J. Stuart B. Wyithe

Self-regulated Growth of Supermassive Black Holes in Galaxies as the Origin of the Optical and X-Ray Luminosity Functions of Quasars

We postulate that supermassive black holes grow in the centers of galaxies until they unbind the galactic gas that feeds them. We show that the corresponding self-regulation condition yields a correlation between black hole mass (Mbh) and galaxy velocity dispersion (σ) as inferred in the local universe and recovers the observed optical and X-ray luminosity functions of quasars at redshifts up to z ~ 6 based on the hierarchical evolution of galaxy halos in a Λ-dominated cold dark matter cosmology. With only one free parameter and a simple algorithm, our model yields the observed evolution in the number density of optically bright or X-ray-faint quasars with 2 z 6 across 3 orders of magnitude in bolometric luminosity and 3 orders of magnitude in comoving density per logarithm of luminosity. The self-regulation condition identifies the dynamical time of galactic disks during the epoch of peak quasar activity (z ~ 2.5) as the origin of the inferred characteristic quasar lifetime of ~107 yr. Since the lifetime becomes comparable to the Salpeter e-folding time at this epoch, the model also implies that the Mbh-σ relation is a product of feedback-regulated accretion during the peak of quasar activity. The mass density in black holes accreted by that time is consistent with the local black hole mass density ρbh ~ (2.3) × 105 M☉ Mpc-3, which we have computed by combining the Mbh-σ relation with the measured velocity dispersion function of Sloan Digital Sky Survey galaxies. Comparison of the local black hole mass function with that inferred from combining the feedback relation with the halo mass function suggests that most massive (>109 M☉) black holes may have already been in place by z ~ 6. Applying a similar self-regulation principle to supernova-driven winds from starbursts, we find that the local ratio between the black hole mass and the stellar mass of galactic spheroids should be ~0.001, independent of mass and in agreement with observations. This ratio increases with redshift, although the Mbh-σ relation is redshift-independent.

Reionization and Cosmology with 21-cm Fluctuations

Measurement of the spatial distribution of neutral hydrogen via the redshifted 21-cm line promises to revolutionize our knowledge of the epoch of reionization and the first galaxies, and may provide a powerful new tool for observational cosmology from redshifts 1<z<4. In this review we discuss recent advances in our theoretical understanding of the epoch of reionization (EoR), the application of 21-cm tomography to cosmology and measurements of the dark energy equation of state after reionization, and the instrumentation and observational techniques shared by 21-cm EoR and postreionization cosmology machines. We place particular emphasis on the expected signal and observational capabilities of first generation 21-cm fluctuation instruments.

The Star Formation Rate in the Reionization Era as Indicated by Gamma-Ray Bursts

High-redshift gamma-ray bursts (GRBs) offer an extraordinary opportunity to study aspects of the early universe, including the cosmic star formation rate (SFR). Motivated by the two recent highest-z GRBs, GRB 080913 at z 6.7 and GRB 090423 at z 8.1, and more than four years of Swift observations, we first confirm that the GRB rate does not trace the SFR in an unbiased way. Correcting for this, we find that the implied SFR to beyond z = 8 is consistent with Lyman Break Galaxy-based measurements after accounting for unseen galaxies at the faint end of the UV luminosity function. We show that this provides support for the integrated star formation in the range 6 z 8 to have been alone sufficient to reionize the universe.

Low-Frequency Gravitational Waves from Massive Black Hole Binaries: Predictions for LISA and Pulsar Timing Arrays

The coalescence of massive black hole (BH) binaries due to galaxy mergers provides a primary source of low-frequency gravitational radiation detectable by pulsar timing measurements and by the proposed the Laser Interferometry Space Antenna (LISA) observatory. We compute the expected gravitational radiation signal from sources at all redshifts by combining the predicted merger rate of galactic halos with recent measurements of the relation between BH mass, MBH, and the velocity dispersion of its host galaxy, ?. Our main findings are as follows: (1) the nHz frequency background is dominated by BH binaries at redshifts z 2, and existing limits from pulsar timing data place tight constraints on the allowed normalization and power-law slope of the MBH-? relation or on the fraction of BH binaries that proceed to coalescence; (2) more than half of all discrete mHz massive BH sources detectable by LISA are likely to originate at redshifts z 7; (3) the number of LISA sources per unit redshift per year should drop substantially after reionization as long as BH formation is triggered by gas cooling in galaxies. Studies of the highest redshift sources among the few hundred detectable events per year will provide unique information about the physics and history of black hole growth in galaxies.

Science with the Murchison Widefield Array

Significant new opportunities for astrophysics and cosmology have been identified at low radio frequencies. The Murchison Widefield Array is the first telescope in the southern hemisphere designed specifically to explore the low-frequency astronomical sky between 80 and 300 MHz with arcminute angular resolution and high survey efficiency. The telescope will enable new advances along four key science themes, including searching for redshifted 21-cm emission from the EoR in the early Universe; Galactic and extragalactic all-sky southern hemisphere surveys; time-domain astrophysics; and solar, heliospheric, and ionospheric science and space weather. The Murchison Widefield Array is located in Western Australia at the site of the planned Square Kilometre Array (SKA) low-band telescope and is the only low-frequency SKA precursor facility. In this paper, we review the performance properties of the Murchison Widefield Array and describe its primary scientific objectives.

The impact of the IGM on high-redshift Lyα emission lines

We calculate the impact of the intergalactic medium (IGM) on the observed Lyα lines emitted by galaxies in an ionized IGM at z 4. Our model accounts for gas clumping in the IGM and for the fact that high-redshift galaxies reside in overdense regions, which causes the velocity field of the IGM to depart from the Hubble flow. The observed shape of the Lyα line varies widely, with dependence on the intrinsic width and systemic velocity of the line, a galaxies star formation rate and the local extragalactic UV background. For large star formation rates and levels of the UV background, absorption in the IGM does not result in a Lyα line that is asymmetric as is common among known high-redshift Lyα emitters (LAEs). For models in which the lines do show the observed strong asymmetry, the IGM typically transmits only 10‐30 per cent of the Lyα flux. The increase in the ionizing background that accompanied the completion of reionization barely increased the IGM transmission, which suggests that LAEs of comparable luminosity should not appear to be significantly dimmer prior to overlap. In this light, we briefly discuss the potential of LAEs as a probe into the epoch of reionization.

Was the Universe Reionized by Massive Metal-free Stars?

The Wilkinson Microwave Anisotropy Probe satellite has measured a large optical depth to electron scattering after the cosmological recombination of τes = 0.17 ± 0.04, implying significant reionization of the primordial gas only ~200 million years after the big bang. However, the most recent overlap of intergalactic H II regions must have occurred at z 9 based on the Lyα forest constraint on the thermal history of the intergalactic medium. Here we argue that a first generation of metal-free stars with a heavy (rather than Salpeter) mass function is therefore required to account for much of the inferred optical depth. This conclusion holds if feedback regulates star formation in early dwarf galaxies as observed in present-day dwarfs.

A large neutral fraction of cosmic hydrogen a billion years after the Big Bang

The fraction of ionized hydrogen left over from the Big Bang provides evidence for the time of formation of the first stars and quasar black holes in the early Universe; such objects provide the high-energy photons necessary to ionize hydrogen. Spectra of the two most distant known quasars show nearly complete absorption of photons with wavelengths shorter than the Lyman α transition of neutral hydrogen, indicating that hydrogen in the intergalactic medium (IGM) had not been completely ionized at a redshift of z ≈ 6.3, about one billion years after the Big Bang. Here we show that the IGM surrounding these quasars had a neutral hydrogen fraction of tens of per cent before the quasar activity started, much higher than the previous lower limits of ∼0.1 per cent. Our results, when combined with the recent inference of a large cumulative optical depth to electron scattering after cosmological recombination therefore suggest the presence of a second peak in the mean ionization history of the Universe.The ionization fraction of cosmic hydrogen, left over from the big bang, provides crucial fossil evidence for when the first stars and quasar black holes formed in the infant universe. Spectra of the two most distant quasars known 1 show nearly complete absorption of photons with wavelengths shorter than the Lyα transition of neutral hydrogen, indicating that hydrogen in the intergalactic medium (IGM) had not been completely ionized at a redshift z ∼ 6.3, about a billion years after the big bang. Here we show that the radii of influence of ionizing radiation from these quasars imply that the surrounding IGM had a neutral hydrogen fraction of tens of percent prior to the quasar activity, much higher than previous lower limits 1,2 of ∼ 0.1%. When combined with the recent inference of a large cumulative optical depth to electron scattering after cosmological recombination from the WMAP data 3 , our result suggests the existence of a second peak in the mean ionization history, potentially due to an early formation episode of the first stars. The detection of a Lyα emitting galaxy at z ∼ 6.56 led to the claim that the IGM must already be highly ionized at that redshift 4. However, it was later shown 5 that the presence of a small ionized region around the galaxy and broadening of the Lyα line,

Luminosity functions of Lyα emitting galaxies and cosmic reionization of hydrogen

Recent observations imply that the observed number counts of Lyα emitters (LAEs) evolved significantly between z = 5.7 and 6.5. It has been suggested that this was due to a rapid evolution in the ionization state, and hence transmission of the intergalactic medium (IGM) which caused Lyα flux from z = 6.5 galaxies to be more strongly suppressed. In this paper, we consider the joint evolution of the Lyα and the ultraviolet luminosity functions (LFs) and show that the IGM transmission evolved between z = 6.5 and 5.7 by a factor 1.1 0.5.

Baryonic acoustic oscillations in 21-cm emission: a probe of dark energy out to high redshifts

Low-frequency observatories are currently being constructed with the goal of detecting red-shifted 21-cm emission from the epoch of reionization. These observatories will also be able to detect intensity fluctuations in the cumulative 21-cm emission after reionization, from hydrogen in unresolved damped Lya absorbers (such as gas-rich galaxies) down to a redshift z ∼ 3.5. The inferred power spectrum of 21-cm fluctuations at all redshifts will show acoustic oscillations, whose comoving scale can be used as a standard ruler to infer the evolution of the equation of state for the dark energy. We find that the first generation of low-frequency experiments (such as MWA or LOFAR) will be able to constrain the acoustic scale to within a few per cent in a redshift window just prior to the end of the reionization era, provided that foregrounds can be removed over frequency bandpasses of ≥ 8 MHz. This sensitivity to the acoustic scale is comparable to the best current measurements from galaxy redshift surveys, but at much higher redshifts. Future extensions of the first-generation experiments (involving an order of magnitude increase in the antennae number of the MWA) could reach sensitivities below 1 per cent in several redshift windows and could be used to study the dark energy in the unexplored redshift regime of 3.5 ≤ z ≤ 12. Moreover, new experiments with antennae designed to operate at higher frequencies would allow precision measurements (≤1 per cent) of the acoustic peak to be made at more moderate redshifts (1.5 ≤ z ≤ 3.5), where they would be competitive with ambitious spectroscopic galaxy surveys covering more than 1000 deg 2 . Together with other data sets, observations of 21-cm fluctuations will allow full coverage of the acoustic scale from the present time out to z ∼ 12.