arXiv subjects

K. E. Heintz

Publications and source records attributed to K. E. Heintz.

At least 19 recordsLinked to original sources

arXiv2026

DeepDive: A deep dive into the physics of the first massive quiescent galaxies in the Universe

We present the DeepDive program, in which we obtained deep JWST/NIRSpec G235M/F170LP spectra for ten primary massive ($\log{(M_\star/M_\odot)}=10.8-11.5$) quiescent galaxies at $z\sim3-4$. A novel reduction procedure was used to extend the nominal wavelength coverage of G235M beyond H$α$ and [NII] at $z\sim 4$, revealing weak, narrow H$α$ lines indicative of low star formation rates. Two out of ten primary targets have broad H$α$ lines, indicating the presence of active galactic nuclei. We also conducted an archival search of quiescent galaxies observed with NIRSpec gratings in the DAWN JWST Archive, providing a statistical context for interpreting the DeepDive targets. This archival search provided a spectroscopic sample of 126 quiescent galaxies spanning $1<z<5$, selected by high Dn4000, UVJ color, or low specific star formation rate, and covering more than an order of magnitude in stellar mass. This sample allowed us to revisit the sample from the different selections, finding $\sim90\%$ overlap between these criteria. The total sample of 136 quiescent galaxies from this study shows that those at $z\sim3-5$, including the DeepDive targets, typically exhibit weaker breaks and bluer colors than their lower-redshift counterparts, indicating generally younger stellar populations. Stacked spectra of sources grouped by the Dn4000 index reveal faint iron and magnesium absorption line features in the stellar continuum even for the low Dn4000 subsample at high redshift ($z\sim3$). In addition, higher Dn4000 subsamples show fainter nebular emission lines. These results demonstrate that medium-resolution NIRSpec spectroscopy is essential for robustly characterizing the diversity and evolution of early quiescent galaxies. The large sample constructed in this paper will allow a statistical census of the properties of quiescent galaxies at high redshift to be obtained.

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arXiv2026

The metallicities of little red dot host galaxies: LRDs are metal poor, but not pristine

Little Red Dots (LRDs) are a population of high-z sources discovered by JWST whose compactness, broad permitted lines, strong absorption features, continuum shapes and luminosities point to accreting supermassive black holes (SMBHs) embedded in dense gas. To date, the metallicity of the hosts of these systems has not been systematically measured. We determine the gas-phase metallicities of LRD host galaxies and test whether their narrow-line emission is consistent with metal poor star formation or AGN activity. We assemble a sample of 24 LRDs at z ~ 2.3-7 with medium and high-resolution JWST/NIRSpec data. We derive oxygen abundances and electron temperatures using the direct Te method applied exclusively to the narrow components of emission lines, and cross-check against widely used strong line calibrations. We derive a sample-averaged abundance of $Z_{T_\mathrm{e}} = 0.08_{-0.03}^{+0.11}\,\mathrm{Z_{\odot}}$ ($T_\mathrm{e} = 23000_{-7000}^{+17000}$\,K), placing LRDs firmly in the metal-poor regime of high redshift star forming galaxies. The R-hat calibration yields a consistent average of $Z_{\hat{\mathrm{R}}} = 0.07_{-0.04}^{+0.07}\,\mathrm{Z_{\odot}}$, with only 4% scatter relative to the direct Te method, providing a robust proxy for systems where the [O III]4363Å line is not detected. We also identify two extremely metal-poor LRDs with metallicities <1.3%. The general population of LRDs are among the lower metallicity galaxies found by JWST at this epoch; they exhibit a narrow range of metallicities with a range of about 0.6 dex, which remains remarkably stable over cosmic time. Such low metallicity may then be a defining property of the class. The fact that LRDs have substantial metallicity across most of the class poses a challenge to models that require formation via pristine gas collapse, while their generally low metallicity indicates that they are not standard AGN.

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arXiv2026

Extended [CII] gas emission in and around a massive quiescent galaxy at z=7.3

We report the discovery of [CII] 158 micron emission in and around the most distant known massive quiescent galaxy RUBIES-UDS-QG-z7 at z = 7.27. Observed with ALMA in band 6, the [CII] line independently confirms the spectroscopic redshift from JWST/NIRSpec spectra at low and medium resolution. The emission extends over an effective radius R_eff,[CII] = 8 +/- 3 kpc, well beyond the compact stellar body traced by JWST/NIRCam (R_eff = 209 (+33/-24) pc), with a significant fraction of approximately 70% of the flux arising from a circumgalactic halo. No dust continuum is detected at rest-frame ~160 micron, setting an upper limit on the infrared luminosity of L_IR < 1.4 x 10^11 Lsun, overall consistent with expectations from rest-frame UV to near-infrared SED modeling under energy balance. Converting the galaxy-scale [CII] emission into cold gas mass, we find log(M_mol/Msun) = 9.53 (+0.32/-0.31) and log(M_HI/Msun) = 9.46-10.34, depending on the assumed calibration and metallicity. Despite being approximately 10x more gas-poor than typical star-forming galaxies at fixed redshift, stellar mass, and [CII] to gas mass conversion, RUBIES-UDS-QG-z7 retains a substantial cold gas reservoir with fractions f_gas >~ 20% and long depletion timescales across most assumptions. The extended [CII] halo carries approximately twice as much gas as the galaxy alone and shows a blueshifted velocity offset consistent with the tentative gas outflow detected in MgII absorption in previous work, suggesting a past episode of AGN-driven gas expulsion possibly linked to the suppression of star formation. The presence of a large gas reservoir in and around a massive quiescent galaxy just 700 Myr after the Big Bang implies that whatever mechanism is suppressing star formation must be remarkably effective at maintaining a low star formation efficiency on ~100 Myr timescales, even in the presence of abundant fuel.

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arXiv2026

Little red dots as young supermassive black holes in dense ionized cocoons

The James Webb Space Telescope (JWST) has uncovered many compact galaxies at high redshift with broad hydrogen and helium lines, including the enigmatic population of little red dots (LRDs). The nature of these galaxies is debated and is attributed to supermassive black holes (SMBHs) or intense star formation. They exhibit unusual properties for SMBHs, such as black holes that are overmassive for their host galaxies and extremely weak X-ray and radio emission. Here we show that in most objects studied with the highest-quality JWST spectra, the lines are broadened by electron scattering with a narrow intrinsic core. The data require very high electron column densities and compact sizes (light days), which, when coupled with their high luminosities, can be explained only by SMBH accretion. The narrow intrinsic line cores imply black hole masses of $10^{5-7}$ $M_{\odot}$, two orders of magnitude lower than previous estimates. These are the lowest mass black holes known at high redshift, to our knowledge, and suggest a population of young SMBHs. They are enshrouded in a dense cocoon of ionized gas producing broad lines from which they are accreting close to the Eddington limit, with very mild neutral outflows. Reprocessed nebular emission from this cocoon dominates the optical spectrum, explaining most LRD spectral characteristics, including the weak radio and X-ray emission.

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arXiv2026

Inside the cocoon: a comprehensive explanation of the spectra of Little Red Dots

JWST has revealed a population of compact galaxies in the early Universe with broad emission lines and strong Balmer breaks; among them the so-called ''little red dots'' (LRDs). Their nature remains uncertain with hypotheses including exotic phenomena. We assemble a sample of LRD-like objects at $z>3$ and use self-consistent radiative-transfer calculations to show that a supermassive black hole accreting from a dense gas cocoon accurately reproduces the detailed spectra. We show that the cocoons must be non-spherical, with comparable amounts of inflowing and outflowing material. And we predict correlations between Balmer break strength, Balmer line-absorption and scattering line width, which we confirm in our observed sample. We reproduce all LRD-like properties without requiring star-like atmospheres and we determine the typical black hole in our sample to be of order a million solar masses, with ionized cocoon masses of tens of solar masses potentially supplied from a much larger cold-gas reservoir.

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arXiv2026

GRB 180728A and SN 2018fip: the nearest high-energy cosmological gamma-ray burst with an associated supernova

The long GRB 180728A, at a redshift of $z = 0.1171$, stands out due to its high isotropic energy of $E_{γ,iso} \sim 2.5 \times 10^{51}$ erg, in contrast with most events at redshift $z<0.2$. We analyze the properties of GRB 180728A's prompt emission, afterglow, and associated supernova SN 2018fip, comparing them with other GRB-SN events. This study employs a dense photometric and spectroscopic follow-up of the afterglow and the SN up to 80 days after the burst, supported by image subtraction to remove the presence of a nearby bright star, and modelling of both the afterglow and the supernova. GRB 180728A lies on the $E_{p,i}-E_{γ,iso}$ plane occupied by classical collapsar events, and the prompt emission is one of the most energetic at $z < 0.2$ after GRB 030329 and GRB 221009A. The afterglow of GRB 180728A is less luminous than that of most long GRBs, showing a shallow early phase that steepens around 5 hours (0.2 days). The GRB exploded in an irregular, low-mass, blue, star-forming galaxy, typical of low-z collapsar events. Because of the relatively faint afterglow, the light curve bump of SN 2018fip dominates the optical emission already after $\sim$3 days and is one of the best sampled to date. The strong suppression below $\sim$ 4000 angstrom and a largely featureless continuum in the early 6--9 days spectra favor aspherical two-component ejecta with a high-velocity collimated component ($> 20,000 km s^{-1}$), dominant early-on, and a more massive, low-velocity component, which dominates at much later epochs. Our findings indicate that asymmetries need to be considered in order to better understand GRB-SNe. In any case, SN 2018fip shares many characteristics with typical GRB-SNe. Its kinetic energy is below the common range of $10^{52}-10^{53}$ erg and does not correlate with the high energy of the GRB, highlighting the diversity of the GRB-SN energy budget partition.

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arXiv2025

On the Dusty Proximate Damped Lyman-α System toward Q2310-3358 at z = 2.40

Quasar absorption systems not only affect the way quasars are selected, but also serve as key probes of galaxies, providing insight into their chemical evolution and interstellar medium (ISM). Recently, a method based on Gaia astrometric measurements has aided the selection of quasars reddened by dust hitherto overlooked. We conducted a spectroscopic study using VLT/X-Shooter on one such dust-reddened quasar, Q2310-3358. This quasar, at $z = 2.3909\pm0.0022$, is associated with a damped Lyman-$α$ absorber (DLA) at nearly the same redshift $2.4007\pm0.0003$, with a neutral hydrogen column density of $\log N(HI)=21.214\pm0.003$. The DLA is very metal-rich (close to solar metallicity after correction for depletion on dust grains). Its properties align with the metal-to-dust ratio and the mass-metallicity relation established in previous large samples of DLAs. Surprisingly, given its proximity to the quasar in redshift, the absorber has strong cold gas characteristics, including CI and H$_2$. Based on the derived kinetic temperature of $71^{+28}_{-15}$~K, we infer the presence of a strong UV radiation field, which in turn suggests that the quasar and the DLA are in close proximity, i.e., part of the same galaxy and not just different objects in the same overdensity of galaxies. We used the line ratios of the fine-structure lines to constrain the density of the cold gas, yielding $n_{\rm H} \sim 10^{3}~\mathrm{cm}^{-3}$. Our analysis extends the understanding of $z_{abs} \approx z_{em}$ absorption line systems and provides valuable constraints on the interplay between dust, metals, and neutral gas in the ISM of early galaxies.

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arXiv2025

Evidence of violation of Case B recombination in Little Red Dots

Little Red Dots (LRDs) are a new class of compact extragalactic objects, with a v-shaped optical spectral energy distribution breaking close to the Balmer break wavelength, and broad, typically exponentially-shaped lines. They are believed to be supermassive black holes surrounded by very dense, ionized gas, leading us to explore for any departures from Case B recombination by examining the ratios of multiple hydrogen Balmer lines: $Hα$, $Hβ$, $Hγ$, and $Hδ$. We analyze a dozen high-S/N LRDs with JWST/NIRSpec, measuring Balmer ratios in the seven objects with coverage of at least three lines. We decompose the line ratios into their respective broad and narrow components. Broad line ratios are consistent with Case B plus dust extinction in all objects but one, RUBIES EGS-49140, which departs from Case B expectations by more than $5σ$. The narrow components are consistent with minimal dust attenuation, while two objects exhibit narrow $Hα$/$Hβ\approx 1.8$. Such low decrements are observed in highly ionized density bounded nebulae, associated with starburst environments. Nevertheless, both flat decrement cases can be reconciled assuming an unresolved absorption feature. RUBIES EGS-49140, shows a high broad $Hα$/$Hβ$, but $Hγ$/$Hα$ and $Hδ$/$Hα$ ratios are lower than expected for extinction-modified Case B, hinting at an unphysically steep dust law. These line ratios may be due to increased optical depth in the Balmer lines, as a direct effect of high density (log$n_e$ > 9) gas surrounding the black hole. If Case B recombination does hold in most LRDs, they must be moderate-to-heavily dust obscured ($A_V\simeq1-8$) while the host-galaxy should be dust-free, suggesting that the extinction in the broad lines is local to the LRD and not due to the general ISM of the host galaxy.

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arXiv2025

Neutral hydrogen in and around galaxies during the Epoch of Reionization

JWST spectra revealing Lyman-$α$ (Ly$α$) absorption in $z\sim 5-14$ galaxies offer a unique probe of reionization's earliest stages. However, disentangling absorption by the increasingly neutral intergalactic medium (IGM) from that in galaxies' interstellar and circumgalactic medium (ISM, CGM) remains challenging due to the poorly constrained nature of neutral hydrogen (HI) in and around galaxies at these redshifts. We use the SERRA high-resolution cosmological zoom-in simulations to characterize the HI distribution and its evolution along sightlines to star-forming regions during reionization, to interpret the contribution of local HI (ISM+CGM) to Ly$α$ absorption in $z>5$ spectra. We analyze $\sim 100$ $z=6-9.5$ galaxies, generating mock sightlines from each galaxy's star formation peak. We study the sightline distribution of HI column densities ($N_{\rm HI}$) and its variation with radius, halo mass and redshift. We find broad sightline variation in $N_{\rm HI}$ (0.5-1.5dex) due to complex ISM morphology driven by bursty star formation, with median $\log(N_{\rm HI}/{\rm cm}^{-2}) \simeq 21-22$. Dense ISM gas is the dominant origin of damped Ly$α$ absorption (DLA) systems along sightlines towards star-forming regions, outweighing gas in the CGM, filaments and proximate absorbers. Median $N_{\rm HI}$ increases with halo mass, scaling roughly with the virial radius, as expected due to larger potentials and more extended CGM, but shows negligible redshift evolution at fixed halo mass. This suggests post-reionization $N_{\rm HI}$ distributions may provide useful priors to interpret IGM damping wings at higher redshifts. To interpret strong $z>5$ DLA candidates found by JWST, we investigate $N_{\rm HI}>10^{22}{\rm cm}^{-2}$ sightlines. These trace dense, metal-enriched ISM within <1 kpc of massive halos, a scenario testable with higher-resolution spectroscopy.

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arXiv2025

A 50 s quasi-periodic oscillation in the early X-ray afterglow of GRB 220711B

It is generally believed that long duration gamma-ray bursts (GRBs) originate from the core collapse of rapidly spinning massive stars and at least some of them are powered by hyper-accreting black holes. However, definite proofs about the progenitor and central engine of these GRBs have not been directly observed in the past. Here we report the existence of a Quasi-Periodic Oscillation (QPO) signature with periodic frequency $\sim$0.02 Hz in the early X-ray afterglow phase of GRB 220711B. Such a low-frequency QPO likely signals the precession of a relativistic jet launched from a GRB hyper-accreting black hole central engine. The energy injection signature from the \textbf{late} X-ray observations (from $5\times 10^2s\sim 1\times10^4s$) is consistent with the precession hypothesis. The prompt $γ$-ray light curve does not show any QPO signature, suggesting that the X-ray flaring emission in the early afterglow phase and prompt emission likely originate from different accretion processess, indicating that the progenitor stars of GRBs have a core-envelope structure with a stratified angular momentum distribution and the late-time accretion disk likely has a misalignment with respect to the rotation axis of the black hole. Such a misalignment is not expected in a canonical collapsar model. As a result, the QPO signature in GRB 220711B may reveal a new formation channel of long GRBs, possibly a stellar-merger-induced core collapse, with the orbital angular momentum of the binary misaligned with the spin axis of the collapsing star.

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arXiv2025

SVOM GRB 250314A at z $\simeq$ 7.3: an exploding star in the era of reionization

Most long Gamma-ray bursts originate from a rare type of massive stellar explosion. Their afterglows, while rapidly fading, can be initially extremely luminous at optical/near-infrared wavelengths, making them detectable at large cosmological distances. Here we report the detection and observations of GRB 250314A by the SVOM satellite and the subsequent follow-up campaign with the near-infrared afterglow discovery and the spectroscopic measurements of its redshift z $\simeq$ 7.3 . This burst happened when the Universe was only $\sim$ 5% of its current age. We discuss the signature of these rare events within the context of the SVOM operating model, and the ways to optimize their identification with adapted ground follow-up observation strategies.

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arXiv2025

Gas outflows in two recently quenched galaxies at z = 4 and 7

Outflows are a key element in the baryon cycle of galaxies, and their properties provide a fundamental test for our models of how star formation quenches in galaxies. Here we report the detection of outflowing gas in two recently quenched, massive ($M_\star\sim10^{10.2}M_\odot$) galaxies at z=4.106 (NS_274) and z=7.276 (RUBIES-UDS-QG-z7) observed with JWST/NIRSpec. The outflows are traced by blue-shifted MgII absorption lines, and in the case of the z=4.1 system, also by FeII and NaI features. The spectra of the two sources are similar to those of local post-starburst galaxies, showing deep Balmer features and minimal star formation on 10 Myr timescales as traced by the lack of bright emission lines, also suggesting the absence of a strong and radiatively efficient AGN. The galaxies' SFHs are consistent with an abrupt quenching of star formation, which continued at rates of $\sim15\,M_\odot$/yr averaged over 100 Myr timescales. Dedicated millimeter observations of NS_274 constrain its dust obscured SFR to $<12\,M_\odot$/yr. Under simple geometrical assumptions, we derive mass loading factors $\lesssim1$ and $>10$ for the z=4.1 and z=7.3 systems, respectively, and similarly different energies carried by the outflows. Supernova feedback can account for the mass and energy of the outflow in NS_274. However, the low mass loading factor and average gas velocity suggest that the observed outflow is likely not the primary factor behind its quenching. SF-related processes seem to be insufficient to explain the extreme mass outflow rate of RUBIES-UDS-QG-z7, which would require an additional ejective mechanism such as an undetected AGN. Finally, the average outflow velocities per unit $M_\star$, SFR, or its surface area are consistent with those of lower-redshift post-starburst galaxies, suggesting that outflows in rapidly quenched galaxies might occur similarly across cosmic time. [Abridged]

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arXiv2025

A large, chemically enriched, neutral gas reservoir in a galaxy at z = 6.782

The chemical characterization of galaxies in the first billion years after the Big Bang is one of the central goals of current astrophysics. Optical/near-infrared spectroscopy of long gamma-ray bursts (GRBs) have been heralded as an effective diagnostic to probe the interstellar medium of their host galaxies and their metal and dust content, up to the highest redshift. An opportunity to fulfill this expectation was provided by the recent blast triggered by the Neil Gehrels Swift Observatory of GRB 240218A at redshift z=6.782. We study a high-redshift galaxy selected in a complementary way with respect to flux-limited surveys, not depending on galaxy luminosity and stellar mass. We present the VLT/X-shooter spectrum of its afterglow enabling the detection of neutral-hydrogen, low-ionization, high-ionization and fine-structure absorption lines. We determine the metallicity, kinematics and chemical abundance pattern, providing the first detailed characterization of the neutral gas of a galaxy at z>6.5. From the analysis of fine-structure lines we estimate the distance of the closest gas clouds as $d_{II}=620^{+230}_{-140}$ pc. We determine a high neutral hydrogen column density, $\log(N(HI)/cm^{-2})=22.5\pm0.3$, which is the highest one at z>6 determined so far for a GRB host galaxy, as well as a surprisingly high metal column density, $\log(N(ZnII)/cm^{-2})>14.3$. The observed metallicity of the host galaxy system is [Zn/H]>-0.8. We find evidence of a high amount of dust depletion and of aluminum overabundance, although a number of transitions are saturated. The high hydrogen column density, metal abundances and dust depletion in the neutral gas align with those of the ionized gas of very high-redshift galaxies unveiled by ALMA and JWST, testifying that a rapid build up of metals and dust, and massive neutral hydrogen reservoirs seem to be common features of galaxies in the early Universe.

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arXiv2025

First IFU observations of two GRB host galaxies at cosmic noon with JWST/NIRSpec

Long gamma-ray bursts (GRBs) serve as powerful probes of distant galaxies. Their luminous afterglow pinpoints galaxies independent of luminosity, in contrast to most flux-limited surveys. Nevertheless, GRB-selected galaxy samples are not free from bias, instead tracing the conditions favoured by the progenitor stars. Characterising the galaxy populations traced by GRBs is therefore important both to effectively use GRBs as probes as well as to place stronger constraints on the progenitor stars capable of forming long GRBs. Spatially-resolved spectroscopic observations with integral field units (IFUs) provide valuable insights into the interstellar medium and stellar populations of GRB host galaxies. In this paper we present results of the first two GRB host galaxies observed with the JWST/NIRSpec IFU with a spatial resolution of ~ 1.6 kpc; the hosts of GRB 150403A and GRB 050820A at redshifts z ~ 2.06 and z ~ 2.61, respectively. The data reveal two complex galaxy environments made up of two or more star forming galaxies that are likely interacting given their small spatial separation (< 20 kpc) and line of sight velocity offsets (< 100 km/s). The measured gas-phase metallicity, star formation rates (SFRs), and key diagnostic line ratios for each of the detected galaxies are overall consistent with the properties of other star forming galaxies and GRB hosts at z > 2. However, differences in the SFR and metallicities of the interacting galaxies highlight the importance of spatially resolved observations in order to accurately characterise the galaxy properties traced by GRBs.

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arXiv2025

A merging pair of massive quiescent galaxies at $z=3.44$ in the Cosmic Vine

We report the spectroscopic confirmation of a merging pair of massive quiescent galaxies at $z=3.44$. Using JWST observations, we confirm that the two galaxies lie at a projected separation of 4.5 kpc with a velocity offset of $\sim 680\, {\rm km\, s^{-1}}\ (δ_z \sim 0.01)$. The pair resides in the core of a known rich overdensity of galaxies, dubbed the "Cosmic Vine". For both pair members, modeling of the Spectral Energy Distributions and faint rest-frame optical emission lines indicate high stellar masses ($\log{(M_\star/M_\odot)}\sim10.9$) and suppressed star formation ($\log{\rm (sSFR/yr^{-1})}<-10$), more than an order of magnitude below the level of the star formation main sequence at this redshift. We then explore the Illustris-TNG simulation and the GAEA and SHARK semi-analytical models to examine whether they produce a pair of massive quiescent galaxies akin to that of the Cosmic Vine. While all models produce close pairs of massive quiescent galaxies at $2 90\%$ of simulated pairs in the models that we analyzed merge by $z=0$, our findings suggest that our observed pair will likely coalesce into a single massive galaxy. The merger, occurring in the dense core of a large-scale structure, might represent a critical event in the formation of a brightest cluster galaxy and the morphological transformation of high-redshift disky quiescent galaxies into early-type ellipticals.

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arXiv2025

The brightest GRB ever detected: GRB 221009A as a highly luminous event at z = 0.151

Context: The extreme luminosity of gamma-ray bursts (GRBs) makes them powerful beacons for studies of the distant Universe. The most luminous bursts are typically detected at moderate/high redshift, where the volume for seeing such rare events is maximized and the star-formation activity is greater than at z = 0. For distant events, not all observations are feasible, such as at TeV energies. Aims: Here we present a spectroscopic redshift measurement for the exceptional GRB 221009A, the brightest GRB observed to date with emission extending well into the TeV regime. Methods: We used the X-shooter spectrograph at the ESO Very Large Telescope (VLT) to obtain simultaneous optical to near-IR spectroscopy of the burst afterglow 0.5 days after the explosion. Results: The spectra exhibit both absorption and emission lines from material in a host galaxy at z = 0.151. Thus GRB 221009A was a relatively nearby burst with a luminosity distance of 745 Mpc. Its host galaxy properties (star-formation rate and metallicity) are consistent with those of LGRB hosts at low redshift. This redshift measurement yields information on the energy of the burst. The inferred isotropic energy release, $E_{\rm iso} > 5 \times 10^{54}$ erg, lies at the high end of the distribution, making GRB 221009A one of the nearest and also most energetic GRBs observed to date. We estimate that such a combination (nearby as well as intrinsically bright) occurs between once every few decades to once per millennium.

astro-ph.HE↗

arXiv2024

Neutral Fraction of Hydrogen in the Intergalactic Medium Surrounding High-Redshift Gamma-Ray Burst 210905A

The Epoch of Reionization (EoR) is a key period of cosmological history in which the intergalactic medium (IGM) underwent a major phase change from being neutral to almost completely ionized. Gamma-ray bursts (GRBs) are luminous and unique probes of their environments that can be used to study the timeline for the progression of the EoR. Here we present a detailed analysis of the ESO Very Large Telescope X-shooter spectrum of GRB 210905A, which resides at a redshift of z ~ 6.3. We focus on estimating the fraction of neutral hydrogen, x_HI, on the line of sight to the host galaxy of GRB 210905A by fitting the shape of the Lyman-alpha damping wing of the afterglow spectrum. The X-shooter spectrum has a high signal to noise ratio, but the complex velocity structure of the host galaxy limits the precision of our conclusions. The statistically preferred model suggests a low neutral fraction with a 3-sigma upper limit of x_HI < 0.15 or x_HI < 0.23, depending on the absence or presence of an ionized bubble around the GRB host galaxy, indicating that the IGM around the GRB host galaxy is mostly ionized. We discuss complications in current analyses and potential avenues for future studies of the progression of the EoR and its evolution with redshift.

astro-ph.HE↗

arXiv2024

Modelling of long gamma-ray burst host galaxies at cosmic noon from damped Lyman-α absorption statistics

We study the properties of long gamma-ray burst (GRB) host galaxies using a statistical modelling framework derived to model damped Lyman-$α$ absorbers (DLAs) in quasar spectra at high redshift. The distribution of NHI for GRB-DLAs is $\sim$10 times higher than what is found for quasar-DLAs at similar impact parameters. We interpret this as a temporal selection effect due to the short-lived GRB progenitor probing its host at the onset of a starburst where the interstellar medium may exhibit multiple overdense regions. Owing to the larger NHI, the dust extinction is larger with 29 per cent of GRB-DLAs exhibiting A(V)>1 mag in agreement with the fraction of 'dark bursts'. Despite the differences in NHI distributions, we find that high-redshift 2 < z < 3 quasar- and GRB-DLAs trace the luminosity function of star-forming host galaxies in the same way. We propose that their differences may arise from the fact that the galaxies are sampled at different times in their star formation histories, and that the absorption sightlines probe the galaxy haloes differently. Quasar-DLAs sample the full H I cross-section, whereas GRB-DLAs sample only regions hosting cold neutral medium. Previous studies have found that GRBs avoid high-metallicity galaxies ($\sim$0.5 $Z_{\odot}$). Since at these redshifts galaxies on average have lower metallicities, our sample is only weakly sensitive to such a threshold. Lastly, we find that the modest detection rate of cold gas (H$_2$ or C I) in GRB spectra can be explained mainly by a low volume filling factor of cold gas clouds and to a lesser degree by destruction from the GRB explosion itself.

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