PDR Model Mapping of Obscured H2 Emission and the Line-of-Sight Structure of M17-SW

TL;DR

This study uses Spitzer-IRS observations and PDR modeling to map the obscuring extinction and line-of-sight structure of the M17-SW region, revealing a high-density, bowl-shaped PDR boundary with a radial extinction gradient.

Contribution

It demonstrates that PDR models can be used to map intervening extinction and infer the three-dimensional structure of the PDR in M17-SW.

Findings

High gas densities up to 10^7 cm^-3 in H2 emission regions

Radial extinction gradient from 15 to 47 mag over 0.9--2.5 pc

PDR surface resembles a concave, bowl-shaped boundary

Abstract

We observed H2 line emission with Spitzer-IRS toward M17-SW and modeled the data with our PDR code. Derived gas density values of up to few times 10^7 cm^-3 indicate that H2 emission originates in high-density clumps. We discover that the PDR code can be utilized to map the amount of intervening extinction obscuring the H2 emission layers, and thus we obtain the radial profile of A_V relative to the central ionizing cluster NGC 6618. The extinction has a positive radial gradient, varying between 15--47 mag over the projected distance of 0.9--2.5 pc from the primary ionizer, CEN 1. These high extinction values are in good agreement with previous studies of A_V toward stellar targets in M17-SW. The ratio of data to PDR model values is used to infer the global line-of-sight structure of the PDR surface, which is revealed to resemble a concave surface relative to NGC 6618. Such a configuration confirms that this PDR can be described as a bowl-shaped boundary of the central H II region in M17. The derived structure and physical conditions are important for interpreting the fine-structure and rotational line emission from the PDR.