Our research revolves around X-ray astronomy and observational cosmology, studying the physics of galaxy clusters, supermassive black holes, dark matter and dark energy.

Galaxy clusters and cosmology

A multi-wavelength view of the cluster MACSJ0911.2+1746. Each panel shows a three-color image of the cluster. Top right: the (smoothed) distribution of cluster galaxies is shown as the yellow overlay. Bottom left: the total mass distribution as reconstructed by a weak lensing analysis is shown in blue. Bottom right: the X-ray emission is indicated in pink. The main cluster is the object at the center of the image, brightly visible in all three mass tracers. However, all maps also indicate that there is a secondary cluster about 3 arcminutes west of MACSJ0911.2+1746. The two clusters are likely to merge in the future to form an even more massive system. (Figure from von der Linden et al., 2012, MNRAS)

Galaxy clusters are the largest objects in the Universe, spanning 10 million light years across and containing as much mass as a million, billion suns. Our research examines the physics of these remarkable systems using the best available multi-wavelength data. We also use the observed distribution and internal properties of clusters, and their variation with mass and redshift, to probe the natures of dark matter, the weakly interacting yet dominant matter component of the Universe, and dark energy, the driving force behind cosmic acceleration. Measurements of galaxy clusters are highly complementary to other cosmological probes.

Most of the normal, baryonic matter in the Universe is in gaseous form. Within galaxy clusters, gravity squeezes this gas, heating it to 10-100 million degrees and causing it to shine brightly at X-ray wavelengths. As well as revealing, in exquisite detail, the thermodynamic and metal enrichment histories of galaxy clusters, X-ray observations made with satellite observatories like the Chandra X-ray Observatory, XMM-Newton and Suzaku allow us to determine their masses, and to separately measure the baryonic and dark, non-baryonic matter within them.

General Relativity predicts that mass concentrations bend light rays passing near to them in a phenomenon known as gravitational lensing. This can both magnify and distort the images of background galaxies. The effects of lensing can be detected in the statistical appearance of background objects seen through clusters (weak lensing). Occasionally, lensing can also lead to large distortions (strong lensing). Our group is at the forefront of using deep, wide-field lensing measurements to measure cluster masses and constrain cosmology.

For further discussion of this work, see e.g.:

Black holes, jets and galaxy formation

Suzaku Shows Clearest Picture Yet Of Perseus Galaxy Cluster (Figure from Simionescu A., Allen S. W., et al.; NASA; March 2011.)

Classical models for galaxy formation predict that the largest galaxies should be much bigger and brighter than we observe. A large and ubiquitous power source must prevent gas from cooling and forming vast numbers of additional stars. Our team has argued that supermassive black holes in the centers of galaxies are responsible for this suppressed star formation. The regions close to these black holes are commonly observed to pump out huge amounts of energy in relativistic jets. These jets inflate cavities in the surrounding gas and drive enormous shock waves and turbulent motions. Using X-ray and radio observations, we have shown that this `feedback' occurs with a high and near-universal efficiency, sufficient to solve the galaxy formation problem, and that turbulent heating may play a key role in balancing cooling. However, significant questions relating to the `duty-cycle' of black hole heating, the accretion process, jet formation and black hole growth remain.

For further discussion of this work, see e.g.:

Future missions

We are involved in the development of powerful, new X-ray satellites such as Athena and X-ray Surveyor, and new, large ground- and space-based optical/near-infrared telescopes and mm-wave surveys, including the Large Synoptic Survey Telescope, Euclid, the South Pole Telescope, and CMB-S4. Together, these projects will push back the boundaries of our understanding of the Universe.

Athena LSST Euclid