arXiv subjects

Matthew McQuinn

Publications and source records attributed to Matthew McQuinn.

At least 19 recordsLinked to original sources

arXiv2026

A First Measurement of Circumgalactic Dust Reddening from Only 4.6 deg$^2$ of the Rubin Observatory's Data Preview 1

We present the first measurement of circumgalactic dust reddening from the Vera C. Rubin Observatory, using only 4.6 deg$^2$ of ComCam Data Preview 1 --- roughly $0.03\%$ of the final LSST footprint. Using photometric redshifts, we stack background-galaxy colors around foreground-galaxy positions and detect a chromatic reddening profile from $r_\perp \simeq 10$ kpc to $1$ Mpc. Interpreting average $E(g-z)$ with a Milky Way extinction curve, we find $A_V = (1.3 \pm 0.4) \times 10^{-1} (r_\perp / 20 \mathrm{kpc})^{-1.8 \pm 0.4}$ within $120$ kpc. The amplitude and radial dependence agree with earlier Sloan Digital Sky Survey (SDSS), KiDS, and Dark Energy Survey (DES) results despite the $\sim1000\times$ smaller survey area and a foreground sample extending 3--6 mag fainter and 1--2 dex lower in stellar mass. The innermost 10--15 kpc bin reaches $A_V \simeq 0.3$ mag, comparable to high-latitude extinction through the Milky Way disk near the solar circle; the steep power-law slope implies a dust distribution that does not simply trace the halo-gas profile. Splitting by rest-frame $g-r$ shows stronger extinction around red foreground galaxies (rest-frame $g-r > 0.5$), although the blue subsample is too noisy to establish a significant color dependence. This red sample, with median halo mass $5 \times 10^{11} M_\odot$, shows substantially more reddening within 50 kpc than previously measured around more massive LRGs and implies a dust-to-stellar-mass ratio of $\sim 2\%$, nearly saturating the dust budget allowed by stellar metal yields. These pathfinder data demonstrate LSST's promise for high-precision galaxy--dust measurements across galaxy mass, environment, and redshift.

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arXiv2026

Radio Recombination Line Contamination in Post-Reionization 21 cm Intensity Mapping

We explore radio recombination line (RRL) contamination in post-reionization 21 cm intensity mapping observations. We develop a formalism to estimate the contamination of the 21 cm auto-power spectrum from the myriad of RRL lines that redshift into a 21 cm map and predict contamination of the 21 cm power spectrum at all redshifts. At $z\lesssim 2$, extrapolation of the conservative upper-bound model of Petrovic & Oh (2011) predicts high contamination to the 21 cm power spectrum, whereas our empirically-calibrated models suggest contamination several orders of magnitude smaller. We also estimate the contribution from carbon RRLs and find it to be subdominant. We find the RRL contamination is highly oscillatory in wavenumber, owing to the dominant contamination from RRLs emitted close in physical space to the 21 cm emission. These oscillations could in principle bias distance measurements derived from Baryon Acoustic Oscillations (BAO), a key science driver of post-reionization 21 cm intensity mapping. We find the shift in the BAO extrema is at least two orders of magnitude smaller than relevant for percent-precision cosmology from BAO.

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arXiv2026

An emulator for the ionizing photon mean free path in ultra-high resolution simulations: the implications of mean free path measurements for the reionization history

Measurements of the mean free path of ionizing photons from high-redshift quasar spectra at $z \sim 5$-$6$ constrain the reionization history, but interpreting them requires modeling the kiloparsec-scale clumping that large-volume reionization simulations cannot resolve. We present a deep learning emulator for the mean free path (MFP) trained on high-resolution cosmological radiative transfer simulations of ionization fronts sweeping through small 2 comoving Mpc/h volumes. Using a residual multi-layer perceptron neural network, we predict the MFP at a given redshift as a function of the reionization redshift, photoionization rate, wavelength, and box-scale density, achieving a median relative error of 1.3\% across nearly four orders of magnitude in MFP. Integrating its predictions over box-scale overdensity and an extended reionization history allows the emulator to predict the global MFP. We apply the emulator to extended reionization histories constrained by observed photoionization rates, finding that models prefer late reionization with substantial neutral fractions persisting at $z \lesssim 6$. Fitting a parametric ionization history yields a midpoint of reionization of $z_{\rm re} = 6.58\pm 1.2$ for reionization durations consistent with Planck and kinetic Sunyaev-Zeldovich constraints, and the universe being $10\%$ neutral still at $z < 5.8 ~(6.3)$ at 1~(2)$σ$. Global ionizing emissivity inferences using measurements of the photoionization rate and MFP plus our emulator, which avoids common power-law assumptions, suggest a factor of $2-3$ decline between $z = 6$ and $4.8$, in agreement with previous studies. Our method provides an efficient (and more converged) alternative to large-volume radiative-hydrodynamic simulations of reionization for interpreting MFP measurements, and can also serve as a subgrid prescription for the ionizing opacity within such simulations.

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arXiv2026

On Cross-Correlating Line Intensity Maps from SPHEREx during Reionization

We have simulated Lyα, Hα, Hβ, [OII], and [OIII] intensity maps which are observable by SPHEREx during cosmic reionization. We simulate these intensity maps including all significant sources of emission for each line, and include radiative transfer for the Lyα intensity maps. We also include a simple model of dust extinction based on observations of galaxies at z<5. One of the main challenges of intensity mapping is interloping lines from galaxies at lower redshifts, which makes producing an auto-power spectrum challenging. We focus on cross-correlations between different lines, as this eliminates such foreground contamination of the signal. We have cross-correlated the simulated SPHEREx intensity maps to find the most observable cases. This includes modeling of interloping lines and masking bright interloping galaxies. Testing a range of cases motivated by observations, we find total signal-to-noise values up to 99 for the highest case of Hα cross-correlated with [OIII] at z=5 assuming no dust extinction. We also find cases which will not be detectable. We find that the dominant noise source in these intensity maps on most scales is from the instrument, except for Lyα and [OII] and then only on the largest scales the interlopers are the dominant source. We find through intensity mapping we can probe galaxies with masses $M<4x10^{10}~M_{\odot}$ which are below the necessary luminosity for a 3σ signal-to-noise direct detection of galaxies by SPHEREx. However, the majority of our observable signal is dominated by large, directly detectable galaxies, rather than the smaller, fainter galaxies. We find marginal detections of the clustering portion of the power spectrum at z=5 for Hαx[OIII]. Detections of the clustering signal from other lines or at z>6 will require more sensitive instruments, such as the Cosmic Dawn Intensity Mapper.

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arXiv2026

A Decade to Map the Diffuse Universe: FRB-QSO Pairs with HST/COS Spectroscopy

Jointly analyzing the sightlines of arcsecond-localized fast radio bursts (FRBs) and UV-bright quasars (QSOs) nearby in projection has the potential to provide strong constraints on the phases, mass distributions, and magnetic structure of the diffuse universe. Each probe supplies what the other cannot: FRBs provide integrated electron columns (DM), line-of-sight magnetic field estimates (RM), and scattering constraints ($τ_{\rm scatt}$) that are independent of gas phase; QSOs provide the redshift- and phase-resolved column densities needed to interpret them. Today, there are only $\sim100$ arcsecond-localized FRBs at $z < 1$, making statistical FRB-QSO pair surveys impossible. By 2035, there will be $\sim10^{5}$. Using the most recent FRB localization forecasts and UV-bright QSO catalogs, we estimate that next-generation interferometers will yield thousands of FRB--QSO pairs at angular separations $θ< 10'$, including $\sim100$ pairs at $θ< 1'$, over a common 20,000\,deg$^2$ footprint by 2035. We outline the science enabled by this sample: constraints on CGM ionization fractions and baryon masses; observational constraints on the role of magnetic fields and turbulence in the CGM and cosmic web; sightline-by-sightline partitioning of the cosmic DM budget; and three-dimensional mapping of the multiphase Milky Way and M31 halos. Together, these measurements directly address the physics of feedback, non-thermal pressure support, and energy balance in the diffuse gas that regulates galaxy growth. HST/COS is the only instrument that can carry out this program, and the 2030s are the only decade in which to do it before Habitable Worlds Observatory (HWO) defines the next era of diffuse universe science.

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arXiv2026

Kiloparsec-scale turbulence driven by reionization may grow intergalactic magnetic fields

The intergalactic medium (IGM) underwent intense heating that resulted in pressure disequilibrium in the wake of ionization fronts during cosmic reionization. The dynamical relaxation to restore pressure balance may have driven small-scale turbulence and, hence, the amplification of intergalactic magnetic fields. We investigate this possibility for the first time using a suite of $\approx 100$ pc resolution radiation-hydrodynamics simulations of IGM gas dynamics. We show that as the spatial resolution improves beyond that achieved with most prior studies, much of the IGM becomes turbulent unless it was pre-heated to $\gg 100~$K before reionization. In our most turbulent simulations, we find that the gas energy spectrum follows the expected $k^{-5/3}$ Kolmogorov scaling to the simulation's resolution, and the eddy turnover time of the turbulence is $< 1$ Gyr at $k \approx 1 ~$kpc$^{-1}$. Turbulence will grow magnetic fields, and we show that the fields grown by reionization-driven turbulence could explain lower limits on the strength of volume-filling B-fields from observations of TeV blazars. As reionization sweeps over the cosmos, this mechanism could create turbulence throughout the cosmic volume with a character that only depends on the amount of IGM preheating.

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arXiv2026

NIAC project report: Solar system-scale VLBI to dramatically improve cosmological distance measurements

We investigate the feasibility and scientific potential of the Cosmic Positioning System (CPS), a space mission concept enabling purely geometric distance measurements to sources at hundreds of megaparsecs by directly detecting electromagnetic wavefront curvature. CPS consists of a constellation of radio antennas distributed across the outer Solar System, operating on baselines of tens of astronomical units. By precisely timing the arrival of repeating fast radio bursts (FRBs), CPS infers source distances via trilateration -- analogous to global navigation satellite systems such as GPS but on cosmological scales. We show that CPS distance measurements could result in sub-percent constraints on the Hubble constant with even a handful of detections, whereas we predict that 10-100 FRB sources are likely visible. We evaluate dominant sources of uncertainty -- wavefront timing precision, interstellar refractive delays, spacecraft positional knowledge, and onboard clock stability -- finding these controllable at required levels using near-term technologies. Our nominal design employs five spacecraft with 8 m deployable antennas, 3-6 GHz receivers with sub-30 K system temperatures, and space-qualified atomic clocks similar to those on GPS satellites, supported by a ground network for ranging calibration and FRB alerts. Beyond cosmic expansion, CPS may enable frontier measurements in astrophysics and fundamental physics, including constraints on small-scale dark matter structure, microhertz gravitational waves (bridging pulsar timing arrays and LISA), and the outer Solar System mass distribution. The most significant viability issue concerns FRB properties at several-GHz frequencies; we recommend observational campaigns to characterize repeating FRBs in this band.

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arXiv2025

Outer Solar System spacecraft to probe the $μ$Hz gravitational wave frontier

The microhertz frequency band of gravitational waves probes the merger of supermassive black holes as well as many other gravitational wave phenomena. However, space-interferometry methods that use test masses would require further development of test-mass isolation systems to detect anticipated astrophysical events. We propose an approach that avoids onboard inertial test masses by situating spacecraft in the low-acceleration environment of the outer Solar System. We show that for Earth-spacecraft and inter-spacecraft distances of $\gtrsim 10$ AU, the accelerations on the spacecraft would be sufficiently small to potentially achieve gravitational wave sensitivities determined by stochastic gravitational wave backgrounds. We further argue, for arm lengths of $10-30$ AU and $\sim 10$ Watt transmissions, that stable phase locks could be achieved with 20 cm mirrors or 5 m radio dishes. We discuss designs that send both laser beams and radio waves between the spacecraft, finding that, despite the $\sim10^4\times$ longer wavelengths, even a design with radio transmissions could reach stochastic background-limited sensitivities at $\lesssim 0.3\times 10^{-4}$ Hz. Operating in the radio significantly reduces many spacecraft design tolerances. Our baseline concepts require two arms to do interferometry. However, if one spacecraft carries a clock with Allan deviations at $10^4$ seconds of $10^{-17}$, a comparable sensitivity could be achieved with a single arm. Finally, we discuss the feasibility of achieving similar gravitational wave sensitivities in a `Doppler tracking' configuration where the single arm is anchored to Earth.

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arXiv2025

CGM cloud sizes from refractive FRB scattering

We explore constraints on the size of cool gas clouds in the circumgalactic medium (CGM) obtainable from the presence, or lack thereof, of refractive scattering in fast radio bursts (FRBs). Our refractive analysis sets the most conservative bounds on parsec-scale CGM clumpiness as it does not make assumptions about the turbulent density cascade. We find that the bulk of low-redshift cool CGM gas, constrained to have densities of $n_{\rm e} \lesssim 10^{-2}\,{\rm cm^{-3}}$, likely cannot produce two refractive images and, hence, scattering. It is only for extremely small cloud sizes $\lesssim 0.1$ pc (about a hundred times smaller than the so-called shattering scale) that such densities could result in detectable scattering. Dense $n_{\rm e} \gtrsim 0.1\,{\rm cm^{-3}}$ gas with shattering-scale cloud sizes is more likely to inhabit the inner several kiloparsecs of the low-redshift CGM: such clouds would result in multiple refractive images and large scattering times $\gtrsim 1 - 10$ ms, but a small fraction FRB sightlines are likely to be affected. We argue that such large scattering times from an intervening CGM would be a signature of sub-parsec clouds, even if diffractive scattering from turbulence contributes to the overall scattering. At redshift $z\sim 3$, we estimate $\sim 0.1\%$ of FRBs to intersect massive proto-clusters, which may be the most likely place to see scattering owing to their ubiquitous $n_{\rm e} \approx 1\,{\rm cm^{-3}}$ cold gas. While much of our discussion assumes a single cloud size, we show similar results hold for a CGM cloud-size distribution motivated by hydrodynamic simulations.

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arXiv2025

Radiative Transfer Simulations of Ly$α$ Intensity Mapping During Cosmic Reionization Including Sources from Galaxies and the Intergalactic Medium

We present new simulations of Lyman-$α$ (Ly$α$) intensity maps that include Ly$α$ radiative transfer in the intergalactic medium (IGM) and all significant sources of Ly$α$ photons. The sources considered include Ly$α$ directly from galaxies, cooling at the edges of ionized bubbles, recombinations within these bubbles, and reprocessing of galaxy continuum emission in the IGM. We also vary astrophysical parameters including the average neutral fraction of the IGM, the dust absorption of Ly$α$ in galaxies, and the ionizing escape fraction. Previous work has suggested that Ly$α$ intensity mapping can be used to constrain the neutral fraction of the IGM when accounting for radiative transfer in the IGM. When radiative transfer is ignored, direct Ly$α$ emission from galaxies has the highest amplitude of power on all scales. When we include radiative transfer in our simulations, we find continuum emission reprocessed as Ly$α$ is comparable to the Ly$α$ emission directly from galaxies on large scales. For high neutral fraction in the IGM, emission from recombinations is comparable to galaxies on large scales. We find that the slope of the power spectrum is sensitive to the neutral fraction of the IGM when radiative transfer is included, suggesting that this may be useful for placing constraints on cosmic reionization. In addition, we find the power of galaxies is decreased across all scales due to dust absorption. We also find the escape fraction must be large for recombinations and bubble edges to contribute significantly to the power. We find the cross power is observable between SPHEREx Ly$α$ intensity maps and a hypothetical galaxy survey is observable with a total signal-to-noise of 4 from $k = 0.035$ Mpc$^{-1}$ to $k = 1$ Mpc$^{-1}$.

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arXiv2025

A semi-analytic estimate for the effective sound speed counterterm in the EFTofLSS

The Effective Field Theory of Large Scale Structure (EFTofLSS) has found tremendous success as a perturbative framework for the evolution of large scale structure, and it is now routinely used to compare theoretical predictions against cosmological observations. The model for the total matter field includes one nuisance parameter at 1-loop order, the effective sound speed, which can be extracted by matching the EFT to full N-body simulations. In this work we first leverage the Layzer-Irvine cosmic energy equation to show that the equation of state can be exactly computed with knowledge of the fully nonlinear power spectrum. When augmented with separate universe methods, we show one can estimate the effective sound speed. This estimate is in good agreement with simulation results, with errors at the few tens of percent level. We apply our method to investigate the cosmology dependence of the effective sound speed and to shed light on what cosmic structures shape its value.

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arXiv2024

The hydrodynamic response of small-scale structure to reionization drives large IGM temperature fluctuations that persist to z = 4

The thermal history and structure of the intergalactic medium (IGM) at $z \geq 4$ is an important boundary condition for reionization, and a key input for studies using the Ly$α$ forest to constrain the masses of alternative dark matter candidates. Most such inferences rely on simulations that lack the spatial resolution to fully resolve the hydrodynamic response of IGM filaments and minihalos to HI reionization heating. In this letter, we use high-resolution hydrodynamic+radiative transfer simulations to study how these affect the IGM thermal structure. We find that the adiabatic heating and cooling driven by the expansion of initially cold gas filaments and minihalos sources significant small-scale temperature fluctuations. These likely persist in much of the IGM until $z \leq 4$. Capturing this effect requires resolving the clumping scale of cold, pre-ionized gas, demanding spatial resolutions of $\leq 2$ $h^{-1}$kpc. Pre-heating of the IGM by X-Rays can slightly reduce the effect. Our preliminary estimate of the effect on the Ly$α$ forest finds that, at $\log(k /[{\rm km^{-1} s}]) = -1.0$, the Ly$α$ forest flux power (at fixed mean flux) can increase $\approx 10\%$ going from $8$ and $2$ $h^{-1}$kpc resolution at $z = 4-5$ for gas ionized at $z < 7$. These findings motivate more careful analyses of how the effects studied here affect the Ly$α$ forest.

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arXiv2024

Detecting Dark Matter Substructures on Small Scales with Fast Radio Bursts

We propose measuring the arrival time difference of Fast Radio Bursts (FRBs) along two adjacent sightlines as a new probe to dark matter substructures on scales down to $\sim 1\,$AU. We discuss two observational scenarios in which it may be possible to place interesting constraints on such models through monitoring repeating FRB sources: 1) By sending radio receivers to space to form a baseline of tens of AU or more and measuring the temporal variation of the arrival time difference between receivers. 2) By measuring the temporal variation of the arrival time difference between two lensed images of one strongly lensed repeater. In both scenarios, obtaining interesting constraints requires correlating the voltage time series to measure the radio-signal arrival time to sub-nanosecond precision. We find that two radio dishes separated by $20\,$AU may be sensitive to the enhancement of small-scale structures at $\sim 10^{-8}M_\odot$ masses in the QCD axion dark matter scenario or from an early epoch of matter-domination with a reheating temperature up to 60 MeV. Other dark matter models such as those composed of $\sim 10^{-13}M_{\odot}$ primordial black holes produced during inflation would also be probed by this method. We further show that a strong lensing situation of multiple images provides an equivalent $\sim 2000\,$AU baseline, which can be much more sensitive but with the uncertainty that intervening ISM decoherence may degrade the timing precision and that spatial variation in the FRB emission spot may result in confounding signals. We show that the lensing magnifications of Type Ia supernovea constrain a similar quantity to such FRB timing, with present limits being equivalent to ruling out the same parameter space that would be probed by a $0.14~$AU baseline.

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arXiv2024

The signatures of density fluctuations and mixing gas in circumgalactic absorption systems

We investigate the prospects for detecting and constraining density and temperature inhomogeneities in the circumgalactic medium (CGM) using absorption measurements of metal ions. Distributions in the gas thermal properties could arise from turbulence, gas cooling from the hot phase, and mixing between the cool and hot phases. Focusing on these physically motivated models, we parameterize each with a single parameter for simplicity and provide empirical and theoretical estimates for reasonable parameter values. We then construct the probability distribution functions for each of these scenarios, calculate the effective ion fractions, and fit our models to the COS-Halos absorption measurements to infer the gas densities and metallicities. We find that the models we consider (i) produce similarly good fits to the observations with or without distributions in the gas thermal properties, and (ii) result in detectable changes in the column densities only at the boundaries of reasonable parameter values. We show that He II self-shielding can have a larger effect on the ion fractions than density and temperature fluctuations. As a result, uncertainties in cloud geometry and their spatial distribution, affecting the details of radiation transfer, may obscure the effect of inhomogeneities.

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arXiv2023

Post-inflationary Contamination of Local Primordial Non-Gaussianity in Galaxy Power Spectra

The scale-dependent bias of galaxy density contrasts is an important signal to be extracted in constraining local primordial non-Gaussianity ($f_{\rm NL}^{\text{local}}$) from observations of large-scale structure. Constraints so obtained rely on the assumption that horizon-scale features in the galaxy power spectrum are exclusively due to primordial physical mechanisms. Yet, post-inflationary effects can induce modulations to the galaxy number density that appear as horizon-scale, scale-dependent bias. We investigate the effect of two such sources of scale-dependent bias - the free-streaming of light relics and fluctuations in the background of ionising radiation - on precision measurements of local primordial non-Gaussianity $f_{\rm NL}^{\text{local}}$ from galaxy power spectrum measurements. Using the SPHEREx survey as a test case survey reaching $σ(f_{\rm NL}^{\rm local}) \lesssim 1$, we show that ignoring the scale-dependent bias induced by free-streaming particles can negatively bias the inferred value of $f_{\rm NL}^{\rm local}$ by $\sim 0.1-0.3σ$. Ignoring the effect of ionising radiation fluctuations can negatively bias the inferred value of $f_{\rm NL}^{\rm local}$ by $ \sim 1σ$. The range of biases depends on the source populations and the ranges of scales used in the analysis, as well as the value of the neutrino mass and the modelling of the impact of ionising radiation. If these sources of scale-dependent bias are included in the analysis, forecasts for $f_{\rm NL}^{\rm local}$ are unbiased but degraded.

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arXiv2023

Cross Correlation of Pencil-Beam Galaxy Surveys and Line-Intensity Maps: An Application of the James Webb Space Telescope

Line-intensity mapping (IM) experiments seek to perform statistical measurements of large-scale structure with spectral lines such as 21cm, CO, and Lyman-$α$ (Ly$α$). A challenge in these observations is to ensure that astrophysical foregrounds, such as galactic synchrotron emission in 21cm measurements, are properly removed. One method that has the potential to reduce foreground contamination is to cross correlate with a galaxy survey that overlaps with the IM volume. However, telescopes sensitive to high-redshift galaxies typically have small field of views (FOVs) compared to IM surveys. Thus, a galaxy survey for cross correlation would necessarily consist of pencil beams which sparsely fill the IM volume. In this paper, we develop the formalism to forecast the sensitivity of cross correlations between IM experiments and pencil-beam galaxy surveys. We find that a random distribution of pencil beams leads to very similar overall sensitivity as a lattice spaced across the IM survey and derive a simple formula for random configurations that agrees with the Fisher matrix formalism. We explore examples of combining high-redshift James Webb Space Telescope (JWST) observations with both a SPHEREx-like Ly$α$ IM survey and a 21cm experiment based on the Hydrogen Epoch of Reionization Array (HERA). We find that the JWST-SPHEREx case is promising, leading to a total signal-to-noise of ${\sim}5$ after 100 total hours of JWST (at $z=7$). We find that HERA is not well-suited for this approach owing to its drift-scan strategy, but that a similar experiment that can integrate down on one field could be.

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arXiv2023

Solar System-scale interferometry on fast radio bursts could measure cosmic distances with sub-percent precision

The light from a source at a distance d will arrive at detectors separated by 100 AU at times that differ by as much as 120 (d/100 Mpc)^{-1} nanoseconds because of the curvature of the wavefront. At gigahertz frequencies, the arrival time difference can be determined to better than a nanosecond with interferometry. If the space-time positions of the detectors are known to a few centimeters, comparable to the accuracy to which very long baseline interferometry baselines and global navigation satellite systems (GNSS) geolocations are constrained, nanosecond timing would allow competitive cosmological constraints. We show that a four-detector constellation at Solar radii of >10 AU could measure distances to individual sources with sub-percent precision and, hence, cosmological parameters such as the Hubble constant to this precision. The precision increases quadratically with baseline length. FRBs are the only known bright extragalactic radio source that are sufficiently point-like. Galactic scattering limits the timing precision at <3 GHz, whereas at higher frequencies the precision is set by removing dispersion. Furthermore, for baselines greater than 100 AU, Shapiro time delays limit the precision, but their effect can be cleaned with two additional detectors. Accelerations that result in ~1 cm uncertainty in detector positions (from variations in the Sun's irradiance, dust collisions and gaseous drag) could be corrected for with weekly GNSS-like trilaterations. Gravitational accelerations from asteroids occur over longer timescales, and so a setup with a precise accelerometer and calibrating the detector positions off of distant FRBs may also be sufficient. The proposed interferometer would also resolve the radio emission region of Galactic pulsars, constrain the mass distribution in the outer Solar System, and reach interesting sensitivities to ~0.01-100 micro-Hz gravitational waves.

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arXiv2023

The Morphology of Reionization in a Dynamically Clumpy Universe

A recent measurement of the Lyman-limit mean free path at $z = 6$ suggests it may have been very short, motivating a better understanding of the role that ionizing photon sinks played in reionization. Accurately modeling the sinks in reionization simulations is challenging because of the large dynamic range required if $\sim 10^4-10^8 M_{\odot}$ gas structures contributed significant opacity. Thus, there is no consensus on how important the sinks were in shaping reionization's morphology. We address this question with a recently developed radiative transfer code that includes a dynamical sub-grid model for the sinks based on radiative hydrodynamics simulations. Compared to assuming a fully pressure-smoothed IGM, our dynamical treatment reduces ionized bubble sizes by $10-20\%$ under typical assumptions about reionization's sources. Near reionization's midpoint, the 21 cm power at $k \sim 0.1$ $h$Mpc$^{-1}$ is similarly reduced. These effects are more modest than the $30-60\%$ suppression resulting from the higher recombination rate if pressure smoothing is neglected entirely. Whether the sinks played a significant role in reionization's morphology depends on the nature of its sources. For example, if reionization was driven by bright ($M_{\rm UV} < -17$) galaxies, the sinks reduce the large-scale 21 cm power by at most $20\%$, even if pressure smoothing is neglected. Conveniently, when bright sources contribute significantly, the morphology in our dynamical treatment can be reproduced accurately with a uniform sub-grid clumping factor that yields the same ionizing photon budget. By contrast, if $M_{\rm UV} \sim -13$ galaxies drove reionization, the uniform clumping model can err by up to $40\%$.

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