The morphology of HII regions during reionization

It is possible that the properties of HII regions during reionization depend sensitively on many poorly constrained quantities (the nature of the ionizing sources, the clumpiness of the gas in the IGM, the degree to which photo-ionizing feedback suppresses the abundance of low mass galaxies, etc.), making it extremely difficult to interpret upcoming observations of this epoch. We demonstrate that the actual situation is more encouraging, using a suite of radiative transfer simulations, post-processed on outputs from a 1024^3, 94 Mpc N-body simulation. Analytic prescriptions are used to incorporate small-scale structures that affect reionization, yet remain unresolved in the N-body simulation. We show that the morphology of the HII regions is most dependent on the global ionization fraction x_i. This is not to say that the bubble morphology is completely independent of all parameters besides x_i. The next most important dependence is that of the nature of the ionizing sources. The rarer the sources, the larger and more spherical the HII regions become. The typical bubble size can vary by as much as a factor of 4 at fixed x_i between different possible source prescriptions. The final relevant factor is the abundance of minihalos or of Lyman-limit systems. These systems suppress the largest bubbles from growing, and the magnitude of this suppression depends on the thermal history of the gas as well as the rate at which minihalos are photo-evaporated. We find that neither source suppression owing to photo-heating nor gas clumping significantly affect the large-scale structure of the HII regions. We discuss how observations of the 21cm line with MWA and LOFAR can constrain properties of reionization, and we study the effect patchy reionization has on the statistics of Lyman-alpha emitting galaxies. [abridged]Comment: 23 pages, 18 figure

The Marginal Labor Supply Disincentives of Welfare: Evidence from Administrative Barriers to Participation

Existing research on the static effects of the manipulation of welfare program benefit parameters on labor supply has allowed only restrictive forms of heterogeneity in preferences. Yet preference heterogeneity implies that the marginal effects on labor supply of welfare expansions and contractions may differ in different time periods with different populations and which sweep out different portions of the distribution of preferences. A new examination of the heavily studied AFDC program uses variation in state-level administrative barriers to entering the program in the late 1980s and early 1990s to estimate the marginal labor supply effects of changes in program participation induced by that variation. The estimates are obtained from a theory-consistent reduced form model which allows for a nonparametric specification of how changes in welfare program participation affect labor supply on the margin. Estimates using a form of local instrumental variables show that the marginal treatment effects are quadratic, rising and then falling as participation rates rise (i.e., becoming more negative then less negative on hours of work). The average work disincentive is not large but that masks some margins where effects are close to zero and some which are sizable. Traditional IV which estimates a weighted average of marginal effects gives a misleading picture of marginal responses. A counterfactual exercise which applies the estimates to three historical reform periods in 1967, 1981, and 1996 when the program tax rate was significantly altered shows that marginal labor supply responses differed in each period because of differences in the level of participation in the period and the composition of who was on the program.NI

Detecting the Rise and Fall of 21 cm Fluctuations with the Murchison Widefield Array

We forecast the sensitivity with which the Murchison Widefield Array (MWA) can measure the 21 cm power spectrum of cosmic hydrogen, using radiative transfer simulations to model reionization and the 21 cm signal. The MWA is sensitive to roughly a decade in scale (wavenumbers of k ~ 0.1 - 1 h Mpc^{-1}), with foreground contamination precluding measurements on larger scales, and thermal detector noise limiting the small scale sensitivity. This amounts primarily to constraints on two numbers: the amplitude and slope of the 21 cm power spectrum on the scales probed. We find, however, that the redshift evolution in these quantities can yield important information about reionization. Although the power spectrum differs substantially across plausible models, a generic prediction is that the amplitude of the 21 cm power spectrum on MWA scales peaks near the epoch when the intergalactic medium (IGM) is ~ 50% ionized. Moreover, the slope of the 21 cm power spectrum on MWA scales flattens as the ionization fraction increases and the sizes of the HII regions grow. Considering detection sensitivity, we show that the optimal MWA antenna configuration for power spectrum measurements would pack all 500 antenna tiles as close as possible in a compact core. The MWA is sensitive enough in its optimal configuration to measure redshift evolution in the slope and amplitude of the 21 cm power spectrum. Detecting the characteristic redshift evolution of our models will confirm that observed 21 cm fluctuations originate from the IGM, and not from foregrounds, and provide an indirect constraint on the volume-filling factor of HII regions during reionization. After two years of observations under favorable conditions, the MWA can constrain the filling factor at an epoch when ~ 0.5 to within roughly +/- 0.1 at 2-sigma.Comment: 14 pages, 9 figures, submitted to Ap

Cosmological Parameter Estimation Using 21 cm Radiation from the Epoch of Reionization

A number of radio interferometers are currently being planned or constructed to observe 21 cm emission from reionization. Not only will such measurements provide a detailed view of that epoch, but, since the 21 cm emission also traces the distribution of matter in the Universe, this signal can be used to constrain cosmological parameters at 6 < z < 20. The sensitivity of an interferometer to the cosmological information in the signal may depend on how precisely the angular dependence of the 21 cm 3-D power spectrum can be measured. Utilizing an analytic model for reionization, we quantify all the effects that break the spherical symmetry of the 3-D 21 cm power spectrum and produce physically motivated predictions for this power spectrum. We find that upcoming observatories will be sensitive to the 21 cm signal over a wide range of scales, from larger than 100 to as small as 1 comoving Mpc. We consider three methods to measure cosmological parameters from the signal: (1) direct fitting of the density power spectrum to the signal, (2) using only the velocity field fluctuations in the signal, (3) looking at the signal at large enough scales such that all fluctuations trace the density field. With the foremost method, the first generation of 21 cm observations should moderately improve existing constraints on cosmological parameters for certain low-redshift reionization scenarios, and a two year observation with the second generation interferometer MWA5000 can improve constraints on Omega_w, Omega_m h^2, Omega_b h^2, Omega_nu, n_s, and alpha_s. If the Universe is substantially ionized by z = 12 or if spin temperature fluctuations are important, we show that it will be difficult to place competitive constraints on cosmological parameters with any of the considered methods.Comment: 20 pages, 12 figures, accepted by Ap

Probing Reionization with the 21 cm-Galaxy Cross Power Spectrum

The cross-correlation between high redshift galaxies and 21 cm emission from the high redshift intergalactic medium (IGM) promises to be an excellent probe of the Epoch of Reionization (EoR). On large scales, the 21 cm and galaxy fields are anti-correlated during most of the reionization epoch. However, on scales smaller than the size of the H II regions around detectable galaxies, the two fields become roughly uncorrelated. Consequently, the 21 cm-galaxy cross power spectrum provides a tracer of bubble growth during reionization, with the signal turning over on progressively larger scales as reionization proceeds. The precise turnover scale depends on the minimum host mass of the detectable galaxies, and the galaxy selection technique. Measuring the turnover scale as a function of galaxy luminosity constrains the characteristic bubble size around galaxies of different luminosities. The cross spectrum becomes positive on small scales if ionizing photons fail to escape from low mass galaxies, and these galaxies are detectable longward of the hydrogen ionization edge, because in this case some identifiable galaxies lie outside of ionized regions. LOFAR can potentially measure the 21 cm-galaxy cross spectrum in conjunction with mild extensions to the existing Subaru survey for $z=6.6$ Lyman-alpha emitters, while the MWA is slightly less sensitive for detecting the cross spectrum. A futuristic galaxy survey covering a sizable fraction of the MWA field of view ($\sim 800$ deg$^2$) can probe the scale dependence of the cross spectrum, constraining the filling factor of H II regions at different redshifts during reionization, and providing other valuable constraints on reionization models.Comment: 17 pages, 12 figures, submitted to Ap

Simulations of core convection in rotating A-type stars: Differential rotation and overshooting

We present the results of 3--D simulations of core convection within A-type stars of 2 solar masses, at a range of rotation rates. We consider the inner 30% by radius of such stars, thereby encompassing the convective core and some of the surrounding radiative envelope. We utilize our anelastic spherical harmonic (ASH) code, which solves the compressible Navier-Stokes equations in the anelastic approximation, to examine highly nonlinear flows that can span multiple scale heights. The cores of these stars are found to rotate differentially, with central cylindrical regions of strikingly slow rotation achieved in our simulations of stars whose convective Rossby number (R_{oc}) is less than unity. Such differential rotation results from the redistribution of angular momentum by the nonlinear convection that strongly senses the overall rotation of the star. Penetrative convective motions extend into the overlying radiative zone, yielding a prolate shape (aligned with the rotation axis) to the central region in which nearly adiabatic stratification is achieved. This is further surrounded by a region of overshooting motions, the extent of which is greater at the equator than at the poles, yielding an overall spherical shape to the domain experiencing at least some convective mixing. We assess the overshooting achieved as the stability of the radiative exterior is varied, and the weak circulations that result in that exterior. The convective plumes serve to excite gravity waves in the radiative envelope, ranging from localized ripples of many scales to some remarkable global resonances.Comment: 48 pages, 16 figures, some color. Accepted to Astrophys. J. Color figures compressed with appreciable loss of quality; a PDF of the paper with better figures is available at http://lcd-www.colorado.edu/~brownim/core_convectsep24.pd

Comparison Of Reionization Models: Radiative Transfer Simulations And Approximate, Semi-Numeric Models

We compare the predictions of four different algorithms for the distribution of ionized gas during the Epoch of Reionization. These algorithms are all used to run a 100 Mpc/h simulation of reionization with the same initial conditions. Two of the algorithms are state-of-the-art ray-tracing radiative transfer codes that use disparate methods to calculate the ionization history. The other two algorithms are fast but more approximate schemes based on iterative application of a smoothing filter to the underlying source and density fields. We compare these algorithms' resulting ionization and 21 cm fields using several different statistical measures. The two radiative transfer schemes are in excellent agreement with each other (with the cross-correlation coefficient of the ionization fields >0.8 for k < 10 h/Mpc and in good agreement with the analytic schemes (>0.6 for k < 1 h/Mpc). When used to predict the 21cm power spectrum at different times during reionization, all ionization algorithms agree with one another at the 10s of percent level. This agreement suggests that the different approximations involved in the ray tracing algorithms are sensible and that semi-numerical schemes provide a numerically-inexpensive, yet fairly accurate, description of the reionization process.Comment: 13 pages, 10 figure

How accurately can 21 cm tomography constrain cosmology?

There is growing interest in using 3-dimensional neutral hydrogen mapping with the redshifted 21 cm line as a cosmological probe, as it has been argued to have a greater long-term potential than the cosmic microwave background. However, its utility depends on many assumptions. To aid experimental planning and design, we quantify how the precision with which cosmological parameters can be measured depends on a broad range of assumptions. We cover assumptions related to modeling of the ionization power spectrum and associated nonlinearity, experimental specifications like array layout and noise, cosmological assumptions about reionization history and inter-galactic medium (IGM) evolution, and assumptions about astrophysical foregrounds. We derive simple analytic approximations for how various assumptions affect the results, and find that ionization power modeling is most important, followed by array layout (crudely, the more compact, the better). We also present an accurate yet robust method for measuring cosmological parameters in practice, separating the physics from the astrophysics by exploiting both gravitationally induced clustering anisotropy and the fact that the ionization power spectra are rather smooth functions that can be accurately fit by 7 phenomenological parameters. For example, a future square kilometer array optimized for 21 cm tomography could improve the sensitivity of the Planck CMB satellite to spatial curvature and neutrino masses by up to two orders of magnitude, to Delta-Omega_k ~ 0.0002 and Delta m_nu ~ 0.007 eV, and give a 4 sigma detection of the spectral index running predicted by the simpliest inflation models.Comment: 20 PRD pages, 9 figures, 13 tables, matches published PRD version, including new explanatory material

Subgenual activation and the finger of blame: individual differences and depression vulnerability.

BACKGROUND: Subgenual cingulate cortex (SCC) responses to self-blaming emotion-evoking stimuli were previously found in individuals prone to self-blame with and without a history of major depressive disorder (MDD). This suggested SCC activation reflects self-blaming emotions such as guilt, which are central to models of MDD vulnerability. METHOD: Here, we re-examined these hypotheses in an independent larger sample. A total of 109 medication-free participants (70 with remitted MDD and 39 healthy controls) underwent fMRI whilst judging self- and other-blaming emotion-evoking statements. They also completed validated questionnaires of proneness to self-blaming emotions including those related to internal (autonomy) and external (sociotropy) evaluation, which were subjected to factor analysis. RESULTS: An interaction between group (remitted MDD v. Control) and condition (self- v. other-blame) was observed in the right SCC (BA24). This was due to higher SCC signal for self-blame in remitted MDD and higher other-blame-selective activation in Control participants. Across the whole sample, extracted SCC activation cluster averages for self- v. other-blame were predicted by a regression model which included the reliable components derived from our factor analysis of measures of proneness to self-blaming emotions. Interestingly, this prediction was solely driven by autonomy/self-criticism, and adaptive guilt factors, with no effect of sociotropy/dependency. CONCLUSIONS: Despite confirming the prediction of SCC activation in self-blame-prone individuals and those vulnerable to MDD, our results suggest that SCC activation reflects blame irrespective of where it is directed rather than selective for self. We speculate that self-critical individuals have more extended SCC representations for blame in the context of self-agency

The influence of non-uniform reionization on the CMB

We investigate the impact of spatial variations in the ionized fraction during reionization on temperature anisotropies in the CMB. We combine simulations of large scale structure to describe the underlying density field with an analytic model based on extended Press-Schechter theory to track the reionization process. We find that the power spectrum of the induced CMB anisotropies depends sensitively on the character of the reionization epoch. Models that differ in the extent of the "patchy phase" could be distinguished by future experiments such as the Atacama Cosmology Telescope (ACT) and the South Pole Telescope (SPT). In our models, the patchy signal peaks at $l\simeq 2000$, where it can be four times larger than the kinetic Sunyaev-Zel'dovich (kSZ)/Ostriker-Vishniac (OV) signal ($\Delta T_{\rm tot} \simeq 2.6 \mu K$). On scales beyond $l \simeq 4000$ the total Doppler signal is dominated by kSZ/OV, but the patchy signal can contribute up to 30% to the power spectrum. The effect of patchy reionization is largest on scales where the primordial CMB anisotropies dominate. Ignoring this contribution could lead to significant biases in the determination of cosmological parameters derived from CMB temperature measurements. Improvements in the theoretical modeling of the reionization epoch will become increasingly important to interpret the results of upcoming experiments.Comment: 12 pages, 9 figures, minor changes, replaced to match version in press at Ap