An irradiated jet in M 17.

Context. M17 is one of the best studied giant HII regions in our galactic neighborhood. It should also provide a suitable environment for the class of fully ionized jets externally irradiated by the presence of nearby hot stars. However, no such jets have been observed thus far in M17. Aims. We report on a visible imaging survey of the M17 nebula with the goal of identifying likely shock-excited nebulosities. Methods. We imaged M17 through narrow-band filters centered on the most intense visible lines of [OIII], Hα+[NII], [SII], and Hα. We obtained follow-up spectroscopy of the only jet-like structure identified in the images.We also used published X-ray observations of M17 obtained with the Chandra X-ray Observatory, as well as infrared data obtained with the Very Large Telescope and with the Spitzer Space Telescope, to look for evidence of a jet-driving source. Results. We have detected what appears to be the first jet identified in M17 visible HII region. The jet is composed of a set of knots, two of which have significant radial velocities with respect to the HII region, and a distant arc-like bright nebulosity that may represent an early episode of intense mass loss by the jet-driving source. The follow-up spectra of the structures composing the jet, including the arc, support this interpretation by revealing intense forbidden lines of [NII] and [SII] due to enhanced collisional excitation in shocks. The presence of a X-ray source, most likely a young stellar object, at a position where the jet launching source should be expected to lie reinforces the interpretation. We identify a tentative near-infrared counterpart of the X-ray source, although it is offset by 1: 009 from the nominal position of the X-ray source, which is almost three times the radius of its positional uncertainty. The very weak [OI] emission in the spectra of the jet knots and the arc suggests that they are nearly fully ionized, in agreement with the environment in which the jet is propagating. [ABSTRACT FROM AUTHOR]