THE NATURE OF STARBURST GALAXIES

Utilizing a large sample of infrared-selected starburst galaxies havin g optical images and long-slit spectra, we explore the interrelationsh ips between the properties of starbursts and relate these properties t o those of the ''host'' galaxy. We find that the half-light radius of the Ha-emitting region (r(e,H alpha)) enters into several correlations that suggest it is physically related to the actual starburst radius. Most suggestively, the effective IR surface brightness (L(IR)/pi r(e, H alpha)(2)) correlates strongly with the far-IR color temperature. Th is can be reproduced roughly with an idealized model of a surrounding dust screen whose far-IR emissivity is determined by the local energy density of UV starburst light. Typical values for r(e,H alpha) are a f ew hundred pc to a few kpc (with the Ha emission being significantly m ore compact than the red starlight). This confirms the ''circumnuclear '' scales of typical starbursts. We show also that starbursts seem to obey a limiting IR surface brightness of about 10(11) L. kpc(2), corre sponding to a maximum star formation rate of about 20 M. yr(-1) kpc(2) for a normal initial mass function. We argue that this upper limit su ggests that starbursts are self-regulating in some way. We show that m ost of these galaxies have relatively normal, symmetric rotation curve s. This implies that the galactic disk need not suffer severe dynamica l damage in order to ''fuel'' a typical starburst. We show also that t he starbursts occur preferentially in the inner region of solid-body r otation. This may reflect both bar-driven inflow of gas to the region between the inner Lindblad resonances and the dominance of gravitation al instability over tidal shear in this region. Most of the starbursts reside in galaxies with rotation speeds of 120-200 km s(-1) (compared to 220 km s(-1) for a fiducial L galaxy like the Milky Way). The lac k of a correlation between galaxy rotation speed and starburst luminos ity means that even relatively modest galaxies (masses approximate to 10% of the Milky Way) can host powerful starbursts. We argue on the ba sis of causality that the internal velocity dispersion in a starburst sets an upper limit to the star formation rate. The most extreme starb ursts approach this limit, but most are well below. Finally, we show t hat the relative narrowness of the nuclear emission lines in starburst s (relative to the galaxy rotation speed) arises because the gas in th e nuclear ''bin'' usually does not sample fully the solid-body part of the rotation curve. The narrow lines do not necessarily imply that th e starburst is not in dynamical equilibrium.