NGC 3576 and NGC 3603: Two Luminous Southern H [CSC]ii[/CSC] Regions Observed at High Resolution with the Australia Telescope Compact Array

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Abstract
NGC 3576 (G291.28-0.71) and NGC 3603 (G291.58-0.43) are optically visible, luminous H II regions located at distances of 3.0 kpc and 6.1 kpc, respectively. We present 3.4 cm Australia Telescope Compact Array (ATCA) observations of these two sources in the continuum and the H90α, He90α, C90α, and H113β recombination lines with an angular resolution of 7'' and a velocity resolution of 2.6 km s-1. All four recombination lines are detected in the integrated profiles of the two sources. Broad radio recombination lines are detected in both NGC 3576 (ΔVFWHM ≥ 50 km s-1) and NGC 3603 (ΔVFWHM ≥ 70 km s-1). In NGC 3576 a prominent north-south velocity gradient (~30 km s-1 pc-1) is observed, and a clear temperature gradient (6000–8000 K) is found from east to west, consistent with a known infrared color gradient in the source. In NGC 3603, the H90α, He90α, and the H113β 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. The H113β/H90α ratio in NGC 3576 is close to the theoretical value, suggesting that local thermodynamic equilibrium exists. This ratio is enhanced for most regions in NGC 3603; enhanced β/α ratios in other sources have been attributed to high optical depth or stimulated emission. We compare the morphology and kinematics of the ionized gas at 3.4 cm with the distribution of stars, 10 μm emission, and H2O, OH, and CH3OH maser emission. These comparisons suggest that both NGC 3576 and NGC 3603 have undergone sequential star formation.
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