High-density, compact H II regions in the starburst galaxies NGC 3628 and IC 694: high-resolution VLA observations of the H92α radio recombination line

Abstract

Using the Very Large Array (VLA), we have observed the H92α line in the nuclear regions of NGC 3628 and Arp 299 (IC 694 + NGC 3690) with an angular resolution of 1". The radio recombination lines could provide quite important constraints to the physical conditions for the ionized gas in the nuclear regions. For the central nuclear components in NGC 3628, IC 694, and NGC 3690, we find that the electron density of the ionized gas is in the range 5 × 10² to 5 × 10⁴ cm_-3, with corresponding geometrical sizes ranging from 25 to 0.01 pc. For the regions with a large line-to-continuum ratio, such as the anomalous kinematic components in IC 694 and NGC 3628, the electron density of the H II regions is well constrained. The derived parameters suggest the existence of ~100 high-density, compact H II regions (n_e ~ 10⁴ cm^-3 and l ~ 1-2 pc) in these two components. The large number of high-density, compact H II regions deduced for these components can be regarded as evidence for enhanced massive star formation in anomalous kinematic structures that are often produced in interacting systems. Our new H92α 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 × 10⁸ M_⊙ exists within a radius R = 120 pc in NGC 3628 and 7 × 10⁸ M_⊙ within R = 200 pc in IC 694. The kinematics of the ionized gas and the models with a collection of H II regions infer that the ratio of H II to dynamical mass in the starburst nuclei varies from 1 × 10^-4 (for the high-density, compact H II region model) to 1 × 10^-2 (for the low-density, large H II region model).