2013ApJ...774L..14S

We observed H₂ line emission with Spitzer-IRS toward M17-SW and modeled the data with our photon-dominated region (PDR) code. Derived gas density values of up to few times 10⁷/cm3 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.