NGC 3576 and NGC 3603: Two Luminous Southern HII Regions Observed at High Resolution with the Australia Telescope Compact Array

Abstract

NGC 3576 (G291.28-0.71; l=291.3o, b=-0.7o) and NGC 3603 (G291.58-0.43; l=291.6o, b=-0.5o) are optically visible, luminous HII regions located at distances of 3.0 kpc and 6.1 kpc, respectively. We present 3.4 cm Australian Telescope Compact Array (ATCA) observations of these two sources in the continuum and the H90a, He90a, C90a and H113b recombination lines with an angular resolution of 7" and a velocity resolution of 2.6 km/s. All four recombination lines are detected in the integrated profiles of the two sources. Broad radio recombination lines are detected in both NGC 3576 (DV_{FWHM}>= 50 km/s) and NGC 3603 (DV_{FWHM}>=70 km/s). In NGC 3576 a prominent N-S velocity gradient (~30 km/s/pc) is observed, and a clear temperature gradient (6000 K to 8000 K) is found from east to west, consistent with a known IR color gradient in the source. In NGC 3603, the H90a, He90a and the H113b lines are detected from 13 individual sources. The Y^+ (He/H) ratios in the two sources range from 0.08+/-0.04 to 0.26+/-0.10. We compare the morphology and kinematics of the ionized gas at 3.4 cm with the distribution of stars, 10 micron emission and H_2O, OH, and CH_3OH maser emission. These comparisons suggest that both NGC 3576 and NGC 3603 have undergone sequential star formation.