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Galaxy Evolution: Regulation of the ISM and Star Formation  
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Paul Alexander
P.Alexander@mrao.cam.ac.uk

Cavendish Astrophysics

Galaxy Evolution

Regulating the ISM and Star Formation

In order to investigate the cosmological evolution of star-forming galaxies it is essential that we understand the physics which controls the evolution and regulation of star formation in the ISM.  Therefore one aspect of our work is to investigate these processes, both observationally and theoretically, in galaxies in the local Universe where observational tools are most powerful.

The interplay between star formation and the inter-stellar medium is both fundamental to how galaxies evolve with cosmic time and highly complex. It has also become clear, especially since the observation of the "Hubble Deep Field", that typical galaxies have undergone many interactions during their evolution. We use a combined observational and numerical approach to study interacting and merging galaxies in the local Universe.

One of the principal results of this work has been to demonstrate the importance of gas-dynamical effects during galaxy-galaxy interactions. Purely tidal forces cannot always explian the gas structures seen around interacting galaxies. An example of this is illustrated below. 

NGC4490
The figure shows the H-alpha emission from hot, ionised gas in star forming regions (red) and the stellar continuum emission (green) in the NGC 4490/85 system. The dramatic offset of the the star forming ISM from the stellar disk in the smaller galaxy is obvious. This stripping has been caused by ram-pressure as the smaller galaxy moved through the extended gas halo of the larger galaxy. Having been stripped of such a large fraction of its gas, the smaller member of this pair of galaxies has completely lost its potential to form stars in the future.

Although the very gas-rich nature of systems like NGC 4490/85 system is rather atypical in the local Universe, such systems are likely to be very common in the early Universe. The importance of processes such as described above in the evolution of galaxies remains unquantified but should be included in future models of galaxy evolution.

As well as detailed studies of individual systems,
such as NGC 4490/85 shown above, star formation in nearby galaxies can also be investigated using statistical studies of the global properties many galaxies. One such ongoing project is the study, via the 21 cm HI line, the (neutral) atomic hydrogen content of many nearby galaxies for which the dust emission at 850 µm has been observed with the SCUBA instrument on the JCMT. This sample of nearby galaxies is the first to be observed in detail in terms of both its atomic and dust components.

Other related studies currently being made include the study if turblence in the ISM of our Galaxy. The high resolution of available observations of atomic hydrogen in our own Galaxy means that it is possible to study, statistical, the structure in the ISM over a wide
range of scales, to provide observational constraints on the underlying turbulent processes at work in the ISM.

University of Cambridge    Last modified: 27/1/2002