Mapping of large scale 158 [CII] line emission: orion A

Abstract

We present the first results of an observational programme undertaken to map the fine structure line emission of singly ionized carbon ([CII] 157.7409 ) over extended regions using a Fabry Perot spectrometer newly installed at the focal plane of a 100 cm balloon-borne far-infrared telescope. This new combination of instruments has a velocity resolution of ~200 km s ^-1 and an angular resolution of 1' 5. During the first flight, an area of 30' ^{× 15}' in Orion A was mapped. These observations extend over a larger area than previous observations, the map is fully sampled and the spectral scanning method used enables reliable estimation of the continuum emission at frequencies adjacent to the [CII] line. The total [CII] line luminosity, calculated by considering up to 20% of the maximum line intensity is 0.04% of the luminosity of the far-infrared continuum. We have compared the [CII] intensity distribution with the velocity-integrated intensity distributions of ¹³CO(1-0), CI(1-0) and CO(3-2) from the literature. Comparison of the [CII], [CI] and the radio continuum intensity distributions indicates that the largescale [CII] emission originates mainly from the neutral gas, except at the position of M 43, where no [CI] emission corresponding to the [CII] emission is seen. Substantial part of the [CII] emission from here originates from the ionized gas. The observed line intensities and ratios have been analyzed using the PDR models by Kaufman et al. (1999) to derive the incident UV flux and volume density at a few selected positions. The models reproduce the observations reasonably well at most positions excepting the [CII] peak (which coincides with the position of θ¹Ori C). Possible reason for the failure could be the simplifying assumption of a homogeneous plane parallel slab in place of a more complicated geometry.