Annu. Rev. Astron. Astrophys. 1996. 34: 155-206 Copyright © 1996 by Annual Reviews. All rights reserved

6. EFFECTS OF THE DYNAMO ENVIRONMENT

6.1. Starbursts

Starburst galaxies are believed to contain regions of strongly enhanced star formation, particularly of massive stars. The rapid evolution of these stars, through phases with energetic stellar winds to supernovae, may possibly make the turbulence more energetic (for example, by increasing the fraction of hot gas and hence the mean sound speed), with several possible consequences for dynamo theory. Any increased turbulent pressure will inflate the disk, and the -effect may be enhanced above the value appropriate to a quiescent galaxy. Both of these effects increase the dynamo number (Section 4.4). This enhancement may be preferentially concentrated in azimuth, perhaps lagging the spiral arms. Ko & Parker (1989) suggested that galactic dynamos may turn on and off in response to changing starburst activity. However, the timescale for starbursts is believed to be less than 10⁸ yr, which is certainly no longer (and possibly considerably shorter) than a dynamo growth time. Thus it is hard to see how significant field growth can be caused by isolated starburst episodes; see also Vallée (1994). Nozakura (1993) presented a local model with several feedback loops, linking star formation via gravitational instability, dynamo action, and energy release into the ISM via supernovae. In some contrast to Ko & Parker, he concluded that there was only a limited parameter range in which strong star formation and dynamo action could coexist: Essentially star formation requires a high surface density of gas and/or a low sound speed, and so a thin disk, giving a smaller dynamo number. These are clearly matters requiring further attention. Moreover, in an active galaxy, fountain flows will be more frequent, enhancing the lifting of field from the disk into the halo - see Section 7.2.