THE NATURE OF STARBURST GALAXIES

摘要

Utilizing a large sample of infrared-selected starburst galaxies having optical images and long-slit spectra, we explore the interrelationships between the properties of starbursts and relate these properties to those of the "host" galaxy. We find that the half-light radius of the Hα-emitting region (r_(e,Hα)) enters into several correlations that suggest it is physically related to the actual starburst radius. Most suggestively, the effective IR surface brightness (L_(IR)/πr_(e,Hα)~2) correlates strongly with the far-IR color temperature. This 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,Hα) are a few hundred pc to a few kpc (with the Hα emission being significantly more 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) solar luminosity kpc~2, corresponding to a maximum star formation rate of about 20 solar mass yr~(-1) kpc~2 for a normal initial mass function. We argue that this upper limit suggests that starbursts are self-regulating in some way. We show that most of these galaxies have relatively normal, symmetric rotation curves. This implies that the galactic disk need not suffer severe dynamical damage in order to " fuel" a typical starburst. We show also that the starbursts occur preferentially in the inner region of solid-body rotation. This may reflect both bar-driven inflow of gas to the region between the inner Lindblad resonances and the dominance of gravitational 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 lack of a correlation between galaxy rotation speed and starburst luminosity means that even relatively modest galaxies (masses ≈ 10% of the Milky Way) can host powerful starbursts. We argue on the basis of causality that the internal velocity dispersion in a starburst sets an upper limit to the star formation rate. The most extreme starbursts approach this limit, but most are well below. Finally, we show that the relative narrowness of the nuclear emission lines in starbursts (relative to the galaxy rotation speed) arises because the gas in the nuclear "bin" usually does not sample fully the solid-body part of the rotation curve. The narrow lines do not necessarily imply that the starburst is not in dynamical equilibrium.