A Two-dimensional Map of Color Excess in NGC 3603

Xiaoying Pang, Anna Pasquali, Eva K. Grebel

Published 2011-08-09Version 1

Using archival HST/WFC3 images centered on the young HD 97950 star cluster in the giant HII region NGC 3603, we computed the pixel-to-pixel distribution of the color excess, E(B-V)g, of the gas associated with this cluster from its H_alpha/Pa_beta flux ratio. At the assumed distance of 6.9 kpc, the resulting median color excess within 1 pc from the cluster center is E(B-V)g =1.51 \pm 0.04 mag. Outside the cluster (at r > 1 pc), the color excess is seen to increase with cluster-centric distance towards both North and South, reaching a value of about 2.2 mag at r = 2 pc from the cluster center. The radial dependence of E(B-V)g westward of the cluster appears rather flat at about 1.55 mag over the distance range 1.2 pc < r < 3 pc. In the eastern direction, E(B-V)g steadily increases from 1.5 mag at r = 1 pc to 1.7 mag at r = 2 pc, and stays nearly constant at 1.7 mag for 2 pc < r < 3 pc. The different radial profiles and the pixel-to-pixel variations of E(B-V)g clearly indicate the presence of significant differential reddening across the 4.9 pc \times 4.3 pc area centered on the HD 97950 star cluster. We interpret the variations of E(B-V)g as the result of stellar radiation and stellar winds interacting with an inhomogeneous dusty local interstellar medium (ISM) whose density varies spatially. From the E(B-V)g values measured along the rims of the prominent pillars MM1 and MM2 in the southwest and southeast of the HD 97950 cluster we estimate an H2 column density of log10(N(H2))=21.7 and extrapolate it to log10(N(H2))=23 in the pillars' interior. We find the pillars to be closer to us than the central ionizing cluster and suggest that star formation may be occurring in the pillar heads.