Jun Koda

Can 21-cm observations discriminate between high-mass and low-mass galaxies as reionization sources?

The prospect of detecting the first galaxies by observing their impact on the intergalactic medium as they reionized it during the first billion years leads us to ask whether such indirect observations are capable of diagnosing which types of galaxies were most responsible for reionization. We attempt to answer this by considering a set of large-scale radiative transfer simulations of reionization in sufficiently large volumes to make statistically meaningful predictions of observable signatures, while also directly resolving all atomically-cooling halos down to 10^8 M_solar. We focus here on predictions of the 21-cm background, to see if upcoming observations are capable of distinguishing a universe ionized primarily by high-mass halos from one in which both high-mass and low-mass halos are responsible, and to see how these results depend upon the uncertain source efficiencies. We find that 21-cm fluctuation power spectra observed by the first generation EoR/21-cm radio interferometer arrays should be able to distinguish the case of reionization by high-mass halos alone from that by both high- and low-mass halos, together. Some reionization scenarios yield very similar power spectra and rms evolution and thus can only be discriminated by their different mean reionization history and 21-cm PDF distributions. We find that the skewness of the 21-cm PDF distribution smoothed over LOFAR-like window shows a clear feature correlated with the rise of the rms due to patchiness. Measurements of the mean photoionization rates are sensitive to the average density of the regions being studied and therefore could be strongly skewed in certain cases. (abridged)

Redshift-space distortion of the 21-cm background from the epoch of reionization – I. Methodology re-examined

The peculiar velocity of the intergalactic gas responsible for the cosmic 21cm background from the epoch of reionization and beyond introduces an anisotropy in the three-dimensional power spectrum of brightness temperature fluctuations. Measurement of this anisotropy by future 21cm surveys is a promising tool for separating cosmology from 21cm astrophysics. However, previous attempts to model the signal have often neglected peculiar velocity or only approximated it crudely. This paper presents a detailed treatment of the effects of peculiar velocity on the 21cm signal. (1) We show that properly accounting for finite optical depth eliminates the unphysical divergence of 21cm brightness temperature in the IGM overdense regions found in previous work that employed the usual optically-thin approximation. (2) We show that previous attempts to circumvent this divergence by capping the velocity gradient result in significant errors in the power spectrum on all scales. (3) We further show that the observed power spectrum in redshift-space remains finite even in the optically-thin approximation if one properly accounts for the redshift-space distortion. However, results that take full account of finite optical depth show that this approximation is only accurate in the limit of high spin temperature. (4) We also show that the linear theory for redshift-space distortion results in a ~30% error in the power spectrum at the observationally relevant wavenumber range, at the 50% ionized epoch. (5) We describe and test two numerical schemes to calculate the 21cm signal from reionization simulations which accurately incorporate peculiar velocity in the optically-thin approximation. One is particle-based, the other grid-based, and while the former is most accurate, we demonstrate that the latter is computationally more efficient and can achieve sufficient accuracy. [Abridged]

ON THE SURVIVAL AND ABUNDANCE OF DISK-DOMINATED GALAXIES

We study the formation of disk-dominated galaxies in a Λ cold dark matter (ΛCDM) universe. Their existence is considered to be a challenge for the ΛCDM cosmology, because galaxy mergers isotropize stellar disks and trigger angular momentum transport in gas disks, thus fostering the formation of central stellar spheroids. Here, we postulate that the formation of stellar spheroids from gas-rich disks is controlled by two parameters that characterize galaxy mergers, the mass ratio of merging dark matter halos, and the virial velocity of the larger merging halo. We utilize merger histories generated from realizations of the cosmological density field to calculate the fraction of dark matter halos that have avoided spheroid formation, and compare the derived statistics with the spheroid occupation fractions in surveys of nearby galaxies. We find, for example, that the survival rate of disk-dominated galaxies in ΛCDM is just high enough to explain the observed fractional representation of disk-dominated galaxies in the universe if the only mergers which lead to central spheroid formation are those with mass ratios M 2/M 1 > 0.3 and virial velocities V vir,1 > 55kms–1. We discuss the physical origin of this criterion.

Gravothermal collapse of isolated self-interacting dark matter haloes: N-body simulation versus the fluid model

We make direct comparisons between Monte Carlo N-body simulations and analytic and numerical solutions of a conduction fluid (gaseous) model, for various isolated selfinteracting dark matter (SIDM) haloes. There is a disagreement between two methods on the sucient strength of collisionality to solve cuspy core problem, but we show that the two agree within 20% for isolated haloes. The N-body agrees very well with the analytical self-similar solution of gravothermal collapse in the fluid model by Balberg et al. (2002) when one free parameter, the coecient of thermal conduction C, is chosen to be 0.75. The density profile evolves self-similarly and the central density and velocity dispersion match the analytical solution perfectly with the predicted exponent fi = 2.19, which is also an asymptotic slope of the density profile. We also initialize the simulation and the 1D numerical calculation of the conducting fluid model with the Plummer’s model, the Hernquist profile and the NFW profile to show that the fluid model is applicable to more realistic density profiles. The central density at maximum core expansion and the collapse time agree within 20% in the long mean free path regime. As the mean free path become comparable to the system size, we see the delay in collapse rate as predicted. In this transitional regime, gravothermal collapse simulation agree with the fluid model if another prefactor of thermal conduction, b, in the short mean free path is set to 0.25. Monte Carlo N-body simulation and conducting fluid model agree with each other if two prefactors of thermal conduction C and b are calibrated by the N-body simulation. Our results validate the use of these two methods for collisional self-gravitating systems. Throughout the paper, the collision is assumed to be isotropic and velocity independent.

The Impact of Nonlinear Structure Formation on the Power Spectrum of Transverse Momentum Fluctuations and the Kinetic Sunyaev–Zel’dovich Effect

Cosmological transverse momentum fields, whose directions are perpendicular to Fourier wave vectors, induce temperature anisotropies in the cosmic microwave background via the kinetic Sunyaev-Zeldovich (kSZ) effect. The transverse momentum power spectrum contains the four-point function of density and velocity fields,

Low redshift baryon acoustic oscillation measurement from the reconstructed 6-degree field galaxy survey

\langle\delta\delta v v\rangle

Simulating cosmic reionization and the radiation backgrounds from the epoch of reionization

. In the post-reionization epoch, nonlinear effects dominate in the power spectrum. We use perturbation theory and cosmological

Power spectrum multipoles on the curved sky: an application to the 6-degree Field Galaxy Survey

N

The Cluster-Merger Shock in 1E 0657–56: Faster than a Speeding Bullet?

-body simulations to calculate this nonlinearity. We derive the next-to-leading order expression for the power spectrum with a particular emphasis on the connected term that has been ignored in the literature. While the contribution from the connected term on small scales (

Improved WiggleZ Dark Energy Survey Distance Measurements to z = 1 with Reconstruction of the Baryonic Acoustic Feature

k>0.1\,h\,\rm{Mpc}^{-1}