Molecular excitation in the Eagle nebula's fingers

Abstract

Context. The M 16 nebula is a relatively nearby Hii region, powered by O stars from the open cluster NGC 6611, which borders to a Giant Molecular Cloud. Radiation from these hot stars has sculpted columns of dense obscuring material on a few arcmin scales. The interface between these pillars and the hot ionised medium provides a textbook example of a Photodissociation Region (PDR). Aims. To constrain the physical conditions of the atomic and molecular material with submillimeter spectroscopic observations. Methods. We used the APEX submillimeter telescope to map a ${\sim} 3'\times3'$ region in the CO $J= 3$–2, 4–3 and 7–6 rotational lines, and a subregion in atomic carbon lines. We also observed C 18 O(3–2) and CO(7–6) with longer integrations on five peaks found in the CO(3–2) map. The large scale structure of the pillars is derived from the molecular lines' emission distribution. We estimate the magnitude of the velocity gradient at the tips of the pillars and use LVG modelling to constrain their densities and temperatures. Excitation temperatures and carbon column densities are derived from the atomic carbon lines. Results. The atomic carbon lines are optically thin and excitation temperatures are of order 60 K to 100 K, well consistent with observations of other Hii region-molecular cloud interfaces. We derive somewhat lower temperatures from the CO line ratios, of order 40 K. The Ci/CO ratio is around 0.1 at the fingers tips.