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Proximity zones of high-redshift quasars are unique probes of supermassive black hole formation, but simultaneously explaining proximity zone sizes and black hole masses has proved to be challenging. We study the robustness of some of the assumptions that are usually made to infer quasar lifetimes from proximity zone sizes. We show that small proximity zones can be readily explained by quasars that vary in brightness with a short duty cycle of $f_\mathrm{duty}\sim 0.1$ and short bright periods of $t_\mathrm{on}\sim 10^4$ yr, even for long lifetimes. We further show that reconciling this with black hole mass estimates requires the black hole to continue to grow and accrete during its obscured phase. The consequent obscured fractions of $\gtrsim$ 0.7 or higher are consistent with low-redshift measurements and models of black hole accretion. Such short duty cycles and long obscured phases are also consistent with observations of large proximity zones, thus providing a simple, unified model for proximity zones of all sizes. The large dynamic range of our simulation, and its calibration to the Lyman-$\alpha$ forest, allows us to investigate the influence of the large-scale topology of reionization and the quasar's host halo mass on proximity zones. We find that incomplete reionization can impede the growth of proximity zones and make them smaller up to 30%, but the quasar host halo mass only affects proximity zones weakly and indirectly. Our work suggests that high-redshift proximity zones can be an effective tool to study quasar variability and black hole growth., Comment: 19 pages, 17 figures, Accepted for publication in MNRAS

Inhomogeneous reionization enhances the 1D Ly α forest power spectrum on large scales at redshifts z ≥ 4. This is due to coherent fluctuations in the ionized hydrogen fraction that arise from large-scale variations in the post-reionization gas temperature, which fade as the gas cools. It is therefore possible to use these relic fluctuations to constrain inhomogeneous reionization with the power spectrum at wavenumbers $\log _{10}(k/{\rm km^{-1}\, s})\lesssim -1.5$. We use the Sherwood-Relics suite of hybrid radiation hydrodynamical simulations to perform a first analysis of new Ly α forest power spectrum measurements at 4.0 ≤ z ≤ 4.6. These data extend to wavenumbers $\log _{10}(k/{\rm km^{-1}\, s})\simeq -3$, with a relative uncertainty of 10–20 per cent in each wavenumber bin. Our analysis returns a 2.7σ preference for an enhancement in the Ly α forest power spectrum at large scales, in excess of that expected for a spatially uniform ultraviolet background. This large-scale enhancement could be a signature of inhomogeneous reionization, although the statistical precision of these data is not yet sufficient for obtaining a robust detection of the relic post-reionization fluctuations. We show that future power spectrum measurements with relative uncertainties of ≲ 2.5 per cent should provide unambiguous evidence for an enhancement in the power spectrum on large scales.

We present the Sherwood-Relics simulations, a new suite of large cosmological hydrodynamical simulations aimed at modelling the intergalactic medium (IGM) during and after the cosmic reionization of hydrogen. The suite consists of over 200 simulations that cover a wide range of astrophysical and cosmological parameters. It also includes simulations that use a new lightweight hybrid scheme for treating radiative transfer effects. This scheme follows the spatial variations in the ionizing radiation field, as well as the associated fluctuations in IGM temperature and pressure smoothing. It is computationally much cheaper than full radiation hydrodynamics simulations and circumvents the difficult task of calibrating a galaxy formation model to observational constraints on cosmic reionization. Using this hybrid technique, we study the spatial fluctuations in IGM properties that are seeded by patchy cosmic reionization. We investigate the relevant physical processes and assess their impact on the z > 4 Lyman-alpha forest. Our main findings are: (i) Consistent with previous studies patchy reionization causes large scale temperature fluctuations that persist well after the end of reionization, (ii) these increase the Lyman-alpha forest flux power spectrum on large scales, and (iii) result in a spatially varying pressure smoothing that correlates well with the local reionization redshift. (iv) Structures evaporated or puffed up by photoheating cause notable features in the Lyman-alpha forest, such as flat-bottom or double-dip absorption profiles., Version accepted for publication in MNRAS, some discussions added, data added to Fig. 8. 24 pages, 15 figures. Images and movies may be viewed at https://www.nottingham.ac.uk/astronomy/sherwood-relics/

The detection of Ly$\alpha$ nebulae around $z\gtrsim 6$ quasars provides evidence for extended gas reservoirs around the first rapidly growing supermassive black holes. Observations of $z > 6$ quasars can be explained by cosmological models provided that the black holes by which they are powered evolve in rare, massive dark matter haloes. Whether these theoretical models also explain the observed extended Ly$\alpha$ emission remains an open question. We post-process a suite of cosmological, radiation-hydrodynamic simulations targeting a quasar host halo at $z>6$ with the Ly$\alpha$ radiative transfer code RASCAS. A combination of recombination radiation from photo-ionised hydrogen and emission from collisionally excited gas powers Ly$\alpha$ nebulae with a surface brightness profile in close agreement with observations. We also find that, even on its own, resonant scattering of the Ly$\alpha$ line associated to the quasar's broad line region can also generate Ly$\alpha$ emission on $\sim 100 \, \rm kpc$ scales, resulting in comparable agreement with observed surface brightness profiles. Even if powered by a broad quasar Ly$\alpha$ line, Ly$\alpha$ nebulae can have narrow line-widths $\lesssim 1000 \, \rm km \, s^{-1}$, consistent with observational constraints. Even if there is no quasar, we find that halo gas cooling produces a faint, extended Ly$\alpha$ glow. However, to light-up extended Ly$\alpha$ nebulae with properties in line with observations, our simulations unambiguously require quasar-powered outflows to clear out the galactic nucleus and allow the Ly$\alpha$ flux to escape and still remain resonant with halo gas. The close match between observations and simulations with quasar outflows suggests that AGN feedback already operates before $z \, = \, 6$ and confirms that high-$z$ quasars reside in massive haloes tracing overdensities., Comment: Submitted to MNRAS (21+3 pages, 11+3 figures)

Proximity zones of high-redshift quasars are unique probes of their central supermassive black holes as well as the intergalactic medium in the last stages of reionization. We present 22 new measurements of proximity zones of quasars with redshifts between 5.8 and 6.6, using the enlarged XQR-30 sample of high-resolution, high-SNR quasar spectra. The quasars in our sample have UV magnitudes of $M_{1450}\sim -27$ and black hole masses of $10^9$$\unicode{x2013}$$10^{10}$ M$_\odot$. Our inferred proximity zone sizes are 2$\unicode{x2013}$7 physical Mpc, with a typical uncertainty of less than 0.5 physical Mpc, which, for the first time, also includes uncertainty in the quasar continuum. We find that the correlation between proximity zone sizes and the quasar redshift, luminosity, or black hole mass, indicates a large diversity of quasar lifetimes. Two of our proximity zone sizes are exceptionally small. The spectrum of one of these quasars, with $z=6.02$, displays, unusually for this redshift, damping wing absorption without any detectable metal lines, which could potentially originate from the IGM. The other quasar has a high-ionization absorber $\sim$0.5 pMpc from the edge of the proximity zone. This work increases the number of proximity zone measurements available in the last stages of cosmic reionization to 87. This data will lead to better constraints on quasar lifetimes and obscuration fractions at high redshift, which in turn will help probe the seed mass and formation redshift of supermassive black holes., Comment: 16 pages, 9 figures, Accepted in MNRAS

We use the Sherwood-Relics suite of hybrid hydrodynamical and radiative transfer simulations to model the effect of inhomogeneous reionisation on the 1D power spectrum of the \Lya forest transmitted flux at redshifts $4.2\leq z \leq 5$. Relative to models that assume a homogeneous UV background, reionisation suppresses the power spectrum at small scales, $k \sim 0.1\rm\,km^{-1}\,s$ by $\sim 10$ per cent because of spatial variations in the thermal broadening kernel and the divergent peculiar velocity field associated with over-pressurised intergalactic gas. On larger scales, $k4$, $0.006 \rm \,km^{-1}\,s\leq k \leq 0.2 \rm\, km^{-1}\,s$, we find inhomogeneous reionisation does not introduce any significant bias in thermal parameter recovery for the current measurement uncertainties of $\sim 10$ per cent. However, for $5$ per cent uncertainties, $\sim 1\sigma$ shifts between the estimated and true parameters occur., Comment: Accepted by MNRAS (21 November 2021), 20 pages, 11 figures

Enhanced ionizing radiation in close proximity to redshift $z\gtrsim 6$ quasars creates short windows of intergalactic Ly$\alpha$ transmission blueward of the quasar Ly$\alpha$ emission lines. The majority of these Ly$\alpha$ near-zones are consistent with quasars that have optically/UV bright lifetimes of $t_{\rm Q}\sim 10^{5}-10^{7}\rm\,yr$. However, lifetimes as short as $t_{\rm Q}\lesssim 10^{4}\rm\,yr$ appear to be required by the smallest Ly$\alpha$ near-zones. These short lifetimes present an apparent challenge for the growth of $\sim 10^{9}\rm\,M_{\odot}$ black holes at $z\gtrsim 6$. Accretion over longer timescales is only possible if black holes grow primarily in an obscured phase, or if the quasars are variable on timescales comparable to the equilibriation time for ionized hydrogen. Distinguishing between very young quasars and older quasars that have experienced episodic accretion with Ly$\alpha$ absorption alone is challenging, however. We therefore predict the signature of proximate 21-cm absorption around $z\gtrsim 6$ radio-loud quasars. For modest pre-heating of intergalactic hydrogen by the X-ray background, where the spin temperature $T_{\rm S} \lesssim 10^{2}\rm\,K$ prior to any quasar heating, we find proximate 21-cm absorption should be observable in the spectra of radio-loud quasars. The extent of the proximate 21-cm absorption is sensitive to the integrated lifetime of the quasar. Evidence for proximate 21-cm absorption from the diffuse intergalactic medium within $2-3\rm\,pMpc$ of a (radio-loud) quasar would be consistent with a short quasar lifetime, $t_{\rm Q}\lesssim 10^{5}\rm\,yr$, and would provide a complementary constraint on models for high redshift black hole growth., Comment: 19 pages, 12 figures, 2 tables. Accepted for publication in MNRAS

We present the current state of models for the $z\sim3$ carbon monoxide (CO) line-intensity signal targeted by the CO Mapping Array Project (COMAP) Pathfinder in the context of its early science results. Our fiducial model, relating dark matter halo properties to CO luminosities, informs parameter priors with empirical models of the galaxy-halo connection and previous CO(1-0) observations. The Pathfinder early science data spanning wavenumbers $k=0.051$-$0.62\,$Mpc$^{-1}$ represent the first direct 3D constraint on the clustering component of the CO(1-0) power spectrum. Our 95% upper limit on the redshift-space clustering amplitude $A_{\rm clust}\lesssim70\,\mu$K$^2$ greatly improves on the indirect upper limit of $420\,\mu$K$^2$ reported from the CO Power Spectrum Survey (COPSS) measurement at $k\sim1\,$Mpc$^{-1}$. The COMAP limit excludes a subset of models from previous literature, and constrains interpretation of the COPSS results, demonstrating the complementary nature of COMAP and interferometric CO surveys. Using line bias expectations from our priors, we also constrain the squared mean line intensity-bias product, $\langle{Tb}\rangle^2\lesssim50\,\mu$K$^2$, and the cosmic molecular gas density, $\rho_\text{H2}, Comment: Paper 5 of 7 in series. 17 pages + appendix and bibliography (30 pages total); 15 figures, 6 tables; accepted for publication in ApJ; v3 reflects the accepted version with minor changes and additions to text

Traditional large-scale models of reionization usually employ simple deterministic relations between halo mass and luminosity to predict how reionization proceeds. We here examine the impact on modelling reionization of using more detailed models for the ionizing sources as identified within the $100~{\rm Mpc/h}$ cosmological hydrodynamic simulation {\sc Simba}, coupled with post-processed radiative transfer. Comparing with simple (one-to-one) models, the main difference of using {\sc Simba} sources is the scatter in the relation between dark matter halos and star formation, and hence ionizing emissivity. We find that, at the power spectrum level, the ionization morphology remains mostly unchanged, regardless of the variability in the number of sources or escape fraction. In particular, the power spectrum shape remains unaffected and its amplitude changes slightly by less than 5-10\%, throughout reionization, depending on the scale and neutral fraction. Our results show that simplified models of ionizing sources remain viable to efficiently model the structure of reionization on cosmological scales, although the precise progress of reionization requires accounting for the scatter induced by astrophysical effects., 13 pages, 6 figures, accepted to ApJ

We present models of CO(1-0) emission from Milky Way-mass galaxies at redshift zero in the FIRE-2 cosmological zoom-in simulations. We calculate the molecular abundances by post-processing the simulations with an equilibrium chemistry solver while accounting for the effects of local sources, and determine the emergent CO(1-0) emission using a line radiative transfer code. We find that the results depend strongly on the shielding length assumed, which in our models sets the attenuation of the incident UV radiation field. At the resolution of these simulations, commonly used choices for the shielding length, such as the Jeans length, result in CO abundances that are too high at a given H$_2$ abundance. We find that a model with a distribution of shielding lengths, which has a median shielding length of $\sim 3$ pc in cold gas ($T < 300$ K) for both CO and H$_{2}$, is able to reproduce both the observed CO(1-0) luminosity and inferred CO-to-H$_{2}$ conversion factor at a given star formation rate compared with observations. We suggest that this short shielding length can be thought of as a subgrid model which controls the amount of radiation that penetrates giant molecular clouds., Accepted to MNRAS. 16 pages, 10 figures

We measure escape fractions, $f_{\rm esc}$, of ionizing radiation from galaxies in the SPHINX suite of cosmological radiation-hydrodynamical simulations of reionization, resolving halos with $M_{\rm vir} \gtrapprox 7.5 \times 10^7 \ M_{\odot}$ with a minimum cell width of $\approx 10$ pc. Our new and largest $20$ co-moving Mpc wide volume contains tens of thousands of star-forming galaxies with halo masses up to a few times $10^{11} \ M_{\odot}$. The simulated galaxies agree well with observational constraints of the UV luminosity function in the Epoch of Reionization. The escape fraction fluctuates strongly in individual galaxies over timescales of a few Myrs, due to its regulation by supernova and radiation feedback, and at any given time a tiny fraction of star-forming galaxies emits a large fraction of the ionizing radiation escaping into the inter-galactic medium. Statistically, $f_{\rm esc}$ peaks in intermediate-mass, intermediate-brightness, and low-metallicity galaxies ($M_{*} \approx 10^7 \ M_{\odot}$, $M_{1500} \approx -17$, $Z\lesssim 5 \times 10^{-3} \ Z_{\odot}$), dropping strongly for lower and higher masses, brighter and dimmer galaxies, and more metal-rich galaxies. The escape fraction correlates positively with both the short-term and long-term specific star formation rate. According to SPHINX, galaxies too dim to be yet observed, with $M_{1500} \gtrapprox -17$, provide about $55$ percent of the photons contributing to reionization. The global averaged $f_{\rm esc}$ naturally decreases with decreasing redshift, as predicted by UV background models and low-redshift observations. This evolution is driven by decreasing specific star formation rates over cosmic time., Comment: 20 figures, 21 pages plus appendices. Accepted for publication in MNRAS

A cosmological zoom-in simulation which develops into a Milky Way-like halo is started at redshift 7. The initial dark matter distribution is seeded with dense star clusters, median mass $5\times 10^5 M_\sun$, placed in the largest sub-halos present, which have a median peak circular velocity of 25 \kms. Three simulations are initialized using the same dark matter distribution, with the star clusters started on approximately circular orbits having initial median radii 6.8 kpc, 0.14 kpc, and, at the exact center of the sub-halos. The simulations are evolved to the current epoch at which time the median galactic orbital radii of the three sets of clusters are 30, 5 and 16 kpc, with the clusters losing about 2, 50 and 15\% of their mass, respectively. Clusters started at small orbital radii have so much tidal forcing that they are often not in equilibrium. Clusters started at larger sub-halo radii have a velocity dispersion that declines smoothly to $\simeq$20\% of the central value at $\simeq$20 half mass radii. The clusters started at the sub-halo centers can show a rise in velocity dispersion beyond 3-5 half mass radii. That is, the clusters formed without local dark matter always have stellar mass dominated kinematics at all radii, whereas about 25\% of the clusters started at sub-halo centers have remnant local dark matter., Comment: ApJ revised submission

We present a new investigation of the intergalactic medium (IGM) near reionization using dark gaps in the Lyman-$\beta$ (Ly$\beta$) forest. With its lower optical depth, Ly$\beta$ offers a potentially more sensitive probe to any remaining neutral gas compared to commonly used Ly$\alpha$ line. We identify dark gaps in the Ly$\beta$ forest using spectra of 42 QSOs at $z_{\rm em}>5.5$, including new data from the XQR-30 VLT Large Programme. Approximately $40\%$ of these QSO spectra exhibit dark gaps longer than $10h^{-1}{\rm Mpc}$ at $z\simeq5.8$. By comparing the results to predictions from simulations, we find that the data are broadly consistent both with models where fluctuations in the Ly$\alpha$ forest are caused solely by ionizing ultraviolet background (UVB) fluctuations and with models that include large neutral hydrogen patches at $z, Comment: 17 pages, 13 figures, accepted for publication in ApJ. The Figure set and machine readable table are available at https://ydzhuastro.github.io/lyb.html

The presence of excess scatter in the Ly-$\alpha$ forest at $z\sim 5.5$, together with the existence of sporadic extended opaque Gunn-Peterson troughs, has started to provide robust evidence for a late end of hydrogen reionisation. However, low data quality and systematic uncertainties complicate the use of Ly-$\alpha$ transmission as a precision probe of reionisation's end stages. In this paper, we assemble a sample of 67 quasar sightlines at $z>5.5$ with high signal-to-noise ratios of $>10$ per $\leq 15$ km s$^{-1}$ spectral pixel, relying largely on the new XQR-30 quasar sample. XQR-30 is a large program on VLT/X-Shooter which obtained deep (SNR $>20$ per pixel) spectra of 30 quasars at $z>5.7$. We carefully account for systematics in continuum reconstruction, instrumentation, and contamination by damped Ly-$\alpha$ systems. We present improved measurements of the mean Ly-$\alpha$ transmission over $4.93.5 \sigma$). Our results indicate that reionisation-related fluctuations, whether in the UVB, residual neutral hydrogen fraction, and/or IGM temperature, persist in the intergalactic medium until at least $z=5.3$ ($t=1.1$ Gyr after the Big Bang). This is further evidence for a late end to reionisation., Comment: Accepted for publication in MNRAS. Full measurement datasets are available on the first author's website

The CO Mapping Array Project (COMAP) aims to use line intensity mapping of carbon monoxide (CO) to trace the distribution and global properties of galaxies over cosmic time, back to the Epoch of Reionization (EoR). To validate the technologies and techniques needed for this goal, a Pathfinder instrument has been constructed and fielded. Sensitive to CO(1-0) emission from $z=2.4$-$3.4$ and a fainter contribution from CO(2-1) at $z=6$-8, the Pathfinder is surveying $12$ deg$^2$ in a 5-year observing campaign to detect the CO signal from $z\sim3$. Using data from the first 13 months of observing, we estimate $P_\mathrm{CO}(k) = -2.7 \pm 1.7 \times 10^4\mu\mathrm{K}^2 \mathrm{Mpc}^3$ on scales $k=0.051-0.62 \mathrm{Mpc}^{-1}$ - the first direct 3D constraint on the clustering component of the CO(1-0) power spectrum. Based on these observations alone, we obtain a constraint on the amplitude of the clustering component (the squared mean CO line temperature-bias product) of $\langle Tb\rangle^2, Comment: Paper 1 of 7 in series. 18 pages, 16 figures, submitted to ApJ

The physical and chemical properties of the circumgalactic medium (CGM) at $z\gtrsim6$ have been studied successfully through the absorption in the spectra of background Quasi-Stellar Objects (QSOs). One of the most crucial questions is to investigate the nature and location of the source galaxies that give rise to these early metal absorbers. Theoretical models suggest that momentum-driven outflows from typical star-forming galaxies can eject metals into the CGM and the intergalactic medium (IGM) at z=5-6. Deep, dedicated surveys have searched for Ly$\alpha$ emission associated with strong CIV absorbers at $z\approx 6$, but only a few Ly$\alpha$ emitter candidates have been detected. Interpreting these detections is moreover ambiguous because Ly$\alpha$ is a resonant line, raising the need for complementary techniques for detecting absorbers' host galaxies. Here, using Atacama Large Millimeter Array (ALMA), we report a [C II] 158$\mu$m emitter associated with a strong low-ionization absorber, OI, at $z=5.978$. The projected impact parameter between OI and [C II] emitter is 20.0 kpc. The measured [C II] luminosity is $7.0\times 10^7$ solar luminosities. Further analysis indicates that strong OI absorbers may reside in the circumgalactic medium of massive halos one to two orders of magnitude more massive than expected values., Comment: Published in Nature Astronomy on Sep 27, 2021; 45 pages; authors' version. Link to the paper: https://www.nature.com/articles/s41550-021-01471-4

Our understanding of the intergalactic medium at redshifts $z=5$-$6$ has improved considerably in the last few years due to the discovery of quasars with $z>6$ that enable Lyman-$\alpha$ forest studies at these redshifts. A realisation from this has been that hydrogen reionization could end much later than previously thought, so that large "islands" of cold, neutral hydrogen could exist in the IGM at redshifts $z=5$-$6$. By using radiative transfer simulations of the IGM, we consider the implications of the presence of these neutral hydrogen islands for the 21-cm power spectrum signal and its potential detection by experiments such as HERA, SKA, LOFAR, and MWA. In contrast with previous models of the 21-cm signal, we find that thanks to the late end of reionization the 21-cm power in our simulation continues to be as high as $\Delta^2_{21}=10~\mathrm{mK}^2$ at $k\sim 0.1~h/$cMpc at $z=5$-$6$. This value of the power spectrum is several orders of magnitude higher than that in the conventional models considered in the literature for these redshifts. Such high values of the 21-cm power spectrum should be detectable by HERA and SKA1-LOW in $\sim 1000$ hours, assuming optimistic foreground subtraction. This redshift range is also attractive due to relatively low sky temperature and potentially greater abundance of multiwavelength data., Comment: 14 pages, 6 figures, 2 tables. Accepted for publication in MNRAS

Line-intensity mapping observations will find fluctuations of integrated line emission are attenuated by varying degrees at small scales due to the width of the line emission profiles. This attenuation may significantly impact estimates of astrophysical or cosmological quantities derived from measurements. We consider a theoretical treatment of the effect of line broadening on both the clustering and shot-noise components of the power spectrum of a generic line-intensity power spectrum using a halo model. We then consider possible simplifications to allow easier application in analysis, particularly in the context of inferences that require numerous, repeated, fast computations of model line-intensity signals across a large parameter space. For the CO Mapping Array Project (COMAP) and the CO(1-0) line-intensity field at $z\sim3$ serving as our primary case study, we expect a $\sim10\%$ attenuation of the spherically averaged power spectrum on average at relevant scales of $k\approx0.2$-$0.3$ Mpc$^{-1}$, compared to $\sim25\%$ for the interferometric Millimetre-wave Intensity Mapping Experiment (mmIME) targeting shot noise from CO lines at $z\sim1$-$5$ at scales of $k\gtrsim1$ Mpc$^{-1}$. We also consider the nature and amplitude of errors introduced by simplified treatments of line broadening, and find that while an approximation using a single effective velocity scale is sufficient for spherically-averaged power spectra, a more careful treatment is necessary when considering other statistics such as higher multipoles of the anisotropic power spectrum or the voxel intensity distribution., 24 pages + appendix and bibliography (33 pages total), 16 figures, 2 tables; accepted for publication in ApJ

We present a detailed overview of the science goals and predictions for the Prime-Cam direct detection camera/spectrometer being constructed by the CCAT-prime collaboration for dedicated use on the Fred Young Submillimeter Telescope (FYST). The FYST is a wide-field, 6-m aperture submillimeter telescope being built (first light in mid-2024) by an international consortium of institutions led by Cornell University and sited at more than 5600 meters on Cerro Chajnantor in northern Chile. Prime-Cam is one of two instruments planned for FYST and will provide unprecedented spectroscopic and broadband measurement capabilities to address important astrophysical questions ranging from Big Bang cosmology through reionization and the formation of the first galaxies to star formation within our own Milky Way galaxy. Prime-Cam on the FYST will have a mapping speed that is over ten times greater than existing and near-term facilities for high-redshift science and broadband polarimetric imaging at frequencies above 300 GHz. We describe details of the science program enabled by this system and our preliminary survey strategies., Comment: 61 pages, 16 figures. Resubmitted to ApJSS July 11, 2022

A late end to reionisation at redshift $z\simeq 5.3$ is consistent with observed spatial variations in the Ly$\alpha$ forest transmission and the deficit of Ly$\alpha$ emitting galaxies around extended Ly$\alpha$ absorption troughs at $z=5.5$. In this model, large islands of neutral hydrogen should persist in the diffuse intergalactic medium (IGM) until $z\simeq 6$. We use a novel, hybrid approach that combines high resolution cosmological hydrodynamical simulations with radiative transfer to predict the incidence of strong 21 cm forest absorbers with optical depths $\tau_{21}>10^{-2}$ from the diffuse IGM in these late reionisation models. We include the effect of redshift space distortions on the simulated 21 cm forest spectra, and treat the highly uncertain heating of the pre-reionisation IGM by soft X-rays as a free parameter. For a model with only modest IGM pre-heating, such that average gas kinetic temperatures in the diffuse IGM remain below $T_{\rm K}\simeq 10^{2} \rm\, K$, we find that strong 21 cm forest absorption lines should persist until $z=6$. For a sample of $\sim 10$ sufficiently radio loud background sources, a null-detection of 21 cm forest absorbers at $z\simeq 6$ with SKA1-low or possibly LOFAR should provide an informative lower limit on the still largely unconstrained soft X-ray background at high redshift and the temperature of the pre-reionisation IGM., Comment: 18 pages, 11 figures, 4 tables. Accepted for publication in MNRAS

The physical and chemical properties of the circumgalactic medium (CGM) at z ≳ 6 have been studied successfully through the absorption in the spectra of background Quasi-Stellar Objects (QSOs) 1–3. One of the most crucial questions is to investigate the nature and location of the source galaxies that give rise to these early metal absorbers4–6. Theoretical models suggest that momentum-driven outflows from typical star-forming galaxies can eject metals into the CGM and the intergalactic medium (IGM), with the projected separation between absorbers and galaxies expected to range from 5 to 50 proper kpc at z=5–6 7–10. Deep, dedicated surveys have searched for Lyα emission associated with strong CIV absorbers at z ≈ 6, but only a few Lyα emitter candidates have been detected. Interpreting these detections is moreover ambiguous because Lyα is a resonant line11–13, raising the need for complementary techniques for detecting absorbers’ host galaxies. Here, using Atacama Large Millimeter Array (ALMA), we report, for the first time, the [C II] 158µm emission and the far-infrared dust continuum associated with a strong low-ionization absorber, O I, at z = 5.978. The [C II] luminosity is 73 million solar luminosities, corresponding to a dark matter halo mass of 100 billion solar masses. This is one to two orders of magnitude more massive than typical values predicted from cosmological simulations9, 16.

We compare a sample of five high-resolution, high S/N Ly$\alpha$ forest spectra of bright $65$ with those obtained from mock absorption spectra from the Sherwood and Sherwood-Relics suites of hydrodynamical simulations of the intergalactic medium (IGM). We use a profile fitting procedure for the inverted transmitted flux, $1-F$, similar to the widely used Voigt profile fitting of the transmitted flux $F$ at lower redshifts, to characterise the transmission spikes that probe predominately underdense regions of the IGM. We are able to reproduce the width and height distributions of the transmission spikes, both with optically thin simulations of the post-reionization Universe using a homogeneous UV background and full radiative transfer simulations of a late reionization model. We find that the width of the fitted components of the simulated transmission spikes is very sensitive to the instantaneous temperature of the reionized IGM. The internal structures of the spikes are more prominant in low temeperature models of the IGM. The width distribution of the observed transmission spikes, which require high spectral resolution ($\leq $ 8 km/s) to be resolved, is reproduced for optically thin simulations with a temperature at mean density of $T_0= (11000 \pm 1600,10500\pm 2100,12000 \pm 2200)$ K at $z= (5.4,5.6,5.8)$. This is weakly dependent on the slope of the temperature-density relation, which is favoured to be moderately steeper than isothermal. In the inhomogeneous, late reionization, full radiative transfer simulations where islands of neutral hydrogen persist to $z\sim5.3$, the width distribution of the observed transmission spikes is consistent with the range of $T_0$ caused by spatial fluctuations in the temperature-density relation., Comment: 20 pages (+10 pages appendices), 12 figures (+14 figures appendices); Accepted in MNRAS; Main results are summarized in Fig. 10, Fig. 12 and Table 3

Fast radio bursts offer the opportunity to place new constraints on the mass and density profile of hot and ionized gas in galactic haloes. We test here the X-ray emission and dispersion measure predicted by different gas profiles for the halo of the Milky Way. We examine a range of models, including entropy stability conditions and external pressure continuity. We find that incorporating constraints from X-ray observations leads to favouring dispersion measures on the lower end of the range given by these models. We show that the dispersion measure of the Milky Way halo could be less than 10 cm$^{-3}$ pc in the most extreme model we consider, which is based on constraints from O VII absorption lines. However, the models allowed by the soft X-ray constraints span more than an order of magnitude in dispersion measures. Additional information on the distribution of gas in the Milky Way halo could be obtained from the signature of a dipole in the dispersion measure of fast radio bursts across the sky, but this will be a small effect for most cases., 5 pages, 4 figures. Accepted to MNRAS Letters

We present an analysis of the evolution of the Lyman-series forest into the epoch of reionization using cosmological radiative transfer simulations in a scenario where reionization ends late. We explore models with different midpoints of reionization and gas temperatures. We find that once the simulations have been calibrated to match the mean flux of the observed Lyman-$\alpha$ forest at $4 < z < 6$, they also naturally reproduce the distribution of effective optical depths of the Lyman-$\beta$ forest in this redshift range. We note that the tail of the largest optical depths that is most challenging to match corresponds to the long absorption trough of ULAS J0148+0600, which we have previously shown to be rare in our simulations. We consider the evolution of the Lyman-series forest out to higher redshifts, and show that future observations of the Lyman-$\beta$ forest at $z>6$ will discriminate between different reionization histories. The evolution of the Lyman-$\alpha$ and Lyman-$\gamma$ forests are less promising as a tool for pushing studies of reionization to higher redshifts due to the stronger saturation and foreground contamination, respectively., Comment: 10 pages, 9 figures, 1 table. Accepted to MNRAS

The thermal state of the post-reionization IGM is sensitive to the timing of reionization and the nature of the ionizing sources. We have modelled here the thermal state of the IGM in cosmological radiative transfer simulations of a realistic, extended, spatially inhomogeneous hydrogen reionization process, carefully calibrated with Ly-alpha forest data. We compare these with cosmological simulations run using a spatially homogeneous ionizing background. The simulations with a realistic growth of ionized regions and a realistic spread in reionization redshifts show, as expected, significant spatial fluctuations in the temperature-density relation (TDR) of the post-reionization IGM. The most recently ionized regions are hottest and exhibit a flatter TDR. In simulations consistent with the average TDR inferred from Ly-alpha forest data, these spatial fluctuations have a moderate but noticeable effect on the statistical properties of the Ly-alpha opacity of the IGM at z ~ 4-6. This should be taken into account in accurate measurements of the thermal properties of the IGM and the free-streaming of dark matter from Ly-alpha forest data in this redshift range. The spatial variations of the TDR predicted by our simulations are, however, smaller by about a factor two than would be necessary to explain the observed large spatial opacity fluctuations on large (> 50 comoving Mpc/h) scales at z > 5.5., 17 pages, 11 figures. Accepted to MNRAS

We present a new investigation of the intergalactic medium (IGM) near the end of reionization using “dark gaps” in the Lyα forest. Using spectra of 55 QSOs at z em > 5.5, including new data from the XQR-30 VLT Large Programme, we identify gaps in the Lyα forest where the transmission averaged over 1 comoving h −1 Mpc bins falls below 5%. Nine ultralong (L > 80 h −1 Mpc) dark gaps are identified at z < 6. In addition, we quantify the fraction of QSO spectra exhibiting gaps longer than 30 h −1 Mpc, F 30, as a function of redshift. We measure F 30 ≃ 0.9, 0.6, and 0.15 at z = 6.0, 5.8, and 5.6, respectively, with the last of these long dark gaps persisting down to z ≃5.3. Comparing our results with predictions from hydrodynamical simulations, we find that the data are consistent with models wherein reionization extends significantly below redshift six. Models wherein the IGM is essentially fully reionized that retain large-scale fluctuations in the ionizing UV background at z ≲6 are also potentially consistent with the data. Overall, our results suggest that signatures of reionization in the form of islands of neutral hydrogen and/or large-scale fluctuations in the ionizing background remain present in the IGM until at least z ≃ 5.3.

A long (110 cMpc/$h$) and deep absorption trough in the Ly$\alpha$ forest has been observed extending down to redshift 5.5 in the spectrum of ULAS J0148+0600. Although no Ly$\alpha$ transmission is detected, Ly$\beta$ spikes are present which has led to claims that the gas along this trough must be ionized. Using high resolution cosmological radiative transfer simulations in large volumes, we show that in a scenario where reionization ends late ($z \sim 5.2$), our simulations can reproduce troughs as long as observed. In this model, we find that the troughs are caused by islands of neutral hydrogen. Small ionized holes within the neutral islands allow for the transmission of Ly$\beta$. We have also modelled the Ly$\alpha$ emitter population around the simulated troughs, and show that there is a deficit of Ly$\alpha$ emitters close to the trough as is observed., Comment: 11 pages, 8 figures. Accepted for publication in MNRAS

High-redshift QSO spectra show large spatial fluctuations in the Ly-alpha opacity of the intergalactic medium on surprisingly large scales at z>~5.5. We present a radiative transfer simulation of cosmic reionization driven by galaxies that reproduces this large scatter and the rapid evolution of the Ly-alpha opacity distribution at 5~6. Reionization is complete at z=5.3 in our model, and 50% of the volume of the Universe is ionized at z=7. Agreement with the Ly-alpha forest data in such a late reionization model requires a rapid evolution of the ionizing emissivity of galaxies that peaks at z~6.8. The late end of reionization results in a large scatter in the photoionisation rate and the neutral hydrogen fraction at redshifts as low as z, Comment: 5 pages, 3 figures; accepted for publication in MNRAS

We present the detailed science case, and brief descriptions of the telescope design, site, and first light instrument plans for a new ultra-wide field submillimeter observatory, CCAT-prime, that we are constructing at a 5600 m elevation site on Cerro Chajnantor in northern Chile. Our science goals are to study star and galaxy formation from the epoch of reionization to the present, investigate the growth of structure in the Universe, improve the precision of B-mode CMB measurements, and investigate the interstellar medium and star formation in the Galaxy and nearby galaxies through spectroscopic, polarimetric, and broadband surveys at wavelengths from 200 m to 2 mm. These goals are realized with our two first light instruments, a large field-of-view (FoV) bolometer-based imager called Prime-Cam (that has both camera and an imaging spectrometer modules), and a multi-beam submillimeter heterodyne spectrometer, CHAI. CCAT-prime will have very high surface accuracy and very low system emissivity, so that combined with its wide FoV at the unsurpassed CCAT site our telescope/instrumentation combination is ideally suited to pursue this science. The CCAT-prime telescope is being designed and built by Vertex Antennentechnik GmbH. We expect to achieve first light in the spring of 2021., Ground-based and Airborne Telescopes VII, June 10-15, 2018, Austin, USA, Series: Proceedings of SPIE; no. 10700

We present the Technicolor Dawn simulations, a suite of cosmological radiation-hydrodynamic simulations of the first 1.2 billion years. By modeling a spatially-inhomogeneous UVB on-the-fly with 24 frequencies and resolving dark matter halos down to $10^8 M_\odot$ within 12 $h^{-1}$ Mpc volumes, our simulations unify observations of the intergalactic and circumgalactic media, galaxies, and reionization into a common framework. The only empirically-tuned parameter, the fraction $f_{\mathrm{esc,gal}}(z)$ of ionizing photons that escape the interstellar medium, is adjusted to match observations of the Lyman-$\alpha$ forest and the cosmic microwave background. With this single calibration, our simulations reproduce the history of reionization; the stellar mass-star formation rate relation of galaxies; the number density and metallicity of damped Lyman-$\alpha$ absorbers (DLAs) at $z\sim5$; the abundance of weak metal absorbers; the ultraviolet background (UVB) amplitude; and the Lyman-$\alpha$ flux power spectrum at $z=5.4$. The galaxy stellar mass and UV luminosity functions are underproduced by $\leq2\times$, suggesting an overly vigorous feedback model. The mean transmission in the Lyman-$\alpha$ forest is underproduced at $z4$ Ryd UVB. Solving this problem by increasing metal yields would overproduce both weak absorbers and DLA metallicities. Instead, the observed strength of high-ionization emission from high-redshift galaxies and absorption from their environments suggest that the ionizing flux from conventional stellar population models is too soft., Comment: 24 pages, 17 figures, accepted to MNRAS

Observations of metal absorption lines in the spectra of QSOs out to z > 6 are providing an important probe into the enrichment and ionization state of the intergalactic medium (IGM) at the tail end of reionization. Using simulations with four different feedback models, including the Illustris and Sherwood simulations, we investigate how the overall incidence rate and equivalent width distribution of metal-line absorbers varies with the galactic wind scheme. The low-ionization absorbers are reasonably insensitive to the feedback implementation, with all models reasonably close to the observed incidence rate of O i absorbers. However, all of our models struggle to reproduce the observations of C iv, which is probing overdensities close to the mean at z ~ 6, suggesting that the metals are not being transported out into the IGM efficiently enough in these simulations.

Line-intensity mapping (LIM or IM) is an emerging field of observational work, with strong potential to fit into a larger effort to probe large-scale structure and small-scale astrophysical phenomena using multiple complementary tracers. Taking full advantage of such complementarity means, in part, undertaking line-intensity surveys with galaxy surveys in mind. We consider the potential for detection of a cross-correlation signal between COMAP and blind surveys based on photometric redshifts (as in COSMOS) or based on spectroscopic data (as with the HETDEX survey of Lyman-$\alpha$ emitters). We find that obtaining $\sigma_z/(1+z)\lesssim0.003$ accuracy in redshifts and $\gtrsim10^{-4}$ sources per Mpc$^3$ with spectroscopic redshift determination should enable a CO-galaxy cross spectrum detection significance at least twice that of the CO auto spectrum. Either a future targeted spectroscopic survey or a blind survey like HETDEX may be able to meet both of these requirements., Comment: 19 pages + appendix (31 pages total), 16 figures, 6 tables; accepted for publication in ApJ

We present models of low- and high-ionization metal-line absorbers (O I, C II, C IV and Mg II) during the end of the reionization epoch, at z ~ 6. Using four cosmological hydrodynamical simulations with different feedback schemes (including the Illustris and Sherwood simulations) and two different choices of hydro-solver, we investigate how the overall incidence rate and equivalent width distribution of metal-line absorbers varies with the galactic wind prescription. We find that the O I and C II absorbers are reasonably insensitive to the feedback scheme. All models, however, struggle to reproduce the observations of C IV and Mg II, which are probing down to lower overdensities than O I and C II at z ~ 6, suggesting that the metals in the simulations are not being transported out into the IGM efficiently enough. The situation is improved but not resolved if we choose a harder (but still reasonable) and/or (locally) increased UV background at z ~ 6., LCK acknowledges the support of an Isaac Newton Studentship, the Cambridge Trust and STFC. Support by the FP7 ERC Advanced Grant Emergence-320596 is gratefully acknowledged. EP acknowledges support from the Kavli Foundation. SB was supported by the National Aeronautics and Space Administration through Einstein Postdoctoral Fellowship Award Number PF5-160133 issued by the Chandra X-ray Observatory Center, which is operated by the Smithsonian Astrophysical Observatory for and on behalf of the National Aeronautics Space Administration under contract NAS8-03060. JSB acknowledges the support of a Royal Society University Research Fellowship. This work used the DIRAC Shared Memory Processing system at the University of Cambridge, operated by the COSMOS Project at the Department of Applied Mathematics and Theoretical Physics on behalf of the STFC DiRAC HPC Facility (www.dirac.ac.uk). This equipment was funded by BIS National E-infrastructure capital grant ST/J005673/1, STFC capital grant ST/H008586/1 and STFC DiRAC Operations grant ST/K00333X/1. This work also used the DiRAC Data Analytic system at the University of Cambridge, operated by the University of Cambridge High Performance Computing Service on behalf of the STFC DiRAC HPC Facility (www.dirac.ac.uk). This equipment was funded by BIS National E-infrastructure capital grant (ST/K001590/1), STFC capital grants ST/H008861/1 and ST/H00887X/1, and STFC DiRAC Operations grant ST/K00333X/1. DiRAC is part of the National E-Infrastructure. We acknowledge PRACE for awarding us access to the Curie supercomputer, based in France at the Tres Grand Centre de Calcul (TGCC), through the 8th regular call., This is the final version of the article. It first appeared from Oxford University Press via http://dx.doi.org/ 10.1093/mnras/stw1306

QSO near-zones are an important probe of the the ionization state of the IGM at z ~ 6-7, at the end of reionization. We present here high-resolution cosmological 3D radiative transfer simulations of QSO environments for a wide range of host halo masses, 10$^{10-12.5}$ M_sun. Our simulated near-zones reproduce both the overall decrease of observed near-zone sizes at 6 < z < 7 and their scatter. The observable near-zone properties in our simulations depend only very weakly on the mass of the host halo. The size of the H II region expanding into the IGM is generally limited by (super-)Lyman Limit systems loosely associated with (low-mass) dark matter haloes. This leads to a strong dependence of near-zone size on direction and drives the large observed scatter. In the simulation centred on our most massive host halo, many sightlines show strong red damping wings even for initial volume averaged neutral hydrogen fractions as low as ~ 10$^{-3}$. For QSO lifetimes long enough to allow growth of the central supermassive black hole while optically bright, we can reproduce the observed near-zone of ULAS J1120+0641 only with an IGM that is initially neutral. Our results suggest that larger samples of z > 7 QSOs will provide important constraints on the evolution of the neutral hydrogen fraction and thus on how late reionization ends., Comment: This is a pre-copyedited, author-produced PDF of an article accepted for publication in MNRAS following peer review. 19 pages, 17 figures and 3 tables

Low ionization metal absorption due to OI has been identified as an important probe of the physical state of the inter-/circumgalactic medium at the tail-end of reionization. We use here high-resolution hydrodynamic simulations to interpret the incidence rate of OI absorbers at z~6 as observed by Becker et al. (2011). We infer weak OI absorbers (EW > 0.1 A) to have typical HI column densities in the range of sub-DLAs, densities of 80 times the mean baryonic density and metallicities of about 1/500 th solar. This is similar to the metallicity inferred at similar overdensities at z~3, suggesting that the metal enrichment of the circumgalactic medium around low-mass galaxies has already progressed considerably by z~6. The apparently rapid evolution of the incidence rates for OI absorption over the redshift range 55. We predict the incidence rate of OI absorbers to continue to rise rapidly with increasing redshift as the IGM becomes more neutral. If the distribution of metals extends to lower density regions, OI absorbers will allow the metal enrichment of the increasingly neutral filamentary structures of the cosmic web to be probed., Comment: 13 pages, 10 figures, 1 table, accepted to MNRAS