Rennan Barkana

In the beginning: the first sources of light and the reionization of the universe

Abstract The formation of the first stars and quasars marks the transformation of the universe from its smooth initial state to its clumpy current state. In popular cosmological models, the first sources of light began to form at a redshift z =30 and reionized most of the hydrogen in the universe by z =7. Current observations are at the threshold of probing the hydrogen reionization epoch. The study of high-redshift sources is likely to attract major attention in observational and theoretical cosmology over the next decade.

Fuzzy cold dark matter: the wave properties of ultralight particles.

There is an asymmetry between the mass of the electric charges, for example proton and electron, can understood by the asymmetrical Planck Distribution Law. This temperature dependent energy distribution is asymmetric around the maximum intensity, where the annihilation of matter and antimatter is a high probability event. The asymmetric sides are creating different frequencies of electromagnetic radiations being in the same intensity level and compensating each other. One of these compensating ratios is the electron – proton mass ratio. The lower energy side has no compensating intensity level, it is the dark energy and the corresponding matter is the dark matter.

Unusually large fluctuations in the statistics of galaxy formation at high redshift

We show that various milestones of high-redshift galaxy formation, such as the formation of the first stars or the complete reionization of the intergalactic medium, occurred at different times in different regions of the universe. The predicted spread in redshift, caused by large-scale fluctuations in the number density of galaxies, is at least an order of magnitude larger than previous expectations, which argued for a sharp end to reionization. This cosmic scatter in the abundance of galaxies introduces new features that affect the nature of reionization and the expectations for future probes of reionization and may help explain the present properties of dwarf galaxies in different environments. The predictions can be tested by future numerical simulations and may be verified by upcoming observations. Current simulations, limited to relatively small volumes and periodic boundary conditions, largely omit cosmic scatter and its consequences. In particular, they artificially produce a sudden end to reionization, and they underestimate the number of galaxies by up to an order of magnitude at redshift 20.

The Photoevaporation of Dwarf Galaxies during Reionization

During the period of reionization, the universe was filled with a cosmological background of ionizing radiation. By that time a significant fraction of the cosmic gas had already been incorporated into collapsed galactic halos with virial temperatures 104 K that were unable to cool efficiently. We show that photoionization of this gas by the fresh cosmic UV background boiled the gas out of the gravitational potential wells of its host halos. We calculate the photoionization heating of gas inside spherically symmetric dark matter halos and assume that gas that is heated above its virial temperature is expelled. In popular cold dark matter models, the Press-Schechter halo abundance implies that ~50%-90% of the collapsed gas was evaporated at reionization. The gas originated from halos below a threshold circular velocity of ~10-15 km s-1. The resulting outflows from the dwarf galaxy population at redshifts z = 5-10 affected the metallicity and the thermal and hydrodynamic states of the surrounding intergalactic medium. Our results suggest that stellar systems with a velocity dispersion 10 km s-1, such as globular clusters or the dwarf spheroidal galaxies of the Local Group, did not form directly through cosmological collapse at high redshifts.

The Reionization of the Universe by the First Stars and Quasars

▪ Abstract The formation of the first stars and quasars marks the transformation of the universe from its smooth initial state to its clumpy current state. In popular cosmological models, the first sources of light began to form at a redshift z = 30 and reionized most of the hydrogen in the universe by z = 7. Current observations are at the threshold of probing the hydrogen reionization epoch. The study of high-redshift sources is likely to attract major attention in observational and theoretical cosmology over the next decade.

Detecting the Earliest Galaxies through Two New Sources of 21 Centimeter Fluctuations

The first galaxies that formed at a redshift z ~ 20-30 emitted continuum photons with energies between the Lyα and Lyman limit wavelengths of hydrogen, to which the neutral universe was transparent except at the Lyman series resonances. As these photons redshifted or scattered into the Lyα resonance, they coupled the spin temperature of the 21 cm transition of hydrogen to the gas temperature, allowing it to deviate from the microwave background temperature. We show that the fluctuations in the radiation emitted by the first galaxies produced strong fluctuations in the 21 cm flux before the Lyα coupling became saturated. The fluctuations were caused by biased inhomogeneities in the density of galaxies, along with Poisson fluctuations in the number of galaxies. Observing the power spectra of these two sources would probe the number density of the earliest galaxies and the typical mass of their host dark matter halos. The enhanced amplitude of the 21 cm fluctuations from the era of Lyα coupling improves considerably the practical prospects for their detection.

A Method for Separating the Physics from the Astrophysics of High-Redshift 21 Centimeter Fluctuations

Fluctuations in the 21 cm brightness from cosmic hydrogen at redshifts z 6 have their source in the primordial density perturbations from inflation, as well as the radiation from galaxies. We propose a method to separate these components based on the angular dependence of the 21 cm fluctuation power spectrum. Peculiar velocities increase the power spectrum by a factor of ~2 compared with density fluctuations alone and introduce an angular dependence in Fourier space. The resulting angular structure relative to the line of sight facilitates a simple separation of the power spectrum into several components, permitting an unambiguous determination of the primordial power spectrum of density fluctuations, and of the detailed properties of all astrophysical sources of 21 cm fluctuations. We also demonstrate that angular multipoles are not useful for measuring large-scale power, and thus, theoretical predictions should focus on the three-dimensional power spectrum of 21 cm fluctuations.

Strong-lensing analysis of a complete sample of 12 MACS clusters at z > 0.5: mass models and Einstein radii

We present the results of a strong-lensing analysis of a complete sample of 12 very luminous X-ray clusters at z > 0:5 using HST/ACS images. Our modelling technique has uncovered some of the largest known critical curves outlined by many accuratelypredicted sets of multiple images. The distribution of Einstein radii has a median value of’ 28 00 (for a source redshift of zs 2), twice as large as other lower-z samples, and extends to 55 00 for MACS J0717.5+3745, with an impressive enclosed Einstein mass of 7:4 10 14 M . We

Constraints on Warm Dark Matter from Cosmological Reionization

We study the constraints that high-redshift structure formation in the universe places on warm dark matter (WDM) dominated cosmological models. We modify the extended Press-Schechter formalism to derive the halo mass function in WDM models. We show that our predictions agree with recent numerical simulations at low redshift over the halo masses of interest. Applying our model to galaxy formation at high redshift, we find that the loss of power on small scales, together with the delayed collapse of low-mass objects, results in strong limits on the root-mean-square velocity dispersion vrms,0 of the WDM particles at redshift zero. For fermions decoupling while relativistic, these limits are equivalent to constraints on the mass mX of the particles. The presence of a ≈4 × 109 M ☉ supermassive black hole at redshift 5.8, believed to power the quasar SDSS 1044-1215, implies mX 0.5 keV (or vrms,0 0.10 km s-1), assuming that the quasar is unlensed and radiating at or below the Eddington limit. Reionization by redshift 5.8 also implies a limit on mX. If high-redshift galaxies produce ionizing photons with an efficiency similar to their redshift-three counterparts, we find mX 1.2 keV (or vrms,0 0.03 s-1). However, given the uncertainties in current measurements from the proximity effect of the ionizing background at redshift three, values of mX as low as 0.75 keV (or vrms,0 = 0.06 s-1) are not ruled out. The limit weakens further to mX 0.4 keV (or vrms,0 0.14 s-1), if, instead, the ionizing-photon production efficiency is 10 times greater at high redshift, but this limit will tighten considerably if reionization is shown in the future to have occurred at higher redshifts. WDM models with mX 1 keV (or vrms,0 0.04 s-1) produce a low-luminosity cutoff in the high-redshift galaxy luminosity function that is directly detectable with the Next Generation Space Telescope, and which serves as a direct constraint on mX.

Large Einstein radii: a problem for ΛCDM

The Einstein radius of a cluster provides a relatively model-independent measure of the mass density of a cluster within a projected radius of ∼150 kpc, large enough to be relatively unaffected by gas physics. We show that the observed Einstein radii of four well-studied massive clusters, for which reliable virial masses are measured, lie well beyond the predicted distribution of Einstein radii in the standard A cold dark matter (ACDM) model. Based on large samples of numerically simulated cluster-sized objects with virial masses ∼10 15 M O , the predicted Einstein radii are only 15-25 arcsec, a factor of 2 below the observed Einstein radii of these four clusters. This is because the predicted mass profile is too shallow to exceed the critical surface density for lensing at a sizable projected radius. After carefully accounting for measurement errors as well as the biases inherent in the selection of clusters and the projection of mass measured by lensing, we find that the theoretical predictions are excluded at a 4σ significance. Since most of the free parameters of the ACDM model now rest on firm empirical ground, this discrepancy may point to an additional mechanism that promotes the collapse of clusters at an earlier time thereby enhancing their central mass density.