arXiv subjects

Steven Furlanetto

Publications and source records attributed to Steven Furlanetto.

At least 55 records · Page 3Linked to original sources

arXiv2007

Reionization and the large-scale 21 cm-cosmic microwave background cross correlation

Of the many probes of reionization, the 21 cm line and the cosmic microwave background (CMB) are among the most effective. We examine how the cross-correlation of the 21 cm brightness and the CMB Doppler fluctuations on large angular scales can be used to study this epoch. We employ a new model of the growth of large scale fluctuations of the ionized fraction as reionization proceeds. We take into account the peculiar velocity field of baryons and show that its effect on the cross correlation can be interpreted as a mixing of Fourier modes. We find that the cross-correlation signal is strongly peaked toward the end of reionization and that the sign of the correlation should be positive because of the inhomogeneity inherent to reionization. The signal peaks at degree scales (l~100) and comes almost entirely from large physical scales (k~0.01 Mpc). Since many of the foregrounds and noise that plague low frequency radio observations will not correlate with CMB measurements, the cross correlation might appear to provide a robust diagnostic of the cosmological origin of the 21 cm radiation around the epoch of reionization. Unfortunately, we show that these signals are actually only weakly correlated and that cosmic variance dominates the error budget of any attempted detection. We conclude that the detection of a cross-correlation peak at degree-size angular scales is unlikely even with ideal experiments.

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arXiv2007

Efficient Simulations of Early Structure Formation and Reionization

We present a method to construct semi-numerical ``simulations'', which can efficiently generate realizations of halo distributions and ionization maps at high redshifts. Our procedure combines an excursion-set approach with first-order Lagrangian perturbation theory and operates directly on the linear density and velocity fields. As such, the achievable dynamic range with our algorithm surpasses the current practical limit of N-body codes by orders of magnitude. This is particularly significant in studies of reionization, where the dynamic range is the principal limiting factor. We test our halo-finding and HII bubble-finding algorithms independently against N-body simulations with radiative transfer and obtain excellent agreement. We compute the size distributions of ionized and neutral regions in our maps. We find even larger ionized bubbles than do purely analytic models at the same volume-weighted mean hydrogen neutral fraction. We also generate maps and power spectra of 21-cm brightness temperature fluctuations, which for the first time include corrections due to gas bulk velocities. We find that velocities widen the tails of the temperature distributions and increase small-scale power, though these effects quickly diminish as reionization progresses. We also include some preliminary results from a simulation run with the largest dynamic range to date: a 250 Mpc box that resolves halos with masses M >~ 2.2 x10^8 M_sun. We show that accurately modeling the late stages of reionization requires such large scales. The speed and dynamic range provided by our semi-numerical approach will be extremely useful in the modeling of early structure formation and reionization.

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arXiv2007

Spin Exchange Rates in Proton-Hydrogen Collisions

The spin temperature of neutral hydrogen, which determines the optical depth and brightness of the 21 cm line, is determined by the competition between radiative and collisional processes. Here we examine the role of proton-hydrogen collisions in setting the spin temperature. We use recent fully quantum mechanical calculations of the relevant cross sections, which allow us to present accurate results over the entire physically relevant temperature range 1-10,000 K. For kinetic temperatures T_K>100 K, the proton-hydrogen rate coefficient exceeds that for hydrogen-hydrogen collisions by about a factor of two. However, at low temperatures (T_K < 5 K) H-p collisions become several thousand times more efficient than H-H and even more important than H-e^- collisions.

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arXiv2006

The Cross-Correlation of High-Redshift 21 cm and Galaxy Surveys

We study the detectability of the cross-correlation between 21 cm emission from the intergalactic medium and the galaxy distribution during (and before) reionization. We show that first-generation 21 cm experiments, such as the Mileura Widefield Array (MWA), can measure the cross-correlation to a precision of several percent on scales k~0.1/Mpc if combined with a deep galaxy survey detecting all galaxies with m>10^10 Msol over the entire ~800 square degree field of view of the MWA. LOFAR can attain even better limits with galaxy surveys covering its ~50 square degree field of view. The errors on the cross-power spectrum scale with the square root of the overlap volume, so even reasonably modest surveys of several square degrees should yield a positive detection with either instrument. In addition to the obvious scientific value, the cross-correlation has four key advantages over the 21 cm signal alone: (1) its signal-to-noise exceeds that of the 21 cm power spectrum by a factor of several, allowing it to probe smaller spatial scales and perhaps to detect inhomogeneous reionization more efficiently; (2) it allows a cleaner division of the redshift-space distortions (although only if the galaxy redshifts are known precisely); (3) by correlating with the high-redshift galaxy population, the cosmological nature of the 21 cm fluctuations can be determined unambiguously; and (4) the required level of foreground cleaning for the 21 cm signal is vastly reduced.

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arXiv2006

Cosmology at Low Frequencies: The 21 cm Transition and the High-Redshift Universe

Observations of the high-redshift Universe with the 21 cm hyperfine line of neutral hydrogen promise to open an entirely new window onto the early phases of cosmic structure formation. Here we review the physics of the 21 cm transition, focusing on processes relevant at high redshifts, and describe the insights to be gained from such observations. These include measuring the matter power spectrum at z~50, observing the formation of the cosmic web and the first luminous sources, and mapping the reionization of the intergalactic medium. The epoch of reionization is of particular interest, because large HII regions will seed substantial fluctuations in the 21 cm background. We also discuss the experimental challenges involved in detecting this signal, with an emphasis on the Galactic and extragalactic foregrounds. These increase rapidly toward low frequencies and are especially severe for the highest redshift applications. Assuming that these difficulties can be overcome, the redshifted 21 cm line will offer unique insight into the high-redshift Universe, complementing other probes but providing the only direct, three-dimensional view of structure formation from z~200 to z~6.

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arXiv2006

The Effects of Dark Matter Decay and Annihilation on the High-Redshift 21 cm Background

The radiation background produced by the 21 cm spin-flip transition of neutral hydrogen at high redshifts can be a pristine probe of fundamental physics and cosmology. At z~30-300, the intergalactic medium (IGM) is visible in 21 cm absorption against the cosmic microwave background (CMB), with a strength that depends on the thermal (and ionization) history of the IGM. Here we examine the constraints this background can place on dark matter decay and annihilation, which could heat and ionize the IGM through the production of high-energy particles. Using a simple model for dark matter decay, we show that, if the decay energy is immediately injected into the IGM, the 21 cm background can detect energy injection rates >10^{-24} eV cm^{-3} sec^{-1}. If all the dark matter is subject to decay, this allows us to constrain dark matter lifetimes <10^{27} sec. Such energy injection rates are much smaller than those typically probed by the CMB power spectra. The expected brightness temperature fluctuations at z~50 are a fraction of a mK and can vary from the standard calculation by up to an order of magnitude, although the difference can be significantly smaller if some of the decay products free stream to lower redshifts. For self-annihilating dark matter, the fluctuation amplitude can differ by a factor <2 from the standard calculation at z~50. Note also that, in contrast to the CMB, the 21 cm probe is sensitive to both the ionization fraction and the IGM temperature, in principle allowing better constraints on the decay process and heating history. We also show that strong IGM heating and ionization can lead to an enhanced H_2 abundance, which may affect the earliest generations of stars and galaxies.

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arXiv2006

Spin Exchange Rates in Electron-Hydrogen Collisions

The spin temperature of neutral hydrogen, which determines the 21 cm optical depth and brightness temperature, is set by the competition between radiative and collisional processes. In the high-redshift intergalactic medium, the dominant collisions are typically those between hydrogen atoms. However, collisions with electrons couple much more efficiently to the spin state of hydrogen than do collisions with other hydrogen atoms and thus become important once the ionized fraction exceeds ~1%. Here we compute the rate at which electron-hydrogen collisions change the hydrogen spin. Previous calculations included only S-wave scattering and ignored resonances near the n=2 threshold. We provide accurate results, including all partial wave terms through the F-wave, for the de-excitation rate at temperatures T_K < 15,000 K; beyond that point, excitation to n>=2 hydrogen levels becomes significant. Accurate electron-hydrogen collision rates at higher temperatures are not necessary, because collisional excitation in this regime inevitably produces Lyman-alpha photons, which in turn dominate spin exchange when T_K > 6200 K even in the absence of radiative sources. Our rates differ from previous calculations by several percent over the temperature range of interest. We also consider some simple astrophysical examples where our spin de-excitation rates are useful.

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arXiv2006

The Scattering of Lyman-series Photons in the Intergalactic Medium

We re-examine scattering of photons near the Lyman-alpha resonance in the intergalactic medium (IGM). We first derive a general integral solution for the radiation field around resonance when spin diffusivity is ignored. Our solution shows explicitly that recoil sources an absorption feature, whose magnitude increases with the relative importance of recoil compared to Doppler broadening. This spectrum depends on the Lyman-alpha line profile, but approximating it with the absorption profile appropriate to the Lorentzian wings of natural broadening accurately reproduces the results for a full Voigt profile so long as T<1000 K in the IGM. This approximation allows us to obtain simple analytic formulae for the total scattering rate of Lyman-alpha photons and the accompanying energy exchange rate. Our power series solutions converge rapidly for photons that redshift into the Lyman-alpha resonance as well as for photons injected at line center. We confirm previous calculations showing that heating through this mechanism is quite slow and probably negligible compared to other sources. We then show that energy exchange during the scattering of higher-order Lyman-series photons can be much more important than naively predicted by recoil arguments. However, the resulting heating is still completely negligible.

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arXiv2006

The 21 Centimeter Forest

We examine the prospects for studying the pre-reionization intergalactic medium (IGM) through the so-called 21 cm forest in spectra of bright high-redshift radio sources. We first compute the evolution of the mean optical depth for models that include X-ray heating of the IGM gas, Wouthuysen-Field coupling, and reionization. Under most circumstances, the spin temperature T_S grows large well before reionization begins in earnest. As a result, the optical depth is less than 0.001 throughout most of reionization, and background sources must sit well beyond the reionization surface in order to experience measurable absorption. HII regions produce relatively large "transmission gaps" and may therefore still be observable during the early stages of reionization. Absorption from sheets and filaments in the cosmic web fades once T_S becomes large and should be rare during reionization. Minihalos can produce strong (albeit narrow) absorption features. Measuring their abundance would yield useful limits on the strength of feedback processes in the IGM as well as their effect on reionization.

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arXiv2006

Redshifted 21 Centimeter Emission from Minihalos Before Reionization

Before reionization, the intergalactic medium (IGM) may have been sufficiently cold for low-mass "minihalos" to condense out of the gas and subsequently affect reionization. Previous work has shown that minihalos generate reasonably large 21 cm fluctuations. Here we consider this signal in its proper cosmological context and show that isolating minihalos from the rest of the IGM is extremely difficult. Using the well-known halo model, we compute the power spectrum of 21 cm fluctuations from minihalos and show that the signal decreases rapidly as feedback increases the Jeans mass. We then show that even a small Lyman-alpha background increases the 21 cm fluctuations of the diffuse IGM well beyond those of the minihalos; because the mass fraction in the IGM is much larger, minihalos will lie buried within the IGM signal. The distinctive signatures of non-linear bias and minihalo structure emerge only at much smaller scales, well beyond the resolution of any upcoming instruments. Using simple, but representative, reionization histories, we then show that the required Lyman-alpha background level is most likely achieved at z>15, while minihalos are still rare, so that they are almost always degenerate with the diffuse IGM.

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arXiv2006

The Global 21 Centimeter Background from High Redshifts

We consider the evolution of the sky-averaged 21 cm background during the early phases of structure formation. Using simple analytic models, we calculate the thermal and ionization histories, assuming that stellar photons dominate the radiation background. The resulting 21 cm spectra can constrain the properties of the first generations of stars and quasars. If Population II stars dominate, Lyman-alpha coupling renders the IGM visible before it is heated by X-rays and long before reionization. Thus the 21 cm background has a strong absorption epoch followed by weaker emission that fades during reionization. The harder spectra of very massive Population III stars compress these transitions into a shorter time interval and decreases the signal amplitude. However, the reionization epoch will remain visible except in extreme cases. The global 21 cm signal will be challenging to observe because of astronomical foregrounds, but it offers an exciting opportunity to study the first sources of light. It also fixes the overall amplitude of the fluctuating background whose detection is a major goal of several next-generation low-frequency radio interferometers.

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arXiv2005

The Evidence of Absence: Galaxy Voids in the Excursion Set Formalism

We present an analytic model for the sizes of voids in the galaxy distribution. Peebles and others have recently emphasized the possibility that the observed characteristics of voids may point to a problem in galaxy formation models, but testing these claims has been difficult without any clear predictions for their properties. In order to address such questions, we build a model to describe the distribution of galaxy underdensities. Our model is based on the "excursion set formalism," the same technique used to predict the dark matter halo mass function. We find that, because of bias, galaxy voids are typically significantly larger than dark matter voids and should fill most of the universe. We show that voids selected from catalogs of luminous galaxies should be larger than those selected from faint galaxies: the characteristic radii range from ~5-10h^{-1} Mpc for galaxies with absolute r-band magnitudes M_r-5 log(h) < -16 to -20. These are reasonably close to, though somewhat smaller than, the observed sizes. The discrepancy may result from the void selection algorithm or from their internal structure. We also compute the halo populations inside voids. We expect small haloes (M<10^{11} Msun) to be up to a factor of two less underdense than the haloes of normal galaxies. Within large voids, the mass function is nearly independent of the size of the underdensity, but finite-size effects play a significant role in small voids (R<7h^{-1} Mpc).

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arXiv2005

Highly-Ionized Oxygen Absorbers in the Intergalactic Medium

Recent ultraviolet and X-ray observations of intergalactic OVI and OVII absorption systems along lines of sight to bright quasars have opened a new window onto the ``warm-hot intergalactic medium'' (WHIM). These systems appear to provide a significant reservoir for baryons in the local universe, and comparison to cosmological simulations suggests that their abundance roughly matches theoretical predictions. Here we use analytic arguments to elucidate the physical properties of the absorbers and their role in structure formation. We first show that if the absorbers result from structure-formation shocks, the observed column densities naturally follow from postshock-cooling models, if we include fast-cooling shocks as well as those that cannot cool within a Hubble time. In this case, the known OVI absorbers should show stronger OVII absorption than expected from collisional-ionization equilibrium (and much more than expected for photoionized systems). We then argue that higher-temperature shocks will be spatially associated with more massive virialized objects even well outside the virial radius. Thus the different oxygen ions will trace different structures; OVII absorbers are the most common because that ion dominates over a wide temperature range (corresponding to a large range in halo mass). If each dark-matter halo is surrounded by a network of shocks with total cross section a few times the size of the virialized systems, then we can reproduce the observed number densities of absorbers with plausible parameters. A simple comparison with simulations shows that these assumptions are reasonable, although the actual distribution of shocked gas is too complex for analytic models to describe fully. Our models suggest that these absorbers cannot be explained as a single-temperature phase.

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arXiv2005

Is Double Reionization Physically Plausible?

Recent observations of z~6 quasars and the cosmic microwave background imply a complex history to cosmic reionization. Such a history requires some form of feedback to extend reionization over a long time interval, but the nature of the feedback and how rapidly it operates remain highly uncertain. Here we focus on one aspect of this complexity: which physical processes can cause the global ionized fraction to evolve non-monotonically with cosmic time? We consider a range of mechanisms and conclude that double reionization is much less likely than a long, but still monotonic, ionization history. We first examine how galactic winds affect the transition from metal-free to normal star formation. Because the transition is actually spatially inhomogeneous and temporally extended, this mechanism cannot be responsible for double reionization given plausible parameters for the winds. We next consider photoheating, which causes the cosmological Jeans mass to increase in ionized regions and hence suppresses galaxy formation there. In this case, double reionization requires that small halos form stars efficiently, that the suppression from photoheating is strong relative to current expectations, and that ionizing photons are preferentially produced outside of previously ionized regions. Finally, we consider H_2 photodissociation, in which the buildup of a soft ultraviolet background suppresses star formation in small halos. This can in principle cause the ionized fraction to temporarily decrease, but only during the earliest stages of reionization. Finally, we briefly consider the effects of some of these feedback mechanisms on the topology of reionization.

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arXiv2004

The Temperature Structure of the Warm-Hot Intergalactic Medium

We study the temperature structure of the intergalactic medium (IGM) using a large cosmological N-body/SPH simulation. We employ a two-temperature model for the thermal evolution of the ionized gas, in which the relaxation process between electrons and ions is explicitly included. In the diffuse, hot IGM, the relaxation time is comparable to the age of the Universe and hence the electron temperature in post-shock regions remains significantly smaller than the ion temperature. We show that, at the present epoch, a large fraction of the warm/hot intergalactic medium (WHIM) has a well-developed two temperature structure, with typical temperature differences of order a factor of a few. Consequently, the fraction of metals in various ionization states such as OVI, OVII, and OVIII, as well as their line emissivities, can differ locally by more than an order of magnitude from those computed with a single temperature model. It is thus necessary to follow the evolution of the electron temperature explicitly to determine absorption and emission by the WHIM. Although equipartition is nearly achieved in the denser intracluster medium (ICM), we find an appreciable systematic deviation between the gas-mass weighted electron temperature and the mean temperature even at half the virial radii of clusters. There is thus a reservoir of warm (Te < 1keV) gas in and around massive clusters. Our results imply that relaxation processes need to be considered in describing and interpreting observational data from existing X-ray telescopes as well as from future missions designed to detect the WHIM, such as the Diffuse Intergalactic Oxygen Surveyor and the Missing Baryon Explorer.

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arXiv2004

Lyman-alpha Emission from Structure Formation

The nature of the interaction between galaxies and the intergalactic medium (IGM) is one of the most fundamental problems in astrophysics. The accretion of gas onto galaxies provides fuel for star formation, while galactic winds transform the nearby IGM in a number of ways. One exciting technique to study this gas is through the imaging of hydrogen Lyman-alpha emission. We use cosmological simulations to study the Lyman-alpha signals expected from the growth of cosmic structure from z=0-5. We show that if dust absorption is negligible, recombinations following the absorption of stellar ionizing photons dominate the total Lyman-alpha photon production rate. However, galaxies are also surrounded by "Lyman-alpha coronae" of diffuse IGM gas. These coronae are composed of a combination of accreting gas and material ejected from the central galaxy by winds. The Lyman-alpha emission from this phase is powered by a combination of gravitational processes and the photoionizing background. While the former dominates at z~0, collisional excitation following photo-heating may well dominate the total emission at higher redshifts. The central regions of these systems are dense enough to shield themselves from the metagalactic ionizing background; unfortunately, in this regime our simulations are no longer reliable. We therefore consider several scenarios for the emission from the central cores, including one in which self-shielded gas does not emit at all. We show that the combination of star formation and cooling IGM gas can explain most of the observed "Lyman-alpha blobs" at z~3, with the important exception of the largest sources. On the other hand, except under the most optimistic assumptions, cooling IGM gas cannot explain the observations on its own.

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arXiv2004

21 cm Tomography of the High-Redshift Universe with the Square Kilometer Array

We discuss the prospects for ``tomography'' of the intergalactic medium (IGM) at high redshifts using the 21 cm transition of neutral hydrogen. Existing observational constraints on the epoch of reionization imply a complex ionization history that may require multiple generations of sources. The 21 cm transition provides a unique tool to probe this era in detail, because it does not suffer from saturation effects, retains full redshift information, and directly probes the IGM gas. Observations in the redshifted 21cm line will allow one to study the history and morphology of reionization in detail. Depending on the characteristics of the first sources, they may also allow us to probe the era before reionization, when the first structures and luminous sources were forming. The construction of high signal-to-noise ratio maps on arcminute scales will require approximately one square kilometer of collecting area.

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arXiv2004

21 Centimeter Fluctuations from Cosmic Gas at High Redshifts

The relatively large Thomson optical depth, tau_e, inferred recently from the WMAP observations suggests that the Universe was reionized in a more complex manner than previously believed. However, the value of tau_e provides only an integral constraint on the history of reionization and, by itself, cannot be used to determine the nature of the sources responsible for this transition. Here, we show that the evolution of the ionization state of the intergalactic medium at high redshifts can be measured statistically using fluctuations in 21 centimeter radiation from neutral hydrogen. By analogy with the mathematical description of anisotropies in the cosmic microwave background, we develop a formalism to quantify the variations in 21 cm emission as a function of both frequency and angular scale. Prior to and following reionization, fluctuations in the 21 cm signal are mediated by density perturbations in the distribution of matter. Between these epochs, pockets of gas surrounding luminous objects become ionized, producing large HII regions. These ``bubbles'' of ionized material imprint features into the 21 cm power spectrum that make it possible to distinguish them from fluctuations produced by the density perturbations. The variation of the power spectrum with frequency can be used to infer the evolution of this process. As has been emphasized previously by others, the absolute 21 cm signal from neutral gas at high redshifts is undetectable owing to contamination by foreground sources. However, we argue that this source of noise can be suppressed by comparing maps closely spaced in frequency, i.e. redshift, so that 21 cm fluctuations from the IGM can be measured against a much brighter, but smoothly varying (in frequency) background.

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