A Two-dimensional Map of Color Excess in NGC 3603

TL;DR

This study maps the spatial distribution of color excess in NGC 3603 using HST images, revealing significant differential reddening and interactions between stellar activity and the interstellar medium, with implications for star formation in pillars.

Contribution

It provides the first pixel-to-pixel two-dimensional map of color excess in NGC 3603, highlighting spatial variations and their relation to the local ISM and star formation processes.

Findings

Median E(B-V)g within 1 pc is 1.51 mag.

Color excess increases with distance from the cluster center.

Pillars have high H2 column densities and may host star formation.

Abstract

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.