NGC 3576 ( G291.28-0.71 ; l = 291.3 ^ { o } , b = -0.7 ^ { o } ) and NGC 3603 ( G291.58-0.43 ; l = 291.6 ^ { o } , b = -0.5 ^ { o } ) 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 H90 \alpha , He90 \alpha , C90 \alpha and H113 \beta recombination lines with an angular resolution of 7 ^ { \prime \prime } 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 ( \Delta V _ { FWHM } \geq 50 km s ^ { -1 } ) and NGC 3603 ( \Delta V _ { FWHM } \geq 70 km s ^ { -1 } ) .
In NGC 3576 a prominent N-S velocity gradient ( \sim 30 km s ^ { -1 } pc ^ { -1 } ) 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 H90 \alpha , He90 \alpha and the H113 \beta lines are detected from 13 individual sources .
The Y ^ { + } ( He/H ) ratios in the two sources range from 0.08 \pm 0.04 to 0.26 \pm 0.10 .
The H113 \beta /H90 \alpha ratio in NGC 3576 is close to the theoretical value , suggesting that local thermodynamic equilibrium ( LTE ) exists .
This ratio is enhanced for most regions in NGC 3603 ; enhanced \beta / \alpha 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 \micron emission and H _ { 2 } O , OH , and CH _ { 3 } OH maser emission .
These comparisons suggest that both NGC 3576 and NGC 3603 have undergone sequential star formation .