroughs in astronomy.

Can Cooling and Heating Functions Be Modeled with Homogeneous Radiation Fields?

Cooling and heating functions describe how radiative processes impact the thermal state of a gas as a function of its temperature and other physical properties. In a most general case the functions depend on the detailed distributions of ionic species and on the radiation spectrum. Hence, these functions may vary on a very wide range of spatial and temporal scales. In this paper, we explore cooling and heating functions between 5 ≤ z ≤ 10 in simulated galaxies from the Cosmic Reionization On Computers project. We compare three functions: (1) the actual cooling and heating rates of hydrodynamic cells as a function of cell temperature, (2) the median cooling and heating functions computed using median interstellar medium (ISM) properties (median ISM), and (3) the median of the cooling and heating functions of all gas cells (instantaneous). We find that the median ISM and instantaneous approaches to finding a median cooling and heating function give identical results within the spread due to cell-to-cell variation. However, the actual cooling (heating) rates experienced by the gas at different temperatures in the simulations do not correspond to either summarized cooling (heating) functions. In other words, the thermodynamics of the gas in the simulations cannot be described by a single set of a cooling plus a heating function with a spatially constant radiation field that could be computed with common tools, such as CLOUDY.

The Infrared Medium-deep Survey. IX. Discovery of Two New z ∼ 6 Quasars and Space Density Down to M 1450 ∼ −23.5 mag

We present the result of the Infrared Medium-deep Survey (IMS) z ∼ 6 quasar survey, using the combination of the IMS near-infrared images and the Canada–France–Hawaii Telescope Legacy Survey optical images. The traditional color selection method results in 25 quasar candidates over 86 deg2. We introduce the corrected Akaike information criterion (AICc) with the high-redshift quasar and late-type star models to prioritize the candidates efficiently. Among the color-selected candidates, seven plausible candidates finally passed the AICc selection, of which three are known quasars at z ∼ 6. The follow-up spectroscopic observations for the remaining four candidates were carried out, and we confirmed that two out of four are z ∼ 6 quasars. With this complete sample, we revisited the quasar space density at z ∼ 6 down to M 1450 ∼ −23.5 mag. Our result supports the low quasar space density at the luminosity where the quasar’s ultraviolet ionizing emissivity peaks, favoring a minor contribution of quasars to the cosmic reionization.

Impact of the turnover in the high-z galaxy luminosity function on the 21-cm signal during Cosmic Dawn and Epoch of Reionization

ABSTRACT The shape of the faint-end of the high-z galaxy luminosity function (LF) informs early star formation and reionization physics during the Cosmic Dawn and Epoch of Reionization. Until recently, based on the strong gravitational lensing cluster deep surveys, the Hubble Frontier Fields (HFF) has found a potential turnover in the ultraviolet (UV) LF at $\mathit{ z}$ ∼ 6. In this paper, we analyse the contribution of extremely faint galaxies with the magnitude larger than the turnover magnitude in LF to cosmic reionization. We apply the measurement from HFF to our suppressed star formation efficiency model, including three free parameters: halo mass threshold Mt, curvature parameter β, and a UV conversion factor lUV. According to our fit of 68 per cent confidence level, the high-redshift star formation in haloes smaller than $M_t=1.82^{+2.86}_{-1.08}\times 10^{10} \, \rm M_{\odot }$ is found to be dampened. The turnover magnitude $\rm \gtrsim -13.99-2.45$, correspondingly the halo mass $\lesssim (4.57+20.03)\times 10^{9} \, \rm M_{\odot }$. We find that the absorption trough in the global 21-cm signal is sensitive to our SFE model parameters. Together with (β, lUV) = ($2.17^{+2.42}_{-1.72}$, $9.33^{+0.43}_{-0.42} \, \rm ~erg~yr ~s^{-1}\, M_{\odot }^{-1})$, the trough locates at ∼$134^{+10}_{-17}$$\rm MHz$ with an amplitude of ∼$-237^{-6}_{+7}$$\rm mK$, compared to (106 MHz, -212 mK) in the absence of turnover. Besides, we find that the star formation of faint galaxies has also an impact on the 21-cm power spectra. The best-fitting peak power decreases by$\sim 4{{\ \rm per\ cent}}$ and shifts towards smaller scales from $0.88 \, h\, \rm Mpc^{-1}$ to $0.91 \, h\, \rm Mpc^{-1}$. According to our calculation, such impact is distinguishable with the forthcoming Square Kilometre Array.

The discovery of the oldest supermassive black hole

<p indent="0mm">On December 20, 2021, the website of <italic>Science News</italic> summarized “6 surprising records science set in 2021”, including the oldest black hole discovered by astronomers. These new findings extend the limits of scientific research and the imagination of human beings. Here we present a brief introduction to the discovery of the oldest black hole in the early Universe and its importance in science, and give some prospects of the related studies in the future. Quasars are luminous active galactic nuclei with supermassive black holes (SMBHs) in their centers, accreting the surrounding gas and emitting huge power. Distant quasars are unique tracers to study the formation of the earliest SMBHs and the history of cosmic reionization. On January 14, 2021, in a paper appeared in the <italic>Astrophysical Journal Letters</italic>, an international team led by Dr. Feige Wang, Dr. Jinyi Yang and Prof. Xiaohui Fan at University of Arizona reported the discovery of a luminous quasar J0313-1806 at redshift 7.64. This discovery breaks the former redshift record, 7.54 for the quasar J1342+0928, making J0313-1806 the most distant quasar discovered so far, with a distance of about 13 billion light years from us. The near-infrared spectra taken with several largest ground based telescopes reveal the existence of a black hole of 1.6 billion solar masses in the center of J0313-1806. It becomes the oldest black hole in the early Universe, with the age of only 670 million years after the Big Bang. How can a black hole grow to 1.6 billion solar masses within 670 million years (only about 5% of the universe’s current age)? The discovery of the oldest black hole provides the most serious challenge to the current understanding of the black hole formation and growth. SMBHs are thought to grow from smaller seed black holes that accrete the surrounding matter. The popular idea is that the seed black holes formed through the collapse of the first generation massive stars (or population III stars), but this can only make the seed black holes up to a few hundred solar masses in most cases, or up to a few thousand solar masses for the extreme cases in the dense stellar clusters. However, even if we assume that J0313-1806’s seed black hole grew as fast as possible, it would have needed a starting mass of at least 10000 solar masses at redshift 30. Among the theoretical models currently known, only the direct collapse black hole (DCBH) model involving the vast amounts of primordial cold hydrogen gas can provide a possible explanation. In addition, J0313-1806 also shows many important features. The rest-frame UV spectrum exhibits broad absorption troughs, which are thought to be produced by the extremely high-velocity outflows launched from the accretion disk around the central black hole. These outflows have a maximum velocity up to 20% of the speed of light, and can provide strong feedbacks to affect the star formation in the quasar host galaxy due to the extremely high kinetic power. J0313-1806’s host galaxy has a very rapid star formation process, with the star formation rate 200 times higher than the Milky Way. Together with its huge luminosity of 36 trillion times higher than the Sun, J0313-1806 has become one of the most important targets to study the formation and growth of the supermassive black holes, as well as their influences on the host galaxies in the early Universe. There will be more observations on the oldest black hole in the near future. The James Webb Space Telescope (JWST) was successfully launched on December 25, 2021. The team who discovered the oldest black hole has been awarded the JWST time of more than two hundred hours. More surprising discoveries are expected to be revealed by the future JWST observations on the most distant black holes, galaxies and quasars in the epoch of cosmic reionization.

New Spectroscopic Confirmations of Lyα Emitters at Z ∼ 7 from the LAGER Survey

We report spectroscopic confirmations of 15 Lyα galaxies at z ∼ 7, implying a spectroscopic confirmation rate of ∼80% on candidates selected from the Lyα Galaxies in the Epoch of Reionization (LAGER), which is the largest (24 deg2) survey aimed at finding Lyα emitters (LAEs) at z ∼ 7 and uses deep narrowband imaging from the Dark Energy Camera at CTIO. LAEs at high redshifts are sensitive probes of cosmic reionization, and narrowband imaging is a robust and effective method for selecting a large number of LAEs. In this work, we present results from the spectroscopic follow-up of LAE candidates in two LAGER fields, COSMOS and WIDE-12, using observations from Keck/LRIS. We report the successful detection of Lyα emission in 15 candidates. Three of these in COSMOS have matching confirmations from a previous spectroscopic follow-up and are part of the overdense region, LAGER-z7OD1. Two other candidates that were not detected with LRIS have prior spectroscopic confirmations from Magellan. Including these, we obtain a spectroscopic confirmation success rate of ∼80% for LAGER LAE candidates. Thorough checks were performed to reject the possibility of these detections being foreground emission resulting with a probability of, at most, one contaminant. We do not detect any other UV nebular lines in our LRIS spectra, apart from Lyα. We estimate a 2σ upper limit for the ratio of N v/Lyα, f NV/f Lyα ≲ 0.27. Including confirmations from this work, a total of 33 LAE sources from LAGER are now spectroscopically confirmed. LAGER has more than doubled the sample of spectroscopically confirmed LAE sources at z ∼ 7.

Reionization time of the Local Group and Local-Group-like halo pairs

ABSTRACT Patchy cosmic reionization resulted in the ionizing UV background asynchronous rise across the Universe. The latter might have left imprints visible in present-day observations. Several numerical simulation-based studies show correlations between the reionization time and overdensities and object masses today. To remove the mass from the study, as it may not be the sole important parameter, this paper focuses solely on the properties of paired haloes within the same mass range as the Milky Way. For this purpose, it uses CoDaII, a fully coupled radiation hydrodynamics reionization simulation of the local Universe. This simulation holds a halo pair representing the Local Group, in addition to other pairs, sharing similar mass, mass ratio, distance separation, and isolation criteria but in other environments, alongside isolated haloes within the same mass range. Investigations of the paired halo reionization histories reveal a wide diversity although always inside-out, given our reionization model. Within this model, haloes in a close pair tend to be reionized at the same time but being in a pair does not bring to an earlier time their mean reionization. The only significant trend is found between the total energy at z = 0 of the pairs and their mean reionization time: Pairs with the smallest total energy (bound) are reionized up to 50 Myr earlier than others (unbound). Above all, this study reveals the variety of reionization histories undergone by halo pairs similar to the Local Group, that of the Local Group being far from an average one. In our model, its reionization time is ∼625 Myr against 660 ± 4 Myr (z ∼ 8.25 against 7.87 ± 0.02) on average.

The prevalence of galaxy overdensities around UV-luminous Lyman 𝛼 emitters in the Epoch of Reionization

ABSTRACT Before the end of the Epoch of Reionization, the Hydrogen in the Universe was predominantly neutral. This leads to a strong attenuation of Ly α lines of z ≳ 6 galaxies in the intergalactic medium. Nevertheless, Ly α has been detected up to very high redshifts (z ∼ 9) for several especially UV luminous galaxies. Here, we test to what extent the galaxy’s local environment might impact the Ly α transmission of such sources. We present an analysis of dedicated Hubble Space Telescope (HST) imaging in the CANDELS/EGS field to search for fainter neighbours around three of the most UV luminous and most distant spectroscopically confirmed Ly α emitters: EGS-zs8-1, EGS-zs8-2, and EGSY-z8p7 at zspec = 7.73, 7.48, and 8.68, respectively. We combine the multiwavelength HST imaging with Spitzer data to reliably select z ∼ 7–9 galaxies around the central, UV-luminous sources. In all cases, we find a clear enhancement of neighbouring galaxies compared to the expected number in a blank field (by a factor ∼3–9×). Our analysis thus reveals ubiquitous overdensities around luminous Ly α emitting sources in the heart of the cosmic reionization epoch. We show that our results are in excellent agreement with expectations from the Dragons simulation, confirming the theoretical prediction that the first ionized bubbles preferentially formed in overdense regions. While three UV luminous galaxies already have spectroscopic redshifts, the majority of the remaining fainter, surrounding sources are yet to be confirmed via spectroscopy. JWST follow-up observations of the neighbouring galaxies identified here will thus be needed to confirm their physical association and to map out the ionized regions produced by these sources.

The Low-Redshift Lyman Continuum Survey

Aims. Combining 66 ultraviolet (UV) spectra and ancillary data from the recent Low-Redshift Lyman Continuum Survey (LzLCS) and 23 LyC observations by earlier studies, we form a statistical sample of star-forming galaxies at z ∼ 0.2 − 0.4 with which we study the role of cold interstellar medium (ISM) gas in the leakage of ionizing radiation. We also aim to establish empirical relations between the H I neutral and low-ionization state (LIS) absorption lines with different galaxy properties. Methods. We first constrain the massive star content (stellar ages and metallicities) and UV attenuation by fitting the stellar continuum with a combination of simple stellar population models. The models, together with accurate LyC flux measurements, allow us to determine the absolute LyC photon escape fraction for each galaxy (fescabs). We then measure the equivalent widths and residual fluxes of multiple H I and LIS lines, and the geometrical covering fraction of the UV emission, adopting the picket-fence model. Results. The LyC escape fraction spans a wide range, with a median fescabs (0.16, 0.84 quantiles) of 0.04 (0.02, 0.20), and 50 out of the 89 galaxies detected in the LyC (1σ upper limits of fescabs ≲ 0.01 for non-detections, typically). The H I and LIS line equivalent widths scale with the UV luminosity and attenuation, and inversely with the residual flux of these lines. Additionally, Lyα equivalent widths scale with both the H I and LIS residual fluxes, but anti-correlate with the corresponding H I or LIS equivalent widths. The H I and LIS residual fluxes are correlated, indicating that the neutral gas is spatially traced by the low-ionization transitions. We find that the observed trends of the absorption lines and the UV attenuation are primarily driven by the geometric covering fraction of the gas. The observed nonuniform gas coverage also demonstrates that LyC photons escape through low-column-density channels in the ISM. The equivalent widths and residual fluxes of both the H I and LIS lines strongly correlate with fescabs: strong LyC leakers (highest fescabs) show weak absorption lines, low UV attenuation, and large Lyα equivalent widths. We provide several empirical calibrations to estimate fescabs from UV absorption lines. Finally, we show that simultaneous UV absorption line and dust attenuation measurements can, in general, predict the escape fraction of galaxies. We apply our method to available measurements of UV LIS lines of 15 star-forming galaxies at z ∼ 4 − 6 (plus 3 high-z galaxy composites), finding that these high-redshift, UV-bright galaxies (MUV ≲ −21) may have low escape fractions, fescabs ≲ 0.1. Conclusions. UV absorption lines trace the cold ISM gas of galaxies, which governs the physics of the LyC escape. We show that, with some assumptions, the absolute LyC escape can be statistically predicted using UV absorption lines, and the method can be applied to study galaxies across a wide redshift range, including in the epoch of cosmic reionization.

Dust Grain Growth and Dusty Supernovae in Low-metallicity Molecular Clouds

We present 3D hydrodynamical models of the evolution of superbubbles powered by stellar winds and supernovae from young coeval massive star clusters within low-metallicity (Z = 0.02 Z ⊙), clumpy molecular clouds. We explore the initial stages of the superbubble evolution, including the occurrence of pair-instability and core-collapse supernovae. Our aim is to study the occurrence of dust grain growth within orbiting dusty clumps, and in the superbubble’s swept-up supershell. We also aim to address the survival of dust grains produced by sequential supernovae. The model accounts for the star cluster gravitational potential and self-gravity of the parent cloud. It also considers radiative cooling (including that induced by dust) and a state-of-the-art population synthesis model for the coeval cluster. As shown before, a superbubble embedded into a clumpy medium becomes highly distorted, expanding mostly due to the hot gas streaming through low-density channels. Our results indicate that in the case of massive (∼107 M ⊙) molecular clouds, hosting a super star cluster (∼5.6 × 105 M ⊙), grain growth increments the dust mass at a rate ∼4.8 × 10−5 M ⊙ yr−1 during the first 2.5 Myr of the superbubble’s evolution, while the net contribution of pair-instability and core-collapse supernovae to the superbubble’s dust budget is ∼1200 M ⊙ (M SC/5.6 × 105 M ⊙), where M SC is the stellar mass of the starburst. Therefore, dust grain growth and dust injection by supernovae lead to the creation of, without invoking a top-heavy initial mass function, massive amounts of dust within low-metallicity star-forming molecular clouds, in accordance with the large dust mass present in galaxies soon after the onset of cosmic reionization.

Predicting Lyman-continuum emission of galaxies using their physical and Lyman-alpha emission properties

Aims.The primary difficulty in understanding the sources and processes that powered cosmic reionization is that it is not possible to directly probe the ionizing Lyman-continuum (LyC) radiation at that epoch as those photons have been absorbed by the intervening neutral hydrogen. It is therefore imperative to build a model to accurately predict LyC emission using other properties of galaxies in the reionization era.Methods.In recent years, studies have shown that the LyC emission from galaxies may be correlated to their Lyman-alpha (Lyα) emission. In this paper we study this correlation by analyzing thousands of simulated galaxies at high redshift in the SPHINX cosmological simulation. We post-process these galaxies with the Lyαradiative transfer code RASCAS and analyze the Lyα– LyC connection.Results.We find that the Lyαand LyC luminosities are strongly correlated with each other, although with dispersion. There is a positive correlation between the escape fractions of Lyαand LyC radiations in the brightest Lyman-alpha emitters (LAEs; escaping LyαluminosityLescLyα&gt; 1041erg s−1), similar to that reported by recent observational studies. However, when we also include fainter LAEs, the correlation disappears, which suggests that the observed relation may be driven by selection effects. We also find that the brighter LAEs are dominant contributors to reionization, withLescLyα&gt; 1040erg s−1galaxies accounting for &gt; 90% of the total amount of LyC radiation escaping into the intergalactic medium in the simulation. Finally, we build predictive models using multivariate linear regression, where we use the physical and Lyαproperties of simulated reionization era galaxies to predict their LyC emission. We build a set of models using different sets of galaxy properties as input parameters and predict their intrinsic and escaping LyC luminosity with a high degree of accuracy (the adjustedR2of these predictions in our fiducial model are 0.89 and 0.85, respectively, whereR2is a measure of how much of the response variance is explained by the model). We find that the most important galaxy properties for predicting the escaping LyC luminosity of a galaxy are itsLescLyα, gas mass, gas metallicity, and star formation rate.Conclusions.These results and the predictive models can be useful for predicting the LyC emission from galaxies using their physical and Lyαproperties and can thus help us identify the sources of reionization.

Exploring the cosmic dawn and epoch of reionization with the 21 cm line

Abstract The dark age of the Universe, when no luminous object existed, ended with the birth of the first stars, galaxies, and black holes. This epoch is called cosmic dawn. Cosmic reionization is the major transition of the intergalactic medium in the Universe driven by ionizing photons emitted from luminous objects. Although the epoch through the dark ages to reionization is a milestone in the Universe, our knowledge of it is not yet sufficient. The cosmic 21 cm signal, which is emitted from neutral hydrogen, is expected to open a new window into this epoch. In this review paper, we first introduce the basic physics of the 21 cm line and how the first stars impact on the 21 cm line signal. Next, we briefly summarize how we extract astrophysical information from the 21 cm line signal by means of statistical and machine-learning approaches. We also discuss the synergy between the 21 cm line signal and other emission lines. Finally, we summarize the current status of 21 cm experiments.

Confirming the energy sources of cosmic reionization

Confirming the energy sources of cosmic reionization