HIGH-DENSITY, COMPACT H-II REGIONS IN THE STARBURST GALAXIES NGC-3628AND IC-694 - HIGH-RESOLUTION VLA-OBSERVATIONS OF THE H92-ALPHA RADIO RECOMBINATION LINE

Using the Very Large Array (VLA),(1) we have observed the H92 alpha li ne in the nuclear regions of NGC 3628 and Arp 299 (IC 694 + NGC 3690) with an angular resolution of 1 ''. The radio recombination lines coul d provide quite important constraints to the physical conditions for t he ionized gas in the nuclear regions. For the central nuclear compone nts in NGC 3628, IC 694, and NGC 3690, we find that the electron densi ty of the ionized gas is in the range 5 x 10(2) to 5 x 10(4) cm(-3), w ith corresponding geometrical sizes ranging from 25 to 0.01 pc. For th e regions with a large line-to-continuum ratio, such as the anomalous kinematic components in IC 694 and NGC 3628, the electron density of t he H II regions is well constrained. The derived parameters suggest th e existence of similar to 100 high-density, compact H Ir regions (n(e) similar to 10(4) cm(-3) and l similar to 1-2 pc) in these two compone nts. The large number of high-density, compact H n regions deduced for these components can be regarded as evidence for enhanced massive sta r formation in anomalous kinematic structures that are often produced in interacting systems. Our new H92 alpha line observations reveal the presence of rotating circumnuclear ionized gas disks in both NGC 3628 and IC 694. From the analysis of the nuclear kinematics, we find that a total dynamical mass of 3 x 10(8) M. exists within a radius R = 120 pc in NGC 3628 and 7 x 10(8) M. within R = 200 pc in IC 694. The kine matics of the ionized gas and the models with a collection of H II reg ions infer that the ratio of H n to dynamical mass in the starburst nu clei varies from 1 x 10(-4) (for the high-density,compact H II region model) to 1 x 10(-2) (for the low-density, large H II region model).