Lyman-alpha Emitters During the Early Stages of Reionization

We investigate the potential of exploiting Lya Emitters (LAEs) to constrain the volume-weighted mean neutral hydrogen fraction of the IGM, x_H, at high redshifts (specifically z~9). We use "semi-numerical'' simulations to efficiently generate density, velocity, and halo fields at z=9 in a 250 Mpc box, resolving halos with masses M>2.2e8 solar masses. We construct ionization fields corresponding to various values of x_H. With these, we generate LAE luminosity functions and "counts-in-cell'' statistics. As in previous studies, we find that LAEs begin to disappear rapidly when x_H > 0.5. Constraining x_H(z=9) with luminosity functions is difficult due to the many uncertainties inherent in the host halo mass Lya luminosity mapping. However, using a very conservative mapping, we show that the number densities derived using the six z~9 LAEs recently discovered by Stark et al. (2007) imply x_H < 0.7. On a more fundamental level, these LAE number densities, if genuine, require substantial star formation in halos with M < 10^9 solar masses, making them unique among the current sample of observed high-z objects. Furthermore, reionization increases the apparent clustering of the observed LAEs. We show that a ``counts-in-cell'' statistic is a powerful probe of this effect, especially in the early stages of reionization. Specifically, we show that a field of view (typical of upcoming IR instruments) containing LAEs has >10% higher probability of containing more than one LAE in a x_H>0.5 universe than a x_H=0 universe with the same overall number density. With this statistic, a fully ionized universe can be robustly distinguished from one with x_H > 0.5 using a survey containing only ~ 20--100 galaxies.Comment: 14 pages, 13 figures, moderate changes to match version accepted for publication in the MNRA

Free-Free Emission at Low Radio Frequencies

We discuss free-free radio emission from ionized gas in the intergalactic medium. Because the emissivity is proportional to the square of the electron density, the mean background is strongly sensitive to the spatial clumping of free electrons. Using several existing models for the clumping of ionized gas, we find that the expected free-free distortion to the cosmic microwave background (CMB) blackbody spectrum is at a level detectable with upcoming experiments such as the Absolute Radiometer for Cosmology, Astrophysics, and Diffuse Emission (ARCADE). However, the dominant contribution to the distortion comes from clumpy gas at z < 3, and the integrated signal does not strongly constrain the epoch of reionization. In addition to the mean emission, we consider spatial fluctuations in the free-free background and the extent to which these anisotropies confuse the search for fluctuations in 21 cm line emission from neutral hydrogen during and prior to reionization. This background is smooth in frequency space and hence can be removed through frequency differencing, but only so long as the 21 cm signal and the free-free emission are uncorrelated. We show that, because the free-free background is generated primarily at low redshifts, the cross-correlation between the two fields is smaller than a few percent. Thus, multifrequency cleaning should be an effective way to eliminate the free-free confusion.Comment: 4 pages, 2 figure

Detecting the redshifted 21cm forest during reionization

The 21cm forest -- HI absorption features in the spectra of high-redshift radio sources -- can potentially provide a unique probe of the largely neutral intergalactic medium (IGM) during the epoch of reionization. We present simulations of the 21cm forest due to the large scale structure of the reionization-era IGM, including a prescription for x-ray heating and the percolation of photoionization bubbles. We show that, if detected with future instruments such as the Square Kilometer Array (SKA), the 21cm forest can provide a significant constraint on the thermal history of the IGM. Detection will be aided by consideration of the sudden increase in signal variance at the onset of 21cm absorption. If radio foregrounds and the intrinsic source spectra are well understood, the flux decrement over wide bandwidths can also improve detection prospects. Our analysis accounts for the possibility of narrow absorption lines from intervening dense regions, but, unlike previous studies, our results do not depend on their properties. Assuming x-ray heating corresponding to a local stellar population, we estimate that a statistically significant detection of 21cm absorption could be made by SKA in less than a year of observing against a Cygnus A-type source at $z \sim 9$, as opposed to nearly a decade for a significant detection of the detailed forest features. We discuss observational challenges due to uncertainties regarding the abundance of background sources and the strength of the 21cm absorption signal.Comment: Submitted to MNRAS. Revised version includes updated and extended calculations, some corrections and added reference

High-redshift voids in the excursion set formalism

Voids are a dominant feature of the low-redshift galaxy distribution. Several recent surveys have found evidence for the existence of large-scale structure at high redshifts as well. We present analytic estimates of galaxy void sizes at redshifts z ~ 5 - 10 using the excursion set formalism. We find that recent narrow-band surveys at z ~ 5 - 6.5 should find voids with characteristic scales of roughly 20 comoving Mpc and maximum diameters approaching 40 Mpc. This is consistent with existing surveys, but a precise comparison is difficult because of the relatively small volumes probed so far. At z ~ 7 - 10, we expect characteristic void scales of ~ 14 - 20 comoving Mpc assuming that all galaxies within dark matter haloes more massive than 10^10 M_sun are observable. We find that these characteristic scales are similar to the sizes of empty regions resulting from purely random fluctuations in the galaxy counts. As a result, true large-scale structure will be difficult to observe at z ~ 7 - 10, unless galaxies in haloes with masses less than ~ 10^9 M_sun are visible. Galaxy surveys must be deep and only the largest voids will provide meaningful information. Our model provides a convenient picture for estimating the "worst-case" effects of cosmic variance on high-redshift galaxy surveys with limited volumes.Comment: 12 pages, 9 figures, 1 table, accepted by MNRA

Constraints on Quasar Lifetimes and Beaming from the HeII Lyman-alpha Forest

We show that comparisons of HeII Lyman-alpha forest lines of sight to nearby quasar populations can strongly constrain the lifetimes and emission geometry of quasars. By comparing the HeII and HI Lyman-alpha forests along a particular line of sight, one can trace fluctuations in the hardness of the radiation field (which are driven by fluctuations in the HeII ionization rate). Because this high-energy background is highly variable - thanks to the rarity of the bright quasars that dominate it and the relatively short attenuation lengths of these photons - it is straightforward to associate features in the radiation field with their source quasars. Here we quantify how finite lifetimes and beamed emission geometries affect these expectations. Finite lifetimes induce a time delay that displaces the observed radiation peak relative to the quasar. For beamed emission, geometry dictates that sources invisible to the observer can still create a peak in the radiation field. We show that both these models produce substantial populations of "bare" peaks (without an associated quasar) for reasonable parameter values (lifetimes ~10^6-10^8 yr and beaming angles <90 degrees). A comparison to existing quasar surveys along two HeII Lyman-alpha forest lines of sight rules out isotropic emission and infinite lifetime at high confidence; they can be accommodated either by moderate beaming or lifetimes ~10^7-10^8 yr. We also show that the distribution of radial displacements between peaks and their quasars can unambiguously distinguish these two models, although larger statistical samples are needed.Comment: submitted to ApJ, 8 pages, 2 figure

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.Comment: submitted to MNRAS, 9 pages, 5 figure

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.Comment: 9 pages, 5 figures, submitted to MNRA

The ionizing background at the end of overlap

One of the most sought-after signatures of reionization is a rapid increase in the ionizing background (usually measured through the Lyman-alpha optical depth toward distant quasars). Conventional wisdom associates this with the "overlap" phase when ionized bubbles merge, allowing each source to affect a much larger volume. We argue that this picture fails to describe the transition to the post-overlap Universe, where Lyman-limit systems absorb ionizing photons over moderate lengthscales (20-100 Mpc). Using an analytic model, we compute the probability distribution of the amplitude of the ionizing background throughout reionization, including both discrete ionized bubbles and Lyman-limit systems (parameterized by an attenuation length). We show that overlap does not by itself cause a rapid increase in the ionizing background or a rapid decrease in the mean Lyman-alpha transmission toward distant quasars. More detailed semi-numeric models support these conclusions. We argue that rapid changes should instead be interpreted as evolution in the attenuation length itself, which may or may not be directly related to overlap.Comment: submitted to MNRAS, 7 pages, 5 figure

Investment-specific technology shocks and consumption

Modern business cycle models systematically underestimate the correlation between consumption and investment. One reason for this failure is that, generally, positive investment-specific technology shocks induce a negative consumption response. The objective of this paper is to investigate whether a positive consumption response to investment-specific technology shocks can be obtained in a modern business cycle model. We find that the answer to this question is yes. With a combination of nominal rigidities and non-separable preferences, the consumption response is positive for very general parameterisations of the model.