Starbursts and Galaxy Evolution

We now know that the Universe began about 13.5 billion years ago with the Big Bang, and that the first stars and galaxies formed 500 million years after that. However, the first stars and galaxies were very different to those we see today. How, then, have galaxies evolved? - this is one of the most important questions concerning modern astronomy.

We know that the average rate at which stars form within galaxies has gradually decreased with time: when the Universe was half its current age the average star formation rate (over the Universe) was more than ten times higher. We also know that the rate with which galaxies have experienced gravitational interactions (close shaves) or collisions with each other must also have decreased over time. It is thought that today's largest elliptical galaxies exhausted their gas reservoirs in intense bursts of star formation (known as starbursts) in the distant past resulting from multiple interactions and/or collisions. Therefore, to understand how galaxies have built up their stars and evolved as structures, we must fully understand the starburst phenomenon and its consequences. However, even though starbursts were much more common in the past (i.e. distant Universe), we can only study them in any detail in the nearest examples.

M82 Dense Pack Pointings

All stars form in groups or clusters. Where there is intense star formation, the probability of forming a very massive cluster is higher, so starbursts tend to contain very young, massive clusters (YMCs). For the same reason, YMCs have large populations of massive stars, and it is these stars that have the most powerful radiation, winds, and supernova explosions. Supernovae are important not just energetically but chemically, since in the explosion elements such as carbon or nitrogen that are produced in the star are blown back into the surrounding gas thus changing its composition. When a starburst is strong enough, the energy release from multiple YMCs will heat up the surrounding gas causing it to expand in the form of a bubble. If this bubble reaches the edge of the galaxy, nothing is left to hold the it together and it 'pops', allowing all the hot, chemically enriched gas to vent out of the galaxy, and potentially into inter-galactic space. An outflow as powerful as this is called a 'supergalactic wind'. Furthermore, the venting of this gas can literally 'blow the starburst out', thus preventing further star formation. The consequences of a starburst are clearly complex and dramatic for the host galaxy - but unfortunately still poorly understood.

Thanks to Mark Westmoquette for this article.

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Page last modified on 02 nov 11 10:48 by Kajal H Nakum