He I $λ$10830 Resonant Scattering in M17: Toward a Diagnostic of Optical Depth and Gas Kinematics

Abstract

The lower level of the He I $λ$10830 transition is metastable and therefore well populated. In H II regions, the line can become optically thick, and its photons then undergo resonant scattering. Draine (2026) predicted that scattering splits the profile into several peaks. We present Magellan/WINERED spectra ($R \approx 70{,}000$) at six pointings along a line from the ionizing source CEN 1 across the southwestern bar of M17, with He I $λ$10830 and Pa$γ$ in the same exposures. Every He I $λ$10830 profile is multi-peaked, while the Pa$γ$ profiles are single-peaked. Because Pa$γ$ does not undergo resonant scattering, it traces gas kinematics; its single peak argues against a purely kinematic origin of the He I peaks, and the shared exposures exclude an instrumental origin. We therefore attribute the He I peaks to resonant scattering, as predicted. We compare the profiles quantitatively with the static, spherically symmetric models of Draine (2026), using width and asymmetry measures independent of the systemic velocity. At two pointings far from CEN 1, no static model of any optical depth reproduces width and asymmetry together: both profiles are more red-weighted than static models of comparable width. Such profiles cannot arise from uniform motion but are expected from scattering in expanding gas. We argue that the profile is shaped by both the kinematics and the optical depth of the He$^{+}$ zone, and resolved He I $λ$10830 profiles could therefore provide a new diagnostic of these properties in H II regions.