The molecular environment of NGC3603 - I. Spatial distribution and kinematic structure

Abstract

We present CS (2-1) and CS (3-2) observations of the molecular gas associated with the Galactic starburst template NGC 3603, over an area of 5.'8 x 16.'7, with the OB cluster at the center. Total velocity integrated maps and channel maps give insight into the spatial distribution and the kinematic structure of the dense gas in the giant molecular cloud from which the starburst cluster originated.

We identify 13 molecular clumps with radii less than 0.8 pc and derive upper limits for their virial masses as well as lower limits for their H-2 column densities: (M-vir) less than or similar to (1.0 +/- 0.6) x 10(3) M-. and N (H-2) greater than or similar to (0.4 +/- 0.2) x 10(23) cm(-2). One of the clumps, MM11, clearly stands out with a mass and column density 4 times higher than average. The CS (3-2) / CS (2-1) map shows higher intensity ratios to the south of the OB cluster than to the north (0.80 +/- 0.08 versus 0.32 +/- 0.11), which indicates a substantial difference in the physical conditions (either opacities or excitation temperatures) of the molecular gas. From the average of the line peak velocities, 14.2 +/- 1.6 km s(-1), we deduce a kinematic distance of 7.7 +/- 0.2 kpc for NGC 3603.

We estimate the star formation efficiency (greater than or similar to 30%) of the central part of the NGC 3603 H II region. If we assume the age of the OB cluster to be less than 3 Myr and the star formation rate to be larger than 1.3 x 10(-3) M-. yr(-1), the derived timescale for gas removal ( 6 Myr) can explain why the starburst cluster itself is nearly void of interstellar material. The remnant clump MM 1 appears to constitute the head of a prominent pillar which still becomes dispersed by ionizing radiation and stellar winds originating from the massive stars of the cluster. Because some of the molecular clumps are associated with near and mid infrared sources as well as OH, H2O and CH3OH maser sources we conclude that star formation is still going on within NGC 3603.