Density structure of the Horsehead nebula photo-dissociation region

We present high angular resolution images of the H2 1–0 S(1) line emission obtained with the Son of ISAAC (SOFI) at the New Technology Telescope (NTT) of the Horsehead nebula. These observations are analysed in combination with Hα line emission, aromatic dust, CO and dust continuum emissions. The Horsehead nebula illuminated by the O9.5V star σ Ori ( 60) presents a typical photodissociation region (PDR) viewed nearly edge-on and offers an ideal opportunity to study the gas density structure of a PDR. The H2 fluorescent emission observations reveal extremely sharp and bright filaments associated with the illuminated edge of the nebula which spatially coincides with the aromatic dust emission. Analysis of the H2 fluorescent emission, sensitive to both the far-UV radiation field and the gas density, in conjunction with the aromatic dust and Hα line emission, brings new constraints on the illumination conditions and the gas density in the outer PDR region. Furthermore, combination of this data with millimeter observations of CO and dust continuum emission allows us to trace the penetration of the far-UV radiation field into the cloud and probe the gas density structure throughout the PDR. From comparison with PDR model calculations, we find that i) the gas density follows a steep gradient at the cloud edge, with a scale length of 0.02 pc (or 10'') and and 105 cm-3 in the H2 emitting and inner cold molecular layers respectively; and ii) this density gradient model is essentially a constant pressure model, with 4 106 K cm-3. The constraints derived here on the gas density profile are important for the study of physical and chemical processes in PDRs and provide new insight into the evolution of interstellar clouds. Also, this work shows the strong influence of the density structure on the PDR spatial stratification and illustrates the use of different tracers to determine this density structure.