I am an astrophysicist working in the mission planning team for the Chandra X-ray Observatory at the Harvard-Smithsonian Center for Astrophysics. My own research speciality is the study of galaxy groups, particularly in the X-ray and radio bands, with interests in elliptical galaxies and galaxy clusters. Until recently I also worked in another field, as a software developer in the Atmospheric Measurements group, preparing for the TEMPO atmospheric pollution monitoring mission, and helping with related projects involving pollution monitoring from space. These pages focus on my astrophysics research.

Recent results

A lot of my recent work has involved the Complete Local-volume Groups Sample, a set of around 50 galaxy groups selected to allow us to explore the population of such systems in the nearby universe, examine their properties, and investigate the role of the supermassive black holes in their central galaxies in regulating the thermal balance of their halos of 10 million K gas. More details of the survey can be found on the CLoGS webpage.

Optical image (yellow/white) of the group-group merger NGC 6338, overlaid with X-ray emission from the groups cores and halos (blue) and with a temperature map tracing the region shock-heated by the collision (red).
SDSS optical image of NGC 6338, with Chandra & XMM X-ray emission in blue, and shocked regions with temperatures >2 keV in red.

I also like to work on individual systems which are exemplars of interesting physical processes. One such system is NGC 6338, shown to the left, an exceptionally violent, high velocity merger between two galaxy groups. Galaxy groups and clusters grow through mergers, and in galaxy clusters we see many examples where the collisions drive supersonic shockwaves through their hot gas halos, visible in X-ray observations as sharp edges and temperature jumps. Fewer mergers between groups have been studied, and most of these are low-velocity encounters without shocks. NGC 6338 is a merger between two groups, traveling toward each other roughly along the line of sight, with an estimated relative velocity of around 1800 km/s, around 4 million miles per hour. The collision has driven Mach 2.3-3.1 shocks into the gas in the two groups, heating it to around 23 million K in a broad figure-eight region between and around the two group cores. The image shows the galaxies and stars in the field of the groups, with X-ray emission in blue, and the high temperature shocked region in red.

Despite the unusual violence of the collision, gas continues to cool in the dense group cores, where we see clouds and filaments of X-ray-emitting plasma and much cooler Hα-emitting ionised gas, as well as cavities caused by past outbursts from the galaxy's central black hole. This work made use of a combination of data from different telescopes: Chandra and XMM-Newton X-ray data, GMRT low-frequency radio observations and Apache Point Observatory optical imaging and spectroscopy. More detail can be found in our paper and the Chandra image release page.

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