A Two-dimensional Map of the Color Excess in NGC 3603

Abstract

Using archival Hubble Space Telescope/Wide Field Camera 3 images centered on the young HD 97950 star cluster in the giant H II 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 Hα/Paβ 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 ± 0.04 mag. Outside the cluster (at r > 1 pc), the color excess is seen to increase with cluster-centric distance toward 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 × 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 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 log _{10}(N_H_2)=21.7 and extrapolate it to log _{10}(N_H_2)=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.