Research Interests

In general terms my work focuses on the structure, formation, and evolution of elliptical galaxies and galaxy groups. There are many things we don't fully understand about these systems: Elliptical galaxies are the end products of galaxy evolution, but we don't know how they form, and in particular how some of them build up halos of multi-million degree gas. Galaxy groups are the environment in which most galaxies reside, but their properties vary hugely, from small associations of a handful of spiral galaxies to massive systems with several elliptical galaxies and an extensive halo of hot gas containing more mass than all the galaxies combined. How groups evolve between these extremes, how they build their hot halos, and the feedback processes that prevent the gas cooling and forming stars, are all areas of active research.

The goal of my current position is to investigate the last of these areas, feedback processes in galaxy groups. The hot gas in the cores of groups is dense enough to cool, but we do not see it cooling as fast as expected. There are various processes that could heat the gas, but at present it appears the most important is heating by the jets of relativistic plasma thrown out by the black holes in the centres of galaxies. These jets drive into the hot gas, causing shocks, and inflate bubbles which heat and lift the gas as they move through it. Investigating these processes requires both excellent X-ray data, to see the gas, and detailed radio observations at multiple frequencies, to see the jets and the plasma in the bubbles - our project uses the Chandra and XMM-Newton X-ray observatories and the GMRT and VLA radio arrays.

Other areas of interest include:

Gas mixing and metal transport in X-ray bright groups

X-ray/radio/optical image of NGC 5044, a group of galaxies in which an infalling galaxy has set the group core ocsillating, transporting metals out into the intra-group gas

The image to the left shows the X-ray brightest group in the sky and its central dominant elliptical, NGC 5044. Using a deep XMM-Newton X-ray observation, we image the hot gas bound in the gravitational well of the group (shown in blue) and mapped the distribution of heavy elements in that gas (primarily Iron, shown in purple). Previous studies had shown that the gas in this group is sloshing, oscillating around its gravitational centre. Our maps shows that this motion has left a spiral trail of metal-enriched gas; the larger purple region in the centre and upper right is the central clump of enriched gas around the elliptical, while the clump to the lower left is the spiral trail viewed end-on. As well as transporting these metals out of the group core, the sloshing motion has influenced an old radio jet and lobe (shown in green, using GMRT 235 MHz data), probably drawing the lobe back toward the group core and distorting the jet and lobe morphology.

Evolving and spiral-rich groups.

X-ray/optical image of Stephan's Quintet, a group of five galaxies, in which an infalling galaxy has shock heated an intergalactic gas filament to several million degrees.

The image to the right shows the well known spiral-rich group Stephan's Quintet. Using some very high-quality Chandra data we were able to confirm the presence of an oblique shock in the system, caused by the collision (at roughly 900 km/s) of an infalling spiral galaxy with a filament of neutral Hydrogen gas - the curving blue region in the middle of the image above is that gas, shock heated to 10 million K. We also raised the possibility that much of the hot gas elsewhere in the group may have been shock heated by previous galactic collisions. The Chandra Education and Public Outreach group picked up the data for a public image release (the image above), and it made it into the 2010 Chandra calendar.

Scaling relations in ellipticals and groups.

The effects of environment on elliptical galaxies, isolated ellipticals and fossil systems.

Dark matter content and distribution in ellipticals.

Mergers and merger shocks in galaxy groups.