I primarily study luminous active galaxies and quasars, which we think are powered by matter falling into black holes which themselves live at the centres of some of the most massive galaxies that we can see. I am also interested in observing samples of quasars as a means of understanding cosmology.

There are many unsolved problems that have arisen from the discovery of active galaxies sixty years ago, and the extremely luminous class of active galaxy known as quasars (or QSOs) a little over forty years ago:

• How do they work - the most powerful quasars can radiate more power than 10,000 billion Suns, yet the source of this power appears to be a region smaller than the distance between the Sun and our nearest star?
• What determines the parts of the electromagnetic spectrum that they radiate in? Active galaxies and quasars produce emission across the whole spectrum: radio, far- and near-infrared, optical, ultraviolet, X-ray and high energy gamma rays - but different active galaxies can radiate vastly different relative amounts in each waveband.
• Why were these extreme objects up to 1000 times more common in the past, when the Universe was about half its present age?
• How is their formation linked to the formation of the galaxies they inhabit?
• What effect do active galaxies have on their immediate surroundings?
• Given that quasars can be seen over immense distances, what can we learn about cosmology by studying their distribution?

I also work on observational study of large-scale structure, both as measured by large redshift surveys of quasars (which can be seen over vast cosmological distances) and from measurements of weak gravitational lensing. Weak gravitational lensing is an effect in which the shapes of galaxies appear distorted as a result of light rays from them having been "bent" as they pass massive cosmic structures (galaxies and clusters of galaxies) on their way to us. The distortion effect is very small, and because we cannot know the intrinsic shape of any one galaxy we can only detect the effect statistically in measurements of large samples of galaxies. However, the method has the great advantage that it probes directly the distribution of all gravitating matter, not just the luminous matter we see, and hence over the past decade study of weak lensing has become a major focus for observational cosmology. I have been interested in the methods we use to make the measurements, and in particular have been applying Bayesian statistical methods to the problems of both measuring galaxy shapes and also estimating galaxy redshifts (a measure of their cosmological distance, crucial for interpreting the shape signal) when spectra are not available (the usual case in faint galaxy surveys).

X-ray observations of active galaxies

The cosmological evolution of quasars and active galaxies

Weak gravitational lensing

The 2dF QSO Redshift Survey (2QZ)

Radio-loud and radio-quiet quasars

The host galaxies of quasars

Strong gravitational lensing of quasars

Publications

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