The structure of photodissociation regions: M17 northern bar

Abstract

Images of part of the photodissociation region and ionization front in the northern bar of M17, in the infrared lines of H 2 , H + and the 3.28-μm emission feature, a tracer of the edge of the molecular cloud, are presented. Spatial coincidence between the 3.28-μm and H 2 emission regions is established to within 1 arsec, which implies a gas density of n > 10 5 cm -3 . The peak observed surface brightness in the H 2 v = 1-0 S(1) line is 1.5 × 10 -7 W/(m 2 × sr) and if we assume a 70° inclination to the line of sight, this intensity implies a density of 10 5 cm -3 , from modelling of one-dimensional, uniform-density photodissociation regions. However, our images reveal clumpy structure down to a scale of 2 arcsec, which means that a clumpy two-component PDR model, one component of high density and the other component at lower density, would be the preferred solution