Search

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 substantial development of test-mass isolation systems to detect anticipated astrophysical events. We propose an approach that avoids on-board 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, although for the laser case this would require lower laser frequency noise relative to the LISA lasers. 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 concept requires 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., Comment: 8 figures, 25 pages + appendix

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., Comment: 17+10 pages, 9 figures. Comments are welcome!

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., Comment: Submitted to ApJ, 11 figures and one table

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$\alpha$ 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$\alpha$ forest finds that, at $\log(k /[{\rm km^{-1} s}]) = -1.0$, the Ly$\alpha$ 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$\alpha$ forest., Comment: 6+1 pages, 2+1 figures, accepted for publication in MNRAS letters. Comments welcome

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., Comment: 26 pages, 7 figures; matches published version

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 $\sigma(f_{\rm NL}^{\rm local}) \lesssim 1$, 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 $\sim 0.1-0.3\sigma$. Ignoring the effect of ionising radiation fluctuations can negatively bias the inferred value of $f_{\rm NL}^{\rm local}$ by $ \sim 1\sigma$. 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., Comment: 17 pages, 11 figures, Updated to match version accepted in Physical Review D

Line-intensity mapping (IM) experiments seek to perform statistical measurements of large-scale structure with spectral lines such as 21cm, CO, and Lyman-$\alpha$ (Ly$\alpha$). 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$\alpha$ 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., Comment: 20 pages, 7 figures, replaced with version accepted by ApJ

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., Comment: 34 pages in preprint format; 3 figures; accepted to ApJ Letters

The distribution of gas in the circumgalactic medium (CGM) of galaxies of all types is poorly constrained. Foreground CGMs contribute an extra amount to the dispersion measure (DM) of fast radio bursts (FRB). We measure this DM excess for the CGMs of $10^{11}-10^{13}\ M_\odot$ halos using the CHIME/FRB first data release, a halo mass range that is challenging to probe in any other way. Because of the uncertainty in the FRBs' angular coordinates, only for nearby galaxies is the localization sufficient to confidently associate them with intersecting any foreground halo. Thus we stack on galaxies within 80 Mpc, optimizing the stacking scheme to approximately minimize the stack's variance and marginalize over uncertainties in FRB locations. The sample has 20-30 FRBs intersecting halos with masses of $10^{11}-10^{12}\ M_\odot$ and also of $10^{12}-10^{13}\ M_\odot$, and these intersections allow a marginal $1-2\,\sigma$ detection of the DM excess in both mass bins. The $10^{11}-10^{12}\ M_\odot$ halos bin also shows a DM excess at 1-2 virial radii. By comparing data with different models for the CGM gas profile, we find that all models are favored by the data up to 2-$\sigma$ level compared to the null hypothesis of no DM excess. With 2000-3000 more bursts from a future CHIME data release, we project a 4-$\sigma$ detection of the CGM. Distinguishing between viable CGM models by stacking FRBs with CHIME-like localization would require tens of thousands of bursts., Comment: submitted to ApJ

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 $40\%$., Comment: 18 pages, 12 figures, accepted for publication in MNRAS

Where the cosmic baryons lie in and around galactic dark matter halos is only weakly constrained. We develop a method to quickly paint on models for their distribution. Our approach uses the statistical advantages of $N$-body simulations, while painting on the profile of gas around individual halos in ways that can be motivated by semi-analytic models or zoom-in hydrodynamic simulations of galaxies. Possible applications of the algorithm include extragalactic dispersion measures to fast radio bursts (FRBs), the Sunyaev-Zeldovich effect, baryonic effects on weak lensing, and cosmic metal enrichment. As an initial application, we use this tool to investigate how the baryonic profile of foreground galactic-mass halos affects the statistics of the dispersion measure (DM) towards cosmological FRBs. We show that the distribution of DM is sensitive to the distribution of baryons in galactic halos, with viable gas profile models having significantly different probability distributions for DM to a given redshift. We also investigate the requirements to statistically measure the circumgalactic electron profile for FRB analyses that stack DM with impact parameter to foreground galaxies, quantifying the size of the contaminating "two-halo" term from correlated systems and the number of FRBs for a high significance detection. Publicly available Python modules implement our CGMBrush algorithm., Comment: 23 pages, 10 figures, public python code at https://github.com/ianw89/cgm-brush

Correlated red noise recently reported from pulsar timing observations may be an indication of stochastic gravitational waves emitted by cosmic strings that formed during a primordial phase transition near the Grand Unification energy scale. Unfortunately, known probes of cosmic strings, namely the Cosmic Microwave Background anisotropies and string lensing of extragalactic galaxies, are not sensitive enough for low string tensions of $G\mu = 10^{-10}-10^{-7}$ that are needed to explain this putative signal. We show that strong gravitational lensing of Fast Radio Bursts (FRBs) by cosmic strings is a potentially unambiguous avenue to probe that range of string tension values. The image pair of string lensing are expected to have identical magnification factor and parity, and have a typical time delay of $\sim 10^2\,\,(G\,\mu/10^{-8})^2$ seconds. The unique spectral fingerprint of each FRB, as well as the possibility to detect correlations in the time series of the electric field of the radio waves, will enable verification of the string lensing interpretation. Very-Long-Baseline Interferometry (VLBI) observations can spatially resolve the image pair and provide a lower bound on the string tension based on the image separation. We calculate the FRB lensing rate as a function of FRB detection number for several different models of the FRB redshift distribution. We find that a survey detecting $\sim 10^5$ FRBs, in line with estimates for the detection rate of the forthcoming survey CHORD, can uncover a strong lensing event for a string tension of $G\mu \simeq 10^{-7}$. Larger FRB surveys, such as Phase 2 of the Square Kilometre Array (SKA), have the potential to significantly improve the sensitivity on the string tension to $G\mu \simeq 10^{-9}$., Comment: 11 pages, 5 figures

If dark matter (DM) is millicharged or darkly charged, collective plasma processes may dominate momentum exchange over direct particle collisions. Plasma streaming instabilities can couple the momentum of the DM to counter-streaming baryons or other DM and result in the counter-streaming fluids coming to rest with each other, just as happens for baryonic collisionless shocks in astrophysical systems. While electrostatic plasma instabilities are highly suppressed by Landau damping when DM is millicharged, in the cosmological situations of interest, electromagnetic instabilities such as the Weibel can couple momenta, assuming that the linear instability saturates in the manner typically found for baryonic plasmas. We find that the streaming of DM in the pre-Recombination universe is affected more strongly by direct collisions than collective processes, validating previous constraints. However, when considering magnetized Weibel and Firehose instabilities, the properties of the Bullet Cluster merger are likely to be substantially altered if $[q_\chi/m_\chi] \gtrsim 10^{-12}-10^{-11}$, where $[q_\chi/m_\chi]$ is the charge-to-mass ratio of the DM relative to that of the proton. The Weibel growth rates are even faster in the case of a dark-$U(1)$ charge, potentially ruling out $[q_\chi/m_\chi] \gtrsim 10^{-14}$ in the Bullet Cluster system, in agreement with previous work. The strongest previous limits on millicharged DM (mDM) arise from considering the spin-down of galactic disks. We show that plasma instabilities or tangled background magnetic fields could lead to diffusive propagation of the DM, weakening these spin-down limits. Thus, plasma instabilities may place some of the most stringent constraints over much of the millicharged, and our results corroborate previous extremely stringent potential constraints on the dark-charged parameter space.

Motivated by integral field units (IFUs) on large ground telescopes and proposals for ultraviolet-sensitive space telescopes to probe circumgalactic medium (CGM) emission, we survey the most promising emission lines and how such observations can inform our understanding of the CGM and its relation to galaxy formation. We tie our emission estimates to both HST/COS absorption measurements of ions around $z\approx 0.2$ Milky Way mass halos and models for the density and temperature of gas. We also provide formulas that simplify extending our estimates to other samples and physical scenarios. We find that OIII 5007 A and NII 6583 A, which at fixed ionic column density are primarily sensitive to the thermal pressure of the gas they inhabit, may be detectable with KCWI and especially IFUs on 30 m telescopes out to half a virial radius. OV 630 A and OVI $1032,1038$ A are perhaps the most promising ultraviolet lines, with models predicting intensities $>100~\gamma$ cm$^{-2}$ s$^{-1}$ sr$^{-1}$ in the inner 100 kpc of Milky Way-like systems. A detection of OVI would confirm the collisionally ionized picture and constrain the density profile of the CGM. Other ultraviolet metal lines constrain the amount of gas that is actively cooling and mixing. We find that CIII 978 A and CIV 1548 A may be detectable if an appreciable fraction of the observed OVI column is associated with mixing or cooling gas. H$\alpha$ emission within $100\;$kpc of Milky Way-like galaxies is within reach of current IFUs even for the minimum signal from ionizing background fluorescence, while Hydrogen $n>2$ Lyman-series lines are too weak to be detectable., Comment: 22 pages; accepted to MNRAS; comments welcome; line emission data used to make Fig.1 is included in the source files

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., Comment: 13 pages, 6 figures, accepted to ApJ

Becker et al. 2021 measured the mean free path of Lyman limit photons in the IGM at $z=6$. The short value suggests that absorptions may have played a prominent role in reionization. Here we study physical properties of ionizing photon sinks in the wake of ionization fronts (I-fronts) using radiative hydrodynamic simulations. We quantify the contributions of gaseous structures to the Lyman limit opacity by tracking the column density distributions in our simulations. Within $\Delta t = 10$ Myr of I-front passage, we find that self-shielding systems ($N_{\rm HI} > 10^{17.2}$ cm$^{-2}$) are comprised of two distinct populations: (1) over-density $\Delta \sim 50$ structures in photo-ionization equilibrium with the ionizing background; (2) $\Delta \gtrsim 100$ density peaks with fully neutral cores. The self-shielding systems contribute more than half of the opacity at these times, but the IGM evolves considerably in $\Delta t \sim 100$ Myr as structures are flattened by pressure smoothing and photoevaporation. By $\Delta t = 300$ Myr, they contribute $\lesssim 10 \%$ to the opacity in an average 1 Mpc$^3$ patch of the Universe. The percentage can be a factor of a few larger in over-dense patches, where more self-shielding systems survive. We quantify the characteristic masses and sizes of self-shielding structures. Shortly after I-front passage, we find $M=10^{4} - 10^8$ M$_\odot$ and effective diameters $d_{\rm eff} = 1 - 20$ ckpc$/h$. These scales increase as the gas relaxes. The picture herein presented may be different in dark matter models with suppressed small-scale power., Comment: 19 pages, 11 figures, 1 table submitted to ApJ

The Lyman-alpha forest is the large-scale structure probe for which we appear to have modeling control to the highest wavenumbers. This makes the Lyman-alpha forest of great interest for constraining the warmness/fuzziness of dark matter and the timing of reionization processes. However, the standard statistic, the Lyman-alpha forest power spectrum, is unable to strongly constrain the IGM temperature-density relation, and this inability further limits how well other high wavenumber-sensitive parameters can be constrained. With the aim of breaking these degeneracies, we measure the power spectrum of the Lyman-beta forest and its cross correlation with the coeval Lyman-alpha forest using the one hundred spectra of z=3.5-4.5 quasars in the VLT/X-Shooter XQ-100 Legacy Survey, motivated by the Lyman-beta transition's smaller absorption cross section that makes it sensitive to somewhat higher densities relative to the Lyman-alpha transition. Our inferences from this measurement for the IGM temperature-density relation appear to latch consistently onto the recent tight lower-redshift Lyman-alpha forest constraints. The z=3.4-4.7 trends we find using the Lyman-alpha$-$Lyman-beta cross correlation show a flattening of the slope of the temperature-density relation with decreasing redshift. This is the trend anticipated from ongoing HeII reionization and there being sufficient time to reach the asymptotic temperature-density slope after hydrogen reionization completes. Furthermore, our measurements provide a consistency check on IGM models that explain the Lyman-alpha forest, with the cross correlation being immune to systematics that are uncorrelated between the two forests, such as metal line contamination.

The first generation (Pop-III) stars can ionize 1-10% of the universe by $z=15$, when the metal-enriched (Pop-II) stars may contribute negligibly to the ionization. This low ionization tail might leave detectable imprints on the large-scale CMB E-mode polarization. However, we show that physical models for reionization are unlikely to be sufficiently extended to detect any parameter beyond the total optical depth through reionization. This result is driven in part by the total optical depth inferred by Planck, indicating a reionization midpoint around $z=8$, which in combination with the requirement that reionization completes by $z\approx 5.5$ limits the amplitude of an extended tail. To demonstrate this, we perform semi-analytic calculations of reionization including Pop-III star formation in minihalos with Lyman-Werner feedback. We find that standard Pop-III models need to produce very extended reionization at $z>15$ to be distinguishable at 2-$\sigma$ from Pop-II-only models, assuming a cosmic variance-limited measurement of the low-$\ell$ EE power spectrum. However, we show that unless there is (1) a late-time quenching mechanism such as from strong X-ray feedback or (2) some other extreme Pop-III scenario, structure formation makes it quite challenging to produce high enough Thomson scattering optical depth from $z>15$, $\tau(z>15)$, and still be consistent with other observational constraints on reionization., Comment: 11 pages 4 figures + appendix

The axion, motivated as a solution to the strong CP problem, is also a viable dark matter candidate. We use N-body simulations to study the formation of substructures from white-noise density fluctuations. The density profiles of our relaxed axion minihalos can be described by the Navarro-Frenk-White profile, and the minihalos' concentration number agrees well with a simple, physically-motivated model. We develop a semi-analytic formula to fit the mass function from our simulation, which agrees broadly at different redshifts and only differs at factor of two level from classic halo mass functions. This analytic mass function allows us to consider uncertainties in the post-inflation axion scenario, as well as extrapolate our high-redshift simulations results to the present. Our work estimates the present-day abundance of axion substructures, as is necessary for predicting their effect on cosmological microlensing caustics and pulsar timing. Our calculations suggest that if pulsar timing and microlensing probes can reach recent sensitivity forecasts, they may be sensitive to the post-inflation axion dark matter scenario, even when accounting for uncertainties pertaining to axion strings. For pulsar timing, the most significant caveat is whether axion minihalos are disrupted by stars, which our estimates show is mildly important at the most relevant masses. Finally, as our gravitational simulations are scale invariant, the results can be extended to models where the dark matter is comprised of other axion-like particles and even clusters of primordial black holes., Comment: 19 pages, 11 figures

Cosmological simulations of reionization often treat radiative transfer by solving for the monopole and dipoles of the intensity field and by making ansatz for the quadrupole moments to close the system of equations. We investigate the accuracy of the most common closure methods, i.e. Eddington tensor choices. We argue that these algorithms are most likely to err after reionization and study quasi-analytic test problems mimicking these situations: large-scale post-reionization ionizing background fluctuations and radiative transfer in a predominantly ionized medium with discrete absorbers. We show that OTVET and M1 over-ionize self-shielding absorbers when fixing the background photoionization rate, leading to 30-40% higher emissivity to balance the increased recombination. This over-ionization results in a simulation run with these algorithms having a factor of ~2 lower average metagalactic photoionization rate relative to truth given an ionizing emissivity. Furthermore, these algorithms are unlikely to reproduce ionizing background fluctuations on scales below the photon mean path: OTVET tends to overpredict the fluctuations there when the simulation box is smaller than twice the mean free path and underpredict otherwise, while M1 drastically underpredicts these fluctuations. As a result, these numerical methods are likely not sufficiently accurate to interpret the Ly$\alpha$ forest opacity fluctuations observed after reionzation. We show that a high number of angular directions need to be followed to capture the post-reionization ionizing background fluctuations accurately with ray-tracing codes. Lastly, we argue that the strong dependence of the post-reionization ionizing background on the value of the reduced speed of light found in many simulations signals that the ionizing photon mean free path is several times larger in such simulations than the observationally measured value., Comment: revision 1, submitted to JCAP

We investigate the amount of primordial information that can be reconstructed from spectroscopic galaxy surveys, as well as what sets the noise in reconstruction at low wavenumbers, by studying a simplified universe in which galaxies are the Zeldovich displaced Lagrangian peaks in the linear density field. For some of this study, we further take an intuitive linearized limit in which reconstruction is a convex problem but where the solution is also a solution to the full nonlinear problem, a limit that bounds the effectiveness of reconstruction. The linearized reconstruction results in similar cross correlation coefficients with the linear input field as our full nonlinear algorithm. The linearized reconstruction also produces similar cross correlation coefficients to those of reconstruction attempts on cosmological N-body simulations, which suggests that existing reconstruction algorithms are extracting most of the accessible information. Our approach helps explain why reconstruction algorithms accurately reproduce the initial conditions up to some characteristic wavenumber, at which point there is a quick transition to almost no correlation. This transition is set by the number of constraints on reconstruction (the number of galaxies in the survey) and not by where shot noise surpasses the clustering signal, as is traditionally thought. We further show that on linear scales a mode can be reconstructed with precision well below the shot noise expectation if the galaxy Lagrangian displacements can be sufficiently constrained. We provide idealized examples of nonlinear reconstruction where shot noise can be outperformed., Comment: 31 JCAP pages; typos corrected and changes to S4.2.2; comments welcome

We examine the impact of baryon-dark matter relative velocities on intergalactic small-scale structure and the 21 cm signal during reionization. Streaming velocities reduced clumping in the intergalactic medium (IGM) on mass scales of $\sim 10^4 - 10^8$ M$_{\odot}$. This effect produced a distinct baryon acoustic oscillation (BAO) feature in the 21 cm power spectrum at wave numbers $k\sim 0.1$ h/Mpc, near which forthcoming surveys will be most sensitive. In contrast to the highly uncertain impact of streaming velocities on star formation, the effect on clumping is better constrained because it is set mainly by cosmology and straightforward gas dynamics. We quantify the latter using coupled radiation-hydrodynamic simulations that capture the Jeans scale of pre-reionization gas. The clumping factor of ionized gas is reduced by 5-10\% in regions with RMS streaming velocities. The suppression peaks $\approx 5$ Myr after a region is reionized, but disappears within 200 Myr due to pressure smoothing. We model the corresponding impact on the 21 cm signal and find that the BAO feature is most likely to appear at $\approx$ 10 \% ionization. During this phase, the feature may appear at the 1 \% (5 \%) level at $k \sim 0.1 (0.06)$ h/Mpc with an amplitude that varies by a factor of $< 10$ across a range of reionization histories. We also provide a model for the signal originating from streaming velocity's impact on ionizing sources, which can vary by 4 orders of magnitude depending on highly uncertain source properties. We find that the clumping signal probably dominates the source one unless Population III star formation in $10^6 - 10^8$ M$_{\odot}$ halos contributed significantly to the first 10\% of reionization., Comment: 28 pages, 12 figures

The intergalactic medium is expected to clump on scales down to $10^4-10^8$ M$_{\odot}$ before the onset of reionization. The impact of these small-scale structures on reionization is poorly understood despite the modern understanding that gas clumpiness limits the growth of H II regions. We use a suite of radiation-hydrodynamics simulations that capture the $\sim 10^4$ $M_\odot$ Jeans mass of unheated gas to study density fluctuations during reionization. Our simulations track the complex ionization and hydrodynamical response of gas in the wake of ionization fronts. The clumping factor of ionized gas (proportional to the recombination rate) rises to a peak value of $5-20$ approximately $\Delta t = 10$ Myr after ionization front passage, depending on the incident intensity, redshift, and degree to which the gas had been pre-heated by the first X-ray sources. The clumping factor reaches its relaxed value of $\approx 3$ by $\Delta t = 300$ Myr. The mean free path of Lyman-limit photons evolves in unison, being up to several times shorter in un-relaxed, recently reionized regions compared to those that were reionized much earlier. Assessing the impact of this response on the global reionizaton process, we find that un-relaxed gaseous structures boost the total number of recombinations by $\approx 50$ % and lead to spatial fluctuations in the mean free path that persist appreciably for several hundred million years after the completion of reionization., Comment: Matches published version

Many works have concentrated on the observable signatures of the dark matter being an ultralight axion-like particle (ALP). We concentrate on a particularly dramatic signature in the late-time cosmological matter power spectrum that occurs if the symmetry breaking that establishes the ALP happens after inflation -- white-noise density fluctuations that dominate at small scales over the adiabatic fluctuations from inflation. These fluctuations alter the early history of nonlinear structure formation. We find that for symmetry breaking scales of $f_A \sim 10^{13}-10^{15}$GeV, which requires a high effective maximum temperature after inflation, ALP dark matter with particle mass of $m_A \sim 10^{-13}-10^{-20}$eV could significantly change the number of high-redshift dwarf galaxies, the reionization history, and the Ly$\alpha$ forest. We consider all three observables. We find that the Ly$\alpha$ forest is the most constraining of current observables, excluding $f_A \gtrsim 10^{15}$GeV ($m_A \lesssim 10^{-17}$eV) in the simplest model for the ALP and considerably lower values in models coupled to a hidden strongly interacting sector ($f_A \gtrsim 10^{13}$GeV and $m_A \lesssim 10^{-13}$eV). Observations that constrain the extremely high-redshift tail of reionization may disfavor similar levels of isocurvature fluctuations as the forest. Future $z\sim 20-30$ 21cm observations have the potential to improve these constraints further using that the supersonic motions of the isocurvature-enhanced abundance of $\sim10^4M_\odot$ halos would shock heat the baryons, sourcing large BAO features., Comment: 16 pages, 8 figures; published in PRD

Present-day galaxies are surrounded by cool and enriched halo gas extending to hundreds of kiloparsecs. This halo gas is thought to be the dominant reservoir of material available to fuel future star formation, but direct constraints on its mass and physical properties have been difficult to obtain. We report the detection of a fast radio burst (FRB 181112) with arcsecond precision, which passes through the halo of a foreground galaxy. Analysis of the burst shows the halo gas has low net magnetization and turbulence. Our results imply predominantly diffuse gas in massive galactic halos, even those hosting active supermassive black holes, contrary to some previous results., Comment: Published in Science on 2019 September 26; Main (3 figures; 1 Table) + Supp (12 figures; 7 Tables)

Reionization leads to large spatial fluctuations in the intergalactic temperature that can persist well after its completion. We study the imprints of such fluctuations on the $z\sim5$ Ly$\alpha$ forest flux power spectrum using a set of radiation-hydrodynamic simulations that model different reionization scenarios. We find that large-scale coherent temperature fluctuations bring $\sim20-60\%$ extra power at $k\sim0.002$ s/km, with the largest enhancements in the models where reionization is extended or ends the latest. On smaller scales ($k\gtrsim0.1$ s/km), we find that temperature fluctuations suppress power by $\lesssim10\%$. We find that the shape of the power spectrum is mostly sensitive to the reionization midpoint rather than temperature fluctuations from reionization's patchiness. However, for all of our models with reionization midpoints of $z\le 8$ ($z\le 12$) the shape differences are $\lesssim20\%$ ($\lesssim40\%$) because of a surprisingly well-matched cancellation between thermal broadening and pressure smoothing that occurs for realistic thermal histories. We also consider fluctuations in the ultraviolet background, finding their impact on the power spectrum to be much smaller than temperature fluctuations at $k\gtrsim0.01$ s/km. Furthermore, we compare our models to power spectrum measurements, finding that none of our models with reionization midpoints of $z<8$ is strongly preferred over another and that all of our models with midpoints of $z\geq8$ are excluded at $2.5\sigma$. Future measurements may be able to distinguish between viable reionization models if they can be performed at lower $k$ or, alternatively, if the error bars on the high-$k$ power can be reduced by a factor of $1.5$., Comment: accepted by MNRAS

We present the science case, reference design, and project plan for the Stage-4 ground-based cosmic microwave background experiment CMB-S4., Comment: 287 pages, 82 figures

The epoch of reionization, when photons from early galaxies ionized the intergalactic medium about a billion years after the Big Bang, is the last major phase transition in the Universe's history. Measuring the characteristics of the transition is important for understanding early galaxies and the cosmic web and for modeling dwarf galaxies in the later Universe. But such measurements require probes of the intergalactic medium itself. Here we describe how the 21-cm line of neutral hydrogen provides a powerful probe of the reionization process and therefore important constraints on both the galaxies and intergalactic absorbers at that time. While existing experiments will make precise statistical measurements over the next decade, we argue that improved 21-cm analysis techniques - allowing imaging of the neutral gas itself - as well as improved theoretical models, are crucial for testing our understanding of this important era., Comment: 8 pages, 4 figures

The early phases of galaxy formation constitute one of the most exciting frontiers in astrophysics. It is during this era that the first luminous sources reionize the intergalactic medium - the moment when structure formation affects every baryon in the Universe. Here we argue that we will obtain a complete picture of this era by combining observations of galaxies with direct measurements of the reionization process: the former will provide a detailed understanding of bright sources, while the latter will constrain the (substantial) faint source population. We further describe how optimizing the comparison of these two measurements requires near-infrared galaxy surveys covering large volumes and retaining redshift information and also improvements in 21-cm analysis, moving those experiments into the imaging regime., Comment: 9 pages, 3 figures

Galaxies evolve under the influence of gas flows between their interstellar medium and their surrounding gaseous halos known as the circumgalactic medium (CGM). The CGM is a major reservoir of galactic baryons and metals, and plays a key role in the long cycles of accretion, feedback, and recycling of gas that drive star formation. In order to fully understand the physical processes at work within galaxies, it is therefore essential to have a firm understanding of the composition, structure, kinematics, thermodynamics, and evolution of the CGM. In this white paper we outline connections between the CGM and galactic star formation histories, internal kinematics, chemical evolution, quenching, satellite evolution, dark matter halo occupation, and the reionization of the larger-scale intergalactic medium in light of the advances that will be made on these topics in the 2020s. We argue that, in the next decade, fundamental progress on all of these major issues depends critically on improved empirical characterization and theoretical understanding of the CGM. In particular, we discuss how future advances in spatially-resolved CGM observations at high spectral resolution, broader characterization of the CGM across galaxy mass and redshift, and expected breakthroughs in cosmological hydrodynamic simulations will help resolve these major problems in galaxy evolution., Comment: Astro2020 Decadal Science White Paper

The hot dense environment of the early universe is known to have produced large numbers of baryons, photons, and neutrinos. These extreme conditions may have also produced other long-lived species, including new light particles (such as axions or sterile neutrinos) or gravitational waves. The gravitational effects of any such light relics can be observed through their unique imprint in the cosmic microwave background (CMB), the large-scale structure, and the primordial light element abundances, and are important in determining the initial conditions of the universe. We argue that future cosmological observations, in particular improved maps of the CMB on small angular scales, can be orders of magnitude more sensitive for probing the thermal history of the early universe than current experiments. These observations offer a unique and broad discovery space for new physics in the dark sector and beyond, even when its effects would not be visible in terrestrial experiments or in astrophysical environments. A detection of an excess light relic abundance would be a clear indication of new physics and would provide the first direct information about the universe between the times of reheating and neutrino decoupling one second later., Comment: 5 pages + references; 1 figure; science white paper submitted to the Astro2020 decadal survey

The discovery of Fast Radio Bursts (FRBs) at cosmological distances has opened a powerful window on otherwise unseen matter in the Universe. In the 2020s, observations of $>10^{4}$ FRBs will assess the baryon contents and physical conditions in the hot/diffuse circumgalactic, intracluster, and intergalactic medium, and test extant compact-object dark matter models., Comment: Science white paper submitted to the Astro2020 Decadal Survey. 15 pages, 3 color figures

Achieving a precise understanding of galaxy formation in a cosmological context is one of the great challenges in theoretical astrophysics, due to the vast range of spatial scales involved in the relevant physical processes. Observations in the millimeter bands, particularly those using the cosmic microwave background (CMB) radiation as a "backlight", provide a unique probe of the thermodynamics of these processes, with the capability to directly measure the density, pressure, and temperature of ionized gas. Moreover, these observations have uniquely high sensitivity into the outskirts of the halos of galaxies and clusters, including systems at high redshift. In the next decade, the combination of large spectroscopic and photometric optical galaxy surveys and wide-field, low-noise CMB surveys will transform our understanding of galaxy formation via these probes., Comment: Submitted to the Astro2020 Decadal Survey

Cosmic dawn and the Epoch of Reionization (EoR) are among the least explored observational eras in cosmology: a time at which the first galaxies and supermassive black holes formed and reionized the cold, neutral Universe of the post-recombination era. With current instruments, only a handful of the brightest galaxies and quasars from that time are detectable as individual objects, due to their extreme distances. Fortunately, a multitude of multi-wavelength intensity mapping measurements, ranging from the redshifted 21 cm background in the radio to the unresolved X-ray background, contain a plethora of synergistic information about this elusive era. The coming decade will likely see direct detections of inhomogenous reionization with CMB and 21 cm observations, and a slew of other probes covering overlapping areas and complementary physical processes will provide crucial additional information and cross-validation. To maximize scientific discovery and return on investment, coordinated survey planning and joint data analysis should be a high priority, closely coupled to computational models and theoretical predictions., Comment: 5 pages, 1 figure, submitted to the Astro2020 Decadal Survey Science White Paper call

Our current understanding of the Universe is established through the pristine measurements of structure in the cosmic microwave background (CMB) and the distribution and shapes of galaxies tracing the large scale structure (LSS) of the Universe. One key ingredient that underlies cosmological observables is that the field that sources the observed structure is assumed to be initially Gaussian with high precision. Nevertheless, a minimal deviation from Gaussianityis perhaps the most robust theoretical prediction of models that explain the observed Universe; itis necessarily present even in the simplest scenarios. In addition, most inflationary models produce far higher levels of non-Gaussianity. Since non-Gaussianity directly probes the dynamics in the early Universe, a detection would present a monumental discovery in cosmology, providing clues about physics at energy scales as high as the GUT scale., Comment: 5 pages + references; Submitted to the Astro2020 call for science white papers. This version: fixed author list

The cosmic dawn and epoch of reionization mark the time period in the universe when stars, galaxies, and blackhole seeds first formed and the intergalactic medium changed from neutral to an ionized one. Despite substantial progress with multi-wavelength observations, astrophysical process during this time period remain some of the least understood with large uncertainties on our existing models of galaxy, blackhole, and structure formation. This white paper outlines the current state of knowledge and anticipated scientific outcomes with ground and space-based astronomical facilities in the 2020s. We then propose a number of scientific goals and objectives for new facilities in late 2020s to mid 2030s that will lead to definitive measurements of key astrophysical processes in the epoch of reionization and cosmic dawn., Comment: Science White paper submitted to Astro2020 Decadal Survey

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., Comment: 13 pages, 8 figures, published in MNRAS

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

During reionization, the intergalactic medium is heated impulsively by supersonic ionization fronts (I-fronts). The peak gas temperatures behind the I-fronts, $T_\mathrm{reion}$, are a key uncertainty in models of the thermal history after reionization. Here we use high-resolution radiative transfer simulations to study the parameter space of $T_\mathrm{reion}$. We show that $T_\mathrm{reion}$ is only mildly sensitive to the spectrum of incident radiation over most of the parameter space, with temperatures set primarily by I-front speeds. We also explore what current models of reionization predict for $T_\mathrm{reion}$ by measuring I-front speeds in cosmological radiative transfer simulations. We find that the post-I-front temperatures evolve toward hotter values as reionization progresses. Temperatures of $T_\mathrm{reion} = 17,000-22,000$ K are typical during the first half of reionization, but $T_\mathrm{reion} = 25,000 - 30,000$ K may be achieved near the end of this process if I-front speeds reach $\sim10^4$ km/s as found in our simulations. Shorter reionization epochs lead to hotter $T_\mathrm{reion}$. We discuss implications for $z>5$ Ly$\alpha$ forest observations, which potentially include sight lines through hot, recently reionized patches of the Universe. Interpolation tables from our parameter space study are made publicly available, along with a simple fit for the dependence of $T_\mathrm{reion}$ on the I-front speed., Comment: Accepted by ApJ. Results unchanged. Added (1) tests showing that conclusions are unaltered by density fluctuations; (2) analytic discussion that provides intuition for numerical results

We present the first simulations of the high-redshift Ly$\alpha$ intensity field that account for scattering in the intergalactic medium (IGM). Using a 3D Monte Carlo radiative transfer code, we find that Ly$\alpha$ scattering smooths spatial fluctuations in the Ly$\alpha$ intensity on small scales and that the spatial dependence of this smoothing depends strongly on the mean neutral fraction of the IGM. Our simulations find a strong effect of reionization on $k=0.1-1~{\rm Mpc^{-1}}$, with $P_{\rm Ly\alpha}\propto k^{-1.75}$ for $\bar{x}_{\rm HI} = 0.63$ and $P_{\rm Ly\alpha} \propto k^{-2.2}$ for $\bar{x}_{\rm HI} = 0.86$ in contrast to $P_{\rm Ly\alpha}\propto k^{-1.5}$ after reionization. At wavenumbers of $k>1 ~ {\rm Mpc^{-1}}$, we find that the signal is sensitive to the emergent Ly$\alpha$ line profiles from galaxies. We also demonstrate that the cross-correlation between a Ly$\alpha$ intensity map and a future galaxy redshift survey could be detected on large scales by an instrument similar to SPHEREx, and over a wide range of scales by a hypothetical intensity mapping instrument in the vein of CDIM., Comment: 7 pages, 4 figures, submitted to ApJL

We develop an effective perturbation theory (and, equivalently, a bias expansion) for the inhomogeneous 21cm radiation field from reionization. Using large-scale simulations of cosmological reionization, we find that this expansion describes the modes in the simulated 21cm signal over much of the wavenumber range probed by upcoming 21cm arrays. This result provides an understanding of the potential signal shapes that compliments the nonlinear numerical modeling that has been the focus of most previous work. We find that the observable signal often can be described with 2-3 bias coefficients that can be interpreted in terms of the source biases, the average neutral fraction, the characteristic size of ionized regions, and the patchiness of reionization. The 21cm signal serves as a wacky example of a bias expansion in cosmology, with our approach synthesizing key results., Comment: 26 JCAP pages, 8 Figures; published in JCAP

We present a semi-analytic model for the Lyman-$\alpha$ forest that is inspired by the Halo Model. This model is built on the absorption line decomposition of the forest. Flux correlations are decomposed into those within each absorption line (the 1-absorber term) and those between separate lines (the 2-absorber term), treating the lines as biased tracers of the underlying matter fluctuations. While the nonlinear exponential mapping between optical depth and flux requires an infinite series of moments to calculate any statistic, we show that this series can be resumed (truncating at the desired order in the linear matter overdensity). We focus on the $z=2-3$ line-of-sight power spectrum. Our model finds that 1-absorber term dominates the power on all scales, with most of its contribution coming from HI columns of $10^{14}-10^{15}\;\mathrm{cm^{-2}}$, while the smaller 2-absorber contribution comes from lower columns that trace overdensities of a few. The prominence of the 1-absorber correlations indicates that the line-of-sight power spectrum is shaped principally by the lines' number densities and their absorption profiles, with correlations between lines contributing to a lesser extent. We present intuitive formulae for the effective optical depth as well as the large-scale limits of 1-absorber and 2-absorber terms, which simplify to integrals over the HI column density distribution with different equivalent-width weightings. With minimalist models for the bias of absorption systems and their peculiar velocity broadening, our model predicts values for the density bias and velocity gradient bias that are consistent with those found in simulations., Comment: 37 pages, 9 figures; added paragraph on comparison with previous Lyman-alpha bias models; version matches the one published in JCAP

A component of the dark matter could consist of two darkly charged particles with a large mass ratio and a massless force carrier. This `atomic' dark sector could behave much like the baryonic sector, cooling and fragmenting down to stellar-mass or smaller scales. Past studies have shown that cosmic microwave background and large-scale structure constraints rule out $\gtrsim 5\%$ of the dark matter to behave in this manner. However, we show that, even with percent level mass fractions, a dark atomic sector could affect some extragalactic and galactic observables. We track the cooling and merger history of an atomic dark component for much of the interesting parameter space. Unlike the baryons, where stellar feedback (driven by nuclear physics) delays the formation and growth of galaxies, cooling dark atomic gas typically results in disks forming earlier, leaving more time for their destruction via mergers. Rather than disks in Milky Way sized halos, we find the end product is typically spheroidal structures on galactic scales or dark atom fragments distributed on halo scales. This result contrasts with previous studies, which had assumed that the dark atoms would result in dark disks. Furthermore the dark atoms condense into dense clumps, analogous to how the baryons fragment on solar-mass scales. We estimate the size of these dark clumps, and use these estimates to show that viable atomic dark matter parameter space is ruled out by stellar microlensing, by the half-light radii of ultra-faint dwarf galaxies, and by Milky Way mass-to-light inferences., Comment: 20 pages and 7 figures, added references and fixed minor typos

Radiation in the extreme ultraviolet (EUV) and soft X-ray holds clues to the location of the missing baryons, the energetics in stellar feedback processes, and the cosmic enrichment history. Additionally, EUV and soft X-ray photons help determine the ionization state of most intergalactic and circumgalactic metals, shaping the rate at which cosmic gas cools. Unfortunately, this band is extremely difficult to probe observationally due to absorption from the Galaxy. In this paper, we model the contributions of various sources to the cosmic EUV and soft X-ray backgrounds. We bracket the contribution from (1) quasars, (2) X-ray binaries, (3) hot interstellar gas, (4) circumgalactic gas, (5) virialized gas, and (6) supersoft sources, developing models that extrapolate into these bands using both empirical and theoretical inputs. While quasars are traditionally assumed to dominate these backgrounds, we discuss the substantial uncertainty in their contribution. Furthermore, we find that hot intrahalo gases likely emit an O(1) fraction of this radiation at low redshifts, and that interstellar and circumgalactic emission potentially contribute tens of percent to these backgrounds at all redshifts. We estimate that uncertainties in the angular-averaged background intensity impact the ionization corrections for common circumgalactic and intergalactic metal absorption lines by ~0.3-1 dex, and we show that local emissions are comparable to the cosmic background only at r_prox = 10-100 kpc from Milky Way-like galaxies., Comment: 13 pages, 9 figures, accepted to ApJ. New calculation added in section 2.3

Observations reveal massive amounts of OVI around star-forming $L_*$ galaxies, with covering fractions of near unity extending to the host halo's virial radius. This OVI absorption is typically kinematically centered upon photoionized gas, with line widths that are suprathermal and kinematically offset from the galaxy. We discuss various scenarios and whether they could result in the observed phenomenology (cooling gas flows, boundary layers, shocks, virialized gas, photoionized clouds in thermal equilibrium). If predominantly collisionally ionized, as we argue is most probable, the OVI observations require that the circumgalactic medium (CGM) of $L_*$ galaxies holds nearly all the associated baryons within a virial radius ($\sim 10^{11}M_\odot$) and hosts massive flows of cooling gas with $\approx30[nT/30{\rm~cm^{-3}K}]~M_\odot~$yr$^{-1}$, which must be largely prevented from accreting onto the host galaxy. Cooling and feedback energetics considerations require $10 <\langle nT\rangle<100{\rm~cm^{-3}K}$ for the warm and hot halo gases. We argue that virialized gas, boundary layers, hot winds, and shocks are unlikely to directly account for the bulk of the OVI. Furthermore, we show that there is a robust constraint on the number density of many of the photoionized $\sim10^4$K absorption systems that yields upper bounds in the range $n<(0.1-3)\times10^{-3}(Z/0.3)$cm$^{-3}$, where $Z$ is the metallicity, suggestive that the dominant pressure in some photoionized clouds is nonthermal. This constraint, which requires minimal ionization modeling, is in accord with the low densities inferred from more complex photoionization modeling. The large amount of cooling gas that is inferred could re-form these clouds in a fraction of the halo dynamical time, as some arguments require, and it requires much of the feedback energy available from supernovae and stellar winds to be dissipated in the CGM., Comment: 16 pages, matches version accepted to ApJ

Despite decades of effort, the timing and duration of He II reionization, as well as its morphology and the properties of the quasars believed to drive it, are still not well constrained. In this paper we present a new method to study both He II reionization and quasars via the thermal proximity effect -- the photoelectric heating of the intergalactic medium around quasars when their hard radiation doubly ionizes helium. We post-process a SPH simulation with 1D radiative calculations, and study how the thermal proximity effect depends on the amount of singly ionized helium, $x_{\rm HeII,0}$, which prevailed in the IGM before the quasar turned on, and the characteristic lifetime $t_{\rm Q}$ for which quasars shine. We find that the amplitude of the temperature boost in the quasar environment depends on $x_{\rm HeII,0}$, with a characteristic value of $\Delta T \simeq 10^4\,{\rm K}$ for an initially singly ionized IGM ($x_{\rm HeII,0} = 1.0$), whereas the size of the thermal proximity zone is sensitive to quasar lifetime $t_{\rm Q}$, with typical sizes of ~100 cMpc for luminous quasars shining for $t_{\rm Q}=10^8$ yr. This temperature boost is manifest as a change in the thermal broadening of H I absorption lines near the quasar. We introduce a new method based on measuring the Ly$\alpha$ forest power spectrum as a function of distance from the quasar, and conduct Bayesian MCMC analysis to demonstrate that the thermal proximity effect should be easily detectable. For a mock dataset of 50 quasars at z~4, we predict that one can measure $x_{\rm HeII,0}$ to a precision $\approx 0.04$, and $t_{\rm Q}$ to a precision of $\approx 0.1$ dex. By applying our formalism to existing high-resolution Ly$\alpha$ forest spectra of quasars at $3.1 \lesssim z \lesssim 5.0$, one should be able to detect the thermal proximity effect, and reconstruct the full reionization history of He II., Comment: 23 pages, 16 Figures, accepted to ApJ, two sections and one figure added

Recent observations have shown that the scatter in opacities among coeval segments of the Lyman-alpha forest increases rapidly at z > 5. In this paper, we assess whether the large scatter can be explained by fluctuations in the ionizing background in the post-reionization intergalactic medium. We find that matching the observed scatter at z ~ 5.5 requires a short spatially averaged mean free path of < 15 comoving Mpc/h, a factor of > 3 shorter than direct measurements at z ~ 5.2. We argue that such rapid evolution in the mean free path is difficult to reconcile with our measurements of the global H I photoionization rate, which stay approximately constant over the interval z ~ 4.8 - 5.5. However, we also show that measurements of the mean free path at z > 5 are likely biased towards higher values by the quasar proximity effect. This bias can reconcile the short values of the mean free path that are required to explain the large scatter in opacities. We discuss the implications of this scenario for cosmological reionization. Finally, we investigate whether other statistics applied to the z > 5 Lyman-alpha forest can shed light on the origin of the scatter. Compared to a model with a uniform ionizing background, models that successfully account for the scatter lead to enhanced power in the line-of-sight flux power spectrum on scales k < 0.1 h/Mpc. We find tentative evidence for this enhancement in observations of the high-redshift Lyman-alpha forest., Comment: Matches version published by MNRAS with clarifications and expanded discussion