Research Description
My main research interests lie in the following broad areas:
(1) Astronomical studies of fundamental constant evolution,
(2) Galaxy evolution, and
(3) Understanding conditions in the Interstellar Medium (ISM) of the Milky Way.
Fundamental constant evolution:
A generic prediction of higher-dimensional theories that attempt to unify the standard model of particle physics and general relativity is that low energy fundamental constants such as the fine structure constant and the ratios of particle masses should evolve with time. Astronomical studies allow one to test for such evolution over cosmological timescales. My research in this field involves using spectroscopic techniques (using combinations of different spectral lines detected in high-redshift galaxies) to probe fundamental constant evolution, as well as devising new techniques for this purpose. Recent results include (1) the strongest present constraint on changes in the proton-electron mass ratio, using inversion and rotational lines, over a lookback time of 7 billion years, (2) stringent constraints on changes in the fine structure constant, the proton-electron mass ratio and the proton g-factor, over the last 10 Gyrs, using combinations of hyperfine and ultraviolet resonance lines and hyperfine and OH lines, and (3) tentative evidence (at 99.1% significance) for a change in one (or more) of the above three constants over the last 3 Gyrs, using OH lines. I am currently attempting to test the above OH result, using significantly deeper spectra from the Arecibo telescope, and am also applying other techniques, based on comparisons between different spectral lines, to significantly improve our sensitivity to changes in the different constants.
Galaxy evolution:
In galaxy evolution, much of my research has focussed on understanding the nature of and evolution of ``damped Lyman-alpha absorbers'' (DLAs), gas-rich galaxies that are detected due to their strong absorption of the light of background quasars. Over the last decade, we have used radio absorption studies of such galaxies to study the evolution of physical conditions in their interstellar media. We have shown that atomic gas in these galaxies is predominantly warm at high redshifts, apparently due to their low metallicity and a paucity of cooling routes, and that their cold gas content increases with decreasing redshift. We have also come up with a new method to image such galaxies, based on quasar sightlines with two DLAs, and using the higher-redshift absorber as a ''blocking filter'' to blank out the background quasar at certain wavelengths, so that one can then image the lower redshift galaxy. Our imaging studies of a large sample of such galaxies with the Keck Telescope and the Hubble Space Telescope (HST) showed that typical DLAs have very low star formation rates at high redshifts. We have recently used the Atacama Large Millimeter/submillimeter Array (ALMA) to detect carbon monoxide (CO) and ionized carbon emission from the hosts of DLAs over a wide range of redshifts, finding that high-metallicity DLAs appear to arise in massive galaxies. We are now using ALMA, HST and Very Large Array imaging studies of high-redshift DLAs to determine their size, total gas content, dynamical mass, etc.
The Interstellar Medium:
My work on the ISM of the Milky Way has aimed to understand the distribution of gas between different temperature phases, and specifically whether gas exists in the ``unstable'' temperature phase, ~ 1000 K. Our studies have found evidence for significant amounts of gas in this unstable phase, which is not expected in standard three-phase models of the ISM. We have also found evidence that the presence of cold gas depends on the total hydrogen column density, with very low cold gas fractions below a threshold column density that is likely to be set by self-shielding against ultraviolet photons. We are currently attempting to carry out a self-consistent modelling of hydrogen absorption and emission spectra, to obtain a better understanding of physical conditions in the ISM.