James Webb Space Telescope Feed Post

JWST observations of the Horsehead photon-dominated region I. First results from multi-band near- and mid-infrared imaging

Illuminated edge of the Horsehead in the H2 line at 2.12 ??m derived from the F212N and F210M filters (red), in the F335M (green) filter, and F187N (red) filter (top). Profiles of the relative brightness after background correction (normalized at a distance of 2?" from the edge) along the four cuts in the H2 line at 2.12 ??m (smoothed with a window size of 5 pixels, corresponding to ~0.3?", in order to limit the noise), in the F335M filter and in the F187N filter (bottom). The three individual images are shown in logarithmic scales. They are used to make the RGB composite map. The four solid lines show the four cuts we use, labeled 1 to 4 from left to right. The position ??=0?" in the profiles coincides with the more external edge of the PDR, as traced by the F335M filter (see §5.1). Negative values of ?? are towards the illuminating star. Abstract: The JWST has captured the sharpest IR images ever taken of the Horsehead nebula, a prototypical moderately irradiated PDR that is fully representative of most of the UV-illuminated molecular gas in the Milky Way and star-forming galaxies. We investigate the impact of FUV radiation of a molecular cloud and constrain the structure of the edge of the PDR and its illumination conditions. We used NIRCam and MIRI to obtain 17 broadband and 6 narrowband maps from 0.7 to 28 µm. We mapped the dust emission, scattered light, and several gas phase lines. We also used HST-WFC3 maps at 1.1 and 1. 6 µm, along with HST-STIS spectroscopic observations of the Ha line. We probed the structure of the edge of the Horsehead and resolved its spatial complexity. We detected a network of faint striated features extending perpendicularly to the PDR front into the H\,II region in filters sensitive to nano-grain emission and light scattered by larger grains. This may indeed figure as the first detection of the entrainment of dust particles in the evaporative flow. The map of the 1-0 S(1) line of H2 presents sharp sub-structures on scales as small as 1.5 arcsec. The ionization and dissociation fronts appear at distances 1-2 arcsec behind the edge of the PDR and seem to spatially coincide, indicating a thickness of the neutral atomic layer below 100 au. All broadband maps present strong color variations which can be explained by dust attenuation. Deviations of the emissions in the Ha, Paa, and Bra lines also indicate dust attenuation. With a very simple model, we derive the main features of the extinction curve. A small excess of extinction at 3 µm may be attributed to icy H2O mantles onto grains. In all lines of sight crossing the inner regions of the Horsehead, it appears that dust attenuation is non-negligible over the entire spectral range.