I am currently Associate Dean (Research) for the Faculty of Science and Professor in the School of Physics at the University of Melbourne.
My primary research interests lie in the field of quasar formation and reionisation in the early universe. In particular I am interested in the evolution of the earliest galaxies and how this evolution may be studied with the next generation of radio telescopes. I also work in the field of gravitational lensing. Specifically, I study problems in quasar microlensing, and the statistical properties of gravitational lensing by galaxies.
The quest to try and understand how the Universe came to look the way it does lies at the heart of astronomy. However when viewing the Universe from a historical perspective astronomers are immediately faced with fundamental unanswered questions. We believe the Universe has a finite age, and as a result, that there must have been an epoch when galaxies appeared for the first time. However we do not know how this first generation of galaxies formed. We do not know what they looked like, or how big they were. Indeed, we do not even know when galaxies first played an important role in the evolution of our Universe.
Over the last decade the composition of the Universe has been determined to high accuracy. Understanding the first galaxies now represents the next great challenge for observational cosmology. Currently, our knowledge of the first galaxies is currently limited to two primary facts, which are represented in the schematic of Figure 1. Astronomers believe that our Universe began with the “Big Bang”, after which the initially very hot Universe expanded and cooled. When the Universe cooled sufficiently that the gas of protons and electrons “recombined” to form atomic hydrogen, light was able to travel freely for the first time. We observe this light today as a diffuse glow on the sky known as the Cosmic Microwave Background, which describes the state of the Universe 380,000 years after the Big Bang. Small ripples of density observed at this time grew under the influence of gravity, forming the sites of modern-day galaxies some 13.7 billion years later. Some of the atomic hydrogen in the early Universe formed stars within galaxies, but most is located in the space between galaxies. The current belief is that the first galaxies appeared a few hundred million years after the Big Bang, resulting in a large UV flux that reionised hydrogen in the Universe. The time when galaxies first became important can be defined as the instant when the combined galaxies in the Universe had produced enough ultra-violet light to reionise all of the hydrogen. Astronomers refer to this as the end of the Dark Ages of the Universe.
There are several key observational areas in which substantial progress will be made in the study of the first galaxies during the coming decade. The first of these are forthcoming programs, with an emphasis on obtaining data beyond the current redshift, or distance frontier, using new surveys and instruments. Following the success of the Hubble Space Telescope (HST), the flagship James Webb Space Telescope (JWST) is scheduled for launch in 2014. JWST is a large infrared optimised telescope that will be used to search for the high redshift galaxies thought to be responsible for the reionisation of the intergalactic hydrogen. The advent of 30-meter class optical/IR telescopes in the next decade, such as the Giant Magellan telescope (GMT), will open a new window on the Universe allowing spectra to be taken of the earliest forming galaxies discovered with JWST. Much emphasis is also based on experiments to measure the redshifted 21 cm radio signal (Furlanetto et al. 2006; Morales & Wyithe 2010), which may provide the first direct probe of the neutral hydrogen in the high redshift Universe. Radio telescopes such as the Murchison Widefield Array (now beginning to take data in Western Australia), will lead efforts in this exciting new field. Next generation ground based surveys such as SkyMapper (Keller et al. 2007), will discover high redshift quasars, thus providing valuable additional targets for studies of the intervening intergalactic hydrogen using quasar absorption spectroscopy. Finally, the Planck surveyor cosmic microwave background experiment will provide tighter limits on fundamental cosmological and astrophysical parameters, providing better constraints on the integrated ionisation history of the IGM. The goal of these observations will be to elucidate the physical history and origin of the first galaxies, which can only be achieved within a sophisticated physical framework.
Within this context, the development of theoretical models that include detailed physics of galaxy formation and intergalactic hydrogen will therefore play a key role. At the highest redshifts astronomers have only theoretical predictions to guide knowledge of the first galaxies and their interaction with intergalactic hydrogen prior to reionisation. Further developing these theoretical models and utilizing them in combination with observational data to better understand the evolution of the IGM during the Epoch of Reionisation underpin my proposed science program.
A full list of papers is available at ADS
Short descriptions of 10 of my most interesting papers are listed below.
This paper recognized the importance of gravitational lensing for observations of the most distant galaxies, and presented evidence for this in the Hubble Ultra-Deep Field. This realization resolves a controversy regarding the findings of different groups studying the Hubble Ultra Deep field, one of the highest profile data sets in astronomy. One of the important motivations for constructing the James Webb Space Telescope (successor to the Hubble Space Telescope) is the study of the buildup of stellar mass in the early Universe and the discovery of the earliest galaxies. This paper illustrates that high redshift surveys do not provide an unbiased census of the build up of stellar mass, but suggests that gravitational lensing could be used in the future to probe galaxy luminosities that are otherwise unreachable. The gravitational lensing already seen by Hubble Space Telescope will be of critical importance to these future studies. The paper reinforces the importance of the James Webb Space Telescope.
2) Wyithe, J.S.B., Loeb, A., Geil, P.M. Baryonic Acoustic Oscillations in 21-cm Emission: A Probe of Dark Energy Out to High Redshifts, 2008, Monthly Notices of the Royal Astronomical Society, 383, 1195-1209
Baryonic acoustic oscillations in the mass-density of the Universe on the largest scales can be used as a standard ruler, providing the most promising method to infer the evolution of the equation of state for the dark energy. This paper showed that low frequency telescopes being built to detect the end of reionization from the redshifted 21cm emission line of hydrogen could be used to study the evolution of this acoustic scale over the first half of cosmic time where it is not accessible to the traditional method of galaxy clustering. It demonstrated that first generation of low-frequency experiments such as the Murchison Widefield Array will be able to constrain the acoustic scale to within a few per cent both prior to and after reionization, comparable to the best current measurements from galaxy redshift surveys, but at much higher redshifts. The paper further showed that future extensions of the first-generation experiments 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.
3) Dijkstra, M., Wyithe, J.S.B., Haiman, Z., Constraints on Reionization of Cosmic Hydrogen from the Ly-alpha Luminosity function of Galaxies, 2007, Monthly Notices of the Royal Astronomical Society, 379, 253-259
High redshift galaxies can be found either by searching for absorption at wavelengths shorter than the Lyman limit of hydrogen in broad-band photometry, or by searching for Ly-alpha emission in a narrow band. Significant observational effort on the world’s largest telescopes has been invested in both approaches. The epoch when the first galaxies finished ionizing all of the hydrogen in the inter-galactic medium remains a sought after observational prize. Because the Ly-alpha radiation can be absorbed by the neutral inter-galactic hydrogen, these galaxies are often suggested as probes of when the reionization era was ended. In this paper we constructed the first model of the luminosity function of Ly-alpha emitters, including both a physical model for transmission of Ly-alpha photons through the inter-galactic medium and the growth of galaxies. Using this model we showed that the observed evolution of the luminosity function does not constrain reionization, as was claimed in the literature.
4) Wyithe, J.S.B., A Log-Quadratic Relation Between the Nuclear Super-Massive Black-Hole Masses and Velocity Dispersions of Galaxies, 2006, Monthly Notices of the Royal Astronomical Society, 365, 1082-1098
The origin tight relation between Super-Massive Black-Holes and their host galaxies remains a great mystery for galaxy formation models, but arguably provides the best evidence for the mechanisms that regulate star-formation and quasar activity. The relation between black-hole mass and galaxy velocity dispersion in particular has received much attention, and is traditionally parameterized by a power-law relation. This paper showed that the relation between the mass of a central black hole and the velocity dispersion of galaxies departs from a pure powerlaw. This eparture is not predicted by simple models, and indicates that there is an additional physical effect at play beyond the hypothesized modes of feedback. Subsequent studies have investigated the origin of the third parameter in this relation.
Early galaxies are thought to have filled the Universe with ionized bubbles, the size and distribution of which provide a signature for studying the first galaxies. The aim of telescopes like the Murchison Widefield Array (MWA), currently under construction in Western Australia is to measure this ionization structure. However these radio telescopes have only a limited range of angular scales over which they are sensitive to the reionization fluctuations. As a result, the size of the ionized bubbles is a critical (unknown) input into the design of the MWA and other similar instruments. This paper presented the first calculation of the maximum size of the ionized regions at the end of reionization from very general considerations of the standard Cold-Dark Matter Model model. We found that the maximum size of bubbles that can be observed is limited to around half a degree, which sets the largest size of the smallest baselines that are required in the radio array in order to ensure sensitivity to the crucial final stages of reionization.
The epoch when the first galaxies finished ionizing all of the hydrogen in the inter-galactic medium remains an unknown milestone in the history of the Universe. One of the most robust methods for studying the ionization state of the inter-galactic medium in order to answer this question is absorption of Ly-alpha radiation from luminous high redshift quasars. Unfortunately only very low atomic fractions can be studied directly, so that the presence of large atomic fractions cannot be detected. This paper introduced a novel approach that used the size of the observed region of ionization around the quasar, combined with modelling of the quasars ionized bubble to constrain the ionized state of the general inter-galactic medium. We demonstrated that the hydrogen in the intergalactic medium was mostly atomic right up until the end of the reionization epoch. This paper motivated a very large number of follow up studies to model the Ly-alpha absorption in the vicinity of the quasar.
7) Wyithe, J.S.B., Loeb, A., Self Regulated Growth of Super-Massive Black-Holes in Galaxies as the Origin of the Optical and X-Ray Luminosity Functions of Quasars, 2003, Astrophysical Journal, 595, 614-623
In the local universe, the Hubble Space Telescope has been used to measure the mass of super-massive black-holes, which appear to be located in the cores of all galaxies. The masses of these black-holes correlate tightly with the properties of their host galaxies. This paper developed the first theory to include feedback in the growth mechanisms of the central Black Holes, to describe the formation and evolution of quasars. The model is based on the hierarchical growth of structure and successfully describes the luminosity functions of quasars, and the origin of quasar lifetimes at high redshift, as well as the local black-hole galaxy relation. This paper contributed to the foundation for the many successful subsequent studies of the hierarchical build up of super-massive black holes via quasar activity.
This paper presented a model of the joint evolution of ionized structure in hydrogen and helium during reionization, including sources both from stars and quasars, as well as population-III stars. The paper made the point that reionization was not a rapid event, and that there was likely an early phase of reionisation by a population-III stars, followed by a period of recombination of the Hydrogen and Helium, and a second phase of reionisation at later times. We suggested that the Universe could have in fact have been reionized twice. In 2003, a major new measurement of the structure in the Cosmic Microwave Background by the Wilkinson Microwave Anisotropy Probe, suggested that the Epoch of Reionisation might have occurred at redshifts as high as 20. This was in sharp contrast to the evidence from absorption features in quasars, which suggested that the Universe was still neutral at a redshift of 6, a billion years after the Big Bang. Our paper provided the basis for the interpretation of this apparently contradictory result by modelling the implications for reionization by population-III stars within the observational constraints. The modelling showed that population-II stars could adequately explain the discrepancy, providing the extended era of reionization demanded by the data.
The existence of super-massive black holes within the first billion years of the Universes history poses a challenge to models for the formation of structures in the early Universe. This paper showed that up to one-third of the most distant quasars known were likely to have had their observed flux magnified by a factor of ten or more, as a consequence of gravitational lensing by galaxies along the line of sight. Based on knowledge of the quasar luminosity function at the time, this implied that their abundance, as well as their luminosity density, could have been substantially overestimated.
10) Wyithe J.S.B., Turner E.L., Spergel, D.N., Gravitational Lens Statistics for Generalized NFW Profiles: Parameter Degeneracy and Implications for Self-Interacting Cold Dark Matter, 2001, Astrophysical Journal, 555, 504-523
There is evidence that the cold dark matter model predicts cores that are denser than those observed in galaxies, groups, and clusters. One possible resolution of the discrepancy is that the dark matter has strong interactions, which leads to lower central densities. In this paper I demonstrated for the first time that gravitational lensing statistics provide a powerful test for the self-interacting dark matter model, since the lower density cores lead to many fewer gravitational cluster lenses. This paper showed that the number of cluster lenses place tight constraints on the interaction strength of dark matter
The Dark-ages, Reionization And Galaxy-formation Observables Numerical Simulation project (DRAGONS) combines a semi-analytic model for galaxy formation designed to accurately represent the growth of galaxies, with a semi-numerical model for the growth and evolution of ionized structure. The goal of DRAGONS is self-consistent modelling of observations of high redshift galaxies and the structure and morphology of reionization.
The Tiamat simulations are a suite of large collisionless N-Body simulations upon which DRAGONS is built. The primary asset setting Tiamat apart from most other large simulation programs is its high density of simulation outputs at high redshift (100 from z=35 to z=5; roughly one every 10 Myrs) enabling the construction of very accurate merger trees at an epoch when galaxy formation is rapid and mergers extremely frequent. We use Tiamat to analyse the dynamical evolution of galaxies at high redshift, drawing comparisons to low-redshift results from other simulations.
Meraxes is a brand new semi-analytic galaxy formation model, specially designed for studying galaxy formation during the Epoch of Reionisation. Meraxes includes a fully temporally and spatially coupled treatment of reionisation and is built upon the Tiamat N-body simulations which possesses both sufficient volume and mass resolution to study reionisation structure as well as the temporal resolution needed to resolve the galaxy and star formation physics relevant to early galaxy formation.
Smaug is a suite of gas hydrodynamic cosmological simulations of used to model galaxy formation at high redshift. The simulations include galaxy formation with strong, thermally coupled supernovae. These models produce enough UV photons to sustain reionization through a significant population of faint, unobserved, galaxies. This predicted population is consistent with extrapolation of the faint end of observed UV luminosity functions. We use these models to inform and calibrate the physics implementations in our Meraxes model
Professor Stuart Wyithe | Head, School of Physics
Room 104, David Caro Building (192)
The University of Melbourne, Victoria 3010 Australia
T: +61 3 8344 5420 and +61 3 8344 5083
F: +61 3 9347 4783