arXiv subjects

Avery Meiksin

Publications and source records attributed to Avery Meiksin.

At least 19 recordsLinked to original sources

arXiv2026

Lyman-$α$ forest 1D flux power spectrum constraints on QSO-assisted reionization models

Recent JWST observations have revealed a large population of faint Quasi-Stellar Objects (QSOs) at redshifts 4 < z < 6, including a highly reddened sub-population of Little Red Dots. We exploit the sensitivity of the Lyman-$α$ forest 1D flux power spectrum to thermal fluctuations in the Intergalactic Medium (IGM) to place statistical constraints on the spectral properties of the QSOs. By post-processing simulated Lyman-$α$ forest spectra from the Sherwood-Relics suite of Lyman-$α$ forest simulations with added HeII reionization by QSOs, in conjunction with published precision measurements of the Lyman-$α$ forest 1D flux power spectrum between 4.2 < z < 5.0, we find that the addition of temperature boosts of the IGM within the HeIII regions improves agreement with the measured power spectra. While the contribution of the Little Red Dot population to HeII reionization is consistent with both a mild temperature boost of $ΔT_b=1\times10^4$ K for soft spectra QSOs and $ΔT_b=2\times10^4$ K for hard spectra QSOs, a contribution from the larger population of faint QSOs found by JWST having $M_{\rm UV} > -21.6$ is excluded at the 2$σ$ level if their spectra are sufficiently hard to boost the IGM temperature in HeIII regions by $ΔT_b=2\times10^4$ K or greater.

astro-ph.GA↗

arXiv2026

The heating rate of the Intergalactic Medium by Lyman-$α$ photon scattering

The strength of the 21-cm Cosmic Dawn radio absorption signature against the Cosmic Microwave Background or against a bright background radio source depends on the temperature of the neutral hydrogen in the intergalactic medium. While x-rays from forming galaxies will likely dominate the heating rate, recent models suggest scenarios in which heating by the scattering of Lyman-$α$ photons sourced by the galaxies contributes non-negligibly as well, and may even dominate. The efficiency of Lyman-$α$ photon heating for a point source at high redshift using a numerical solution to the exact radiative transfer equation is provided here. It is found to be a factor of several smaller than the rate computed in the commonly used diffusion approximation to the radiative transfer equation.

astro-ph.CO↗

arXiv2026

Overview of 21cm Experiments at high redshift with SKAO

We provide an overview of the eight SKAO Science Book chapters that motivate the Epoch of Reionisation and Cosmic Dawn experiments with SKA-Low. We describe the individual SKA-Low experiments and expected sensitivity - power spectrum, tomography, 21-cm forest, cross-correlations, building on the broad observational plan laid out in the 2015 SKA Science Book. Finally, we outline features of the telescope that will be critical for the success of EoR/CD science, e.g., beam apodization, substations, and multi-beaming.

astro-ph.CO↗

arXiv2026

Illuminating the Physics of Cosmic Origin and Evolution: A UK Space Frontiers 2035 White Paper

Understanding the Universe's origins and evolution remains one of the most fundamental challenges in modern cosmology. This white paper explores three key science priorities in this field: unravelling the physics of cosmic inflation, investigating the accelerating expansion of the Universe, and precisely measuring the sum of the neutrino masses. Achieving these goals requires a dedicated survey to map the large-scale structure at high redshift in unprecedented detail. We describe how this can be achieved through a mission concept called SIRMOS, providing a high-throughput, highly multiplexed spectroscopic capability to obtain accurate redshifts for over 100 million galaxies over a wide sky area. Such a survey would leverage the deepest existing wide-area photometric catalogues for targeting, with spectra offering continuous 1.25-2.5~$μ$m wavelength coverage at moderate resolution, allowing precise redshift measurements in the $1<z<4$ range with minimal bias. We outline the scientific opportunities this presents. Recent years have seen significant advances in instrumentation, including digital micromirror devices, complex telescope mirrors, large detector arrays, and data processing pipelines. While these technologies have been demonstrated in terrestrial applications, such a survey is a unique opportunity to apply these proven capabilities in space to address fundamental questions in cosmology. Participation in such a mission will simultaneously deliver a compelling science case, help align UK Space Agency and STFC strategies, demonstrate the UK's growing capability in end-to-end space missions, and strengthen the national space economy through high-value industrial participation.

astro-ph.IM↗

arXiv2025

The 21-cm signature of X-ray heated halos around galaxies during cosmic dawn

X-rays emitted by high mass X-ray binaries (HMXBs) and supernovae-driven winds in the first galaxies during Cosmic Dawn are expected to warm the intergalactic medium prior to its reionization. While most of the heating will be uniform on measurable scales, exceptionally bright sources will produce a warm ring around them with a distinctive 21-cm signature. The detection of such systems would confirm X-rays are a source of IGM heating during Cosmic Dawn and provide a test of models predicting higher X-ray luminosities per star formation rate compared with present-day galaxies. We illustrate the effect for a star-forming galaxy in a $10^{11}\, M_\odot$ halo at $z=12$, treating the photoionizing radiation and X-rays using a novel fully time-dependent 3D ray-tracing radiative transfer code. We consider a range in possible spectra for the HMXBs and star formation efficiencies, as well as the possible effect of an extended halo around the galaxy. We find detection of the signal would require integration times of a few thousand hours using SKA1-Low except for a bright galaxy like a starburst, but only a thousand hours for the expected noise levels of SKA2-Low. Depending on the surrounding gas density profile, the 21-cm signature of X-ray heating may still require an exceptionally high star formation rate, either intrinsic to the source or provided by other systems clustered near it, to avoid dominance of the signal by absorption from the surrounding gas.

astro-ph.CO↗

arXiv2024

The effect of helium reionization on the Lyman-alpha forest hydrogen flux statistics

We assess the impact of QSOs on the high redshift (z > 4) Intergalactic Medium using Monte Carlo realisations of QSO populations and the HeIII regions they generate, applied to the Sherwood-Relics simulations, allowing for uncertainties in the QSO luminosity function, its evolution, and QSO spectra and ages. While QSO luminosity functions based on optical-infrared selection are unable to reproduce the broadening HI Lyman-alpha optical depth distributions at z > 5, much broader distributions are found for the higher numbers of QSOs based on x-ray selection, suggesting a large QSO contribution to the ultra-violet background at z > 5 may offer an alternative to late reionization models to account for the broad HI Lyman-alpha optical depth distributions. Realisations using QSOs based on the higher QSO counts also much better recover the measured pixel flux auto-correlation function at z > 5. The HeIII regions from QSO sources according to both types of luminosity function suppress the pixel flux power spectrum on small scales, k > 0.02 s/km, while enhancing it on larger, both by amounts of up to tens of percent at z > 4, with the magnitude increasing with the intergalactic HeIII filling factor and the boost in temperature within the HeIII regions.

astro-ph.CO↗

arXiv2023

Observational constraints on the metagalactic Ly$α$ photon scattering rate at high redshift

The scattering of Ly$α$ photons from the first radiating sources in the Universe plays a pivotal role in 21-cm radio detections of Cosmic Dawn and the Epoch of Reionization through the Wouthuysen-Field effect. New data from JWST show the Ly$α$ photon scattering rate exceeds that required to decouple the intergalactic hydrogen spin temperature from that of the Cosmic Microwave Background up to $z\sim14$ and render the neutral hydrogen visible over the main redshift range expected for the Epoch of Reionization.

astro-ph.CO↗

arXiv2023

The Sherwood-Relics simulations: overview and impact of patchy reionization and pressure smoothing on the intergalactic medium

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.

astro-ph.CO↗

arXiv2023

A comparison of numerical methods for computing the reionization of intergalacitc hydrogen and helium by a central radiating source

We compare numerical methods for solving the radiative transfer equation in the context of the photoionization of intergalactic gaseous hydrogen and helium by a central radiating source. Direct integration of the radiative transfer equation and solutions using photon packets are examined, both for solutions to the time-dependent radiative transfer equation and in the infinite-speed-of-light approximation. The photon packet schemes are found to be more generally computationally efficient than a direct integration scheme. Whilst all codes accurately describe the growth rate of hydrogen and helium ionization zones, it is shown that a fully time-dependent method is required to capture the gas temperature and ionization structure in the near zone of a source when an ionization front expands at a speed close to the speed of light. Applied to Quasi-Stellar Objects in the Epoch of Reionization (EoR), temperature differences as high as $5\times10^4$ K result in the near-zone for solutions of the time-dependent radiative transfer equation compared with solutions in the infinite-speed-of-light approximation. Smaller temperature differences are found following the nearly full photoionization of helium in gas in which the hydrogen was already ionized and the helium was singly ionized. Variations found in the temperature and ionization structure far from the source, where the gas is predominantly neutral, may affect some predictions for 21-cm EoR experiments.

astro-ph.IM↗

arXiv2022

The effect of ionizing background fluctuations on the spatial correlations of high redshift Ly$α$-emitting galaxies

We investigate the possible influence of fluctuations in the metagalactic photoionizing ultra-violet background (UVBG) on the clustering of Ly$α$-emitting galaxies through the modulation of the ionization level of the gas surrounding the systems. At redshifts $z > 5$, even when assuming the reionization of the intergalactic medium has completed, the fluctuations are sufficiently large that they may non-negligibly enhance, and possibly even dominate, the angular correlation function on scales up to a few hundred arcsecs. Whilst a comparison with observations at $z \sim 5.7$ is statistically consistent with no influence of UVBG fluctuations, allowing for the fluctuations opens up the range of acceptable models to include those with relatively low bias factors for the Ly$α$-emitting galaxies. In this case, the evolution in the bias factor of Ly$α$-emitters over the approximate redshift range $3 < z < 7$ corresponds to a nearly constant halo mass for Ly$α$-emitting galaxies of $\sim 10^{10.5}\,M_\odot$.

astro-ph.GA↗

arXiv2022

Radio Power from Direct-Collapse Black Holes

Direct-collapse black holes (DCBHs) forming at $z \sim$ 20 are currently the leading candidates for the seeds of the first quasars, over 200 of which have now been found at $z >$ 6. Recent studies suggest that DCBHs could be detected in the near infrared by the James Webb Space Telescope, Euclid, and the Roman Space Telescope. However, new radio telescopes with unprecedented sensitivities such as the Square Kilometer Array (SKA) and the Next-Generation Very Large Array (ngVLA) may open another window on the properties of DCBHs in the coming decade. Here we estimate the radio flux from DCBHs at birth at $z =$ 8 - 20 with several fundamental planes of black hole accretion. We find that they could be detected at $z \sim$ 8 by the SKA-FIN all-sky survey. Furthermore, SKA and ngVLA could discover 10$^6$ - 10$^7$ $M_{\odot}$ BHs out to $z \sim$ 20, probing the formation pathways of the first quasars in the Universe.

astro-ph.GA↗

arXiv2021

Radio Power from a Direct-Collapse Black Hole in CR7

The leading contenders for the seeds of the first quasars are direct collapse black holes (DCBHs) formed during catastrophic baryon collapse in atomically-cooled halos at $z \sim$ 20. The discovery of the Ly$α$ emitter CR7 at $z =$ 6.6 was initially held to be the first detection of a DCBH, although this interpretation has since been challenged on the grounds of Spitzer IRAC and Very Large Telescope X-Shooter data. Here we determine if radio flux from a DCBH in CR7 could be detected and discriminated from competing sources of radio emission in the halo such as young supernovae and H II regions. We find that a DCBH would emit a flux of 10 - 200 nJy at 1.0 GHz, far greater than the sub-nJy signal expected for young supernovae but on par with continuum emission from star-forming regions. However, radio emission from a DCBH in CR7 could be distinguished from free-free emission from H II regions by its spectral evolution with frequency and could be detected by the Square Kilometer Array in the coming decade.

astro-ph.GA↗

arXiv2020

Origin of Weak MgII and Higher Ionization Absorption Lines in an Outflow from an Intermediate-Redshift Dwarf Satellite Galaxy

Observations at intermediate redshifts reveal the presence of numerous, compact, weak MgII absorbers with near to super-solar metallicities, often surrounded by more extended regions that produce CIV and/or OVI absorption in the circumgalactic medium at large impact parameters from luminous galaxies. Their origin and nature remains unclear. We hypothesize that undetected, satellite dwarf galaxies are responsible for producing some of these weak MgII absorbers. We test our hypothesis using gas dynamical simulations of galactic outflows from a dwarf satellite galaxy with a halo mass of $5\times10^{9}$ M$_{\odot}$, which could form in a larger $L^{*}$ halo at z=2, to study the gas interaction in the halo. We find that thin, filamentary, weak MgII absorbers are produced in two stages: 1) when shocked core collapse supernova (SNII) enriched gas descending in a galactic fountain gets shock compressed by upward flows driven by subsequent SNIIs and cools (phase 1), and later, 2) during an outflow driven by Type Ia supernovae that shocks and sweeps up pervasive SNII enriched gas, which then cools (phase 2). The width of the filaments and fragments are $\lesssim~100$ pc, and the smallest ones cannot be resolved at 12.8 pc resolution. The MgII absorbers in our simulations are continuously generated for >150 Myr by shocks and cooling, though each cloud survives for only ~60 Myr. Their metallicity is 10-20% solar metallicity and column density is $<10^{12}$ cm$^{-2}$. They are also surrounded by larger (0.5-1 kpc) CIV absorbers that seem to survive longer. In addition, larger-scale (>1 kpc) CIV and OVI clouds are produced in both expanding and shocked SNII enriched gas which is photoionized by the UV metagalactic radiation at intermediate redshift. Our simulation highlights the possibility of dwarf galactic outflows producing highly enriched multiphase gas.

astro-ph.GA↗

arXiv2020

The impact of Lyman-$α$ emission line heating and cooling on the cosmic dawn 21-cm signal

Allowing for enhanced Ly$α$ photon line emission from Population III dominated stellar systems in the first forming galaxies, we show the 21-cm cosmic dawn signal at $10<z<30$ may substantially differ from standard scenarios. Energy transfer by Ly$α$ photons emerging from galaxies may heat intergalactic gas if HII regions within galaxies are recombination bound, or cool the gas faster than by adiabatic expansion if reddened by winds internal to the haloes. In some cases, differential 21-cm antenna temperatures near $-500$ mK may be achieved at $15<z<25$, similar to the signature detected by the EDGES 21-cm cosmic dawn experiment.

astro-ph.CO↗

arXiv2020

Modelling intergalactic low ionisation metal absorption line systems near the epoch of reionization

We interpret observations of intergalactic low ionisation metal absorption systems at redshifts z $\gtrsim$5 in terms of pressure-confined clouds. We find clouds confined by the expected pressure of galactic haloes with masses $11<\log M_h/h^{-1}M_\odot<12$ provide a good description of the column density ratios between low ionisation metal absorbers. Some of the ratios, however, require extending conventional radiative transfer models of irradiated slabs to spherical (or cylindrical) clouds to allow for lines of sight passing outside the cores of the clouds. Moderate depletion of silicon onto dust grains is also indicated in some systems. The chemical abundances inferred span the range between solar and massive-star dominated stellar populations as may arise in starburst galaxies. The typical HI column densities matching the data correspond to Damped Lyman-$α$ Absorbers (DLAs) or sub-DLAs, with sizes of 40 pc to 3 kpc, gas masses $3.5<\log M_c/M_\odot<8$ and metallicites $0.001-0.01Z_\odot$. Such systems continue to pose a challenge for galaxy-scale numerical simulations to reproduce.

astro-ph.CO↗

arXiv2019

The influence of metagalactic ultra-violet background fluctuations on the high-redshift Ly$α$ forest

Under the assumption that galaxies and Quasi-Stellar Objects (QSOs) dominate the metagalactic ultra-violet (UV) background, it is shown that at high redshifts fluctuations in the UV background are dominated by QSO shot noise and have an auto-correlation length of a few to several comoving Mpcs, depending on the bright end of the QSO luminosity function. The correlations create long range spatial coherence in the neutral hydrogen fraction. Using a semi-analytic model, it is demonstrated that the coherence may account for the broad distribution in effective optical depths measured in the Ly$α$ forest spectra of background QSOs, for line-of-sight segments of comoving length $50h^{-1}$ Mpc at redshifts $5<z<6$. Capturing the fluctuations in a numerical simulation requires a comoving box size of $\sim1h^{-1}$ Gpc, although a box half this size may be adequate if sufficient random realizations of the QSO population are performed.

astro-ph.CO↗

arXiv2019

Estimates for the impact of Ultraviolet Background fluctuations on galaxy clustering measurements

Spatial fluctuations in ultraviolet backgrounds can subtly modulate the distribution of extragalactic sources, a potential signal and systematic for large-scale structure surveys. While this modulation has been shown to be significant for 3D Ly alpha forest surveys, its relevance for other large-scale structure probes has been hardly explored, despite being the only astrophysical process that likely can affect clustering measurements on the scales of >~Mpc. We estimate that the background fluctuations, modulating the amount of H I, have a fractional effect of (0.03-0.3) x (k/[10^-2 Mpc^-1])^-1 on the power spectrum of 21 cm intensity maps at z = 1-3. We find a smaller effect for H alpha and Ly alpha intensity mapping surveys of (0.001-0.1) x (k/[10^-2 Mpc^-1])-1 and even smaller effect for more traditional surveys that correlate the positions of individual H alpha or Ly alpha emitters. We also estimate the effect of backgrounds on low-redshift galaxy surveys in general based on a simple model in which background fluctuations modulate the rate halo gas cools, modulating star formation: We estimate a maximum fractional effect on the power of ~0.01 (k/[10-2 Mpc-1])-1 at z = 1. We compare sizes of these imprints to cosmological parameter benchmarks for the next generation of redshift surveys: We find that ionizing backgrounds could result in a bias on the squeezed triangle non-Gaussianity parameter fNL that can be larger than unity for power spectrum measurements with a SPHEREx-like galaxy survey, and typical values of intensity bias. Marginalizing over a shape of the form k-1PL, where PL is the linear matter power spectrum, removes much of this bias at the cost of ~ 40% larger statistical errors.

astro-ph.CO↗

arXiv2018

Time-dependent fluctuations in the metagalactic photoionization background

We present a formalism for computing time-dependent fluctuations in the cosmological photoionizing radiation background, extending background fluctuations models beyond the steady-state approximation. We apply this formalism to estimate fluctuations in the HI LyA flux redshift space power spectrum and its spatial correlation function at redshifts 2<z<4, assuming the photoionization background is dominated by Quasi-stellar Objects (QSOs) and/or galaxies. We show the shot noise in the power spectrum due to discrete sources is strongly suppressed relative to the steady-state value at low wavenumbers by a factor proportional to the lifetime of the sources, and that this suppression may be used to constrain QSO lifetimes. The total HI LyA power spectrum including shot noise is affected at tens of percent on short scales, and by as much as an order of magnitude or more on scales exceeding the mean free path. The spatial correlation function is similarly found to be sensitive to the shot noise, although moderately insensitive to the effects of time-dependence on the non-shotnoise contribution. Photoionization rate fluctuations substantially modify the shape of the Baryonic Acoustic Oscillation peak in the correlation function, including a small increase in its position that must be accounted for to avoid biasing estimates of cosmological parameters based on the peak position. We briefly investigate solving the full frequency dependent equation, finding that it agrees with the frequency-independent to better than percent accuracy. Simple formulas are provided for the power spectrum of fluctuations in the photoionization rate that approximate the full computations.

astro-ph.CO↗