Starburst galaxies are common throughout the Universe, and are the seats of massive bursts of star-formation that can dramatically alter the structure of the host galaxy and input large amounts of energy and mass into the intergalactic medium via a superwind. Understanding this feedback mechanism, particularly at high redshifts, is a key topic in understanding the structure and development of the Universe. With the recent launch of both the Chandra and XMM X-ray observatories we are studying the properties of starbursts and superwinds in a range of nearby galaxies, at X-ray, optical and radio wavelengths.

Research in this area at Birmingham includes:

• Studies of the X-ray properties of superwinds from M82 and other starburst galaxies, with Chandra and XMM-Newton. Density and temperature profiles can be derived through the wind, allowing estimates of mass and energy loss rates to be derived. The observed properties can also be compared with the results of hydrodynamic modelling to explore the physical properties of starbursts and winds.
• Dwarf starbursts: these objects are smaller galaxies undergoing strong star-formation. Outflows from these small but numerous galaxies will be very important in polluting the intergalactic medium with heavier elements.
• We are also studying super-star clusters (SSCs) both in our own neighbourhood (objects such NGC3603 and 30 Doradus) and in external galaxies (such as in dwarf starbursts like NGC1569 and NGC5253). SSCs are very massive and dense clusters of stars - possibly 1 million solar masses in a cluster only a few pc in diameter, and are very common in starburst galaxies.

M82 is the prototypical starburst galaxy. There has been a recent massive burst of star-formation in the heart of the galaxy. The combined action of supersonic winds and supernovae leads to the formation of a hot, X-ray emitting superbubble. This bubble expands and eventually breaks out of the galaxy to give rise to a spectacular bipolar superwind. In this picture we show recent XMM-Newton results on the M82 superwind. In the left panel is an adaptively smoothed image of M82 in the 0.2-10 keV waveband. The extent of the optical galaxy is shown by the ellipse. In the right panel is a three-colour image of M82. The 0.2-0.5 keV, 0.5-0.9 keV and 0.9-2.0 keV bands are shown by red, green and blue respectively. These observations show several new morphological features of the superwind, such as a bridge connecting the main superwind to emission associated with the ``Halpha cap'', as well as a ridge of emission and a dark lane.

If the starburst galaxy is a member of a group or cluster, and is moving rapidly through surrounding hot intracluster gas, the starburst winds can be strongly affected by this gas. We are studying this process using X-ray observations of starburst galaxies in groups. An example is the spiral NGC 2276, where gas is being stripped from the galaxy due to its motion through a group. The starburst has probably been triggered by this motion, and starburst outflows seem to be a major contributor to the tail of stripped gas seen to the left in the image.

In order to understand starburst driven winds, and to interpret the observational data, it is necessary to model them computationally.

The theoretical research projects we are currently engaged in include:

• Development of the hydrodynamical code used in the simulations.
• Generating artificial X-ray data from the simulations, allowing a direct comparison with observations taken with the Chandra and XMM satellites.
• Simulations of the X-ray emitting bubbles in the interstellar medium blown by hot young stars.
• Young starburst galaxies, where the wind has not had time to break out of the galaxy (both modelling and observation).
• Mature starburst galaxies, with clear evidence for galactic winds (both modelling and observation).

A numerical simulation of a starburst driven wind. The picture shows the gas density (a logarithmic plot, red represents high density, blue represents low density). Energy and mass deposited into the Interstellar Medium of the galaxy by the starburst region (in the bottom centre of the image) heats the gas up to millions of degrees, where it emits X-rays The hot gas expands and eventually breaks out of the disk of the galaxy, escaping into the halo of the galaxy at approximately 1000 km per second.

Researchers: Ian Stevens, Trevor Ponman, Jesper Rasmussen