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Universität zu Köln
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Mathematisch-Naturwissenschaftliche Fakultät
Fachgruppe Physik

I. Physikalisches Institut

The origin of [CII] emission on large scales

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Betreuer: Volker Ossenkopf und Jürgen Stutzki

The [CII] line at 158 micron is next to the [OI] 63 micron line the most important cooling line of the interstellar medium in the Milky Way (Fixsen et al.) and in external galaxies (Stacey et al. 1991, Malhotra et al.). The new observatories SOFIA and Herschel will allow to map the [CII] line at unprecedented spatial resolutions and, for (almost) the first time, allow to resolve its velocity structure.

In prepartion of these studies, at least two problems need to be addressed:

  1. [CII] emission from the different phases of the ISM
    [CII] is emitted not only from surfaces of dense molecular clouds, but also from the cold, neutral HI clouds and the ionized components of the interstellar medium. While the average densities and temperatures thus vary strongly between the various phases of the ISM, the main common gas heating mechanism may be the photo-electric effect on dust grains. Thus, models of photon dominated regions like CLOUDY (or in fact KOSMA-tau) may be apropriate to model the [CII] emission from all phases.
    The [NII] lines at 205 and 122 micron stem only from the ionized components. Taking into account the N/C abundances and excitation conditions, observations of the [NII] have been used to estimate the relative importance of the dense and diffuse ionized phases and the contribution of these phases to the total observed [CII] emission (Carral et al. 1994).
    Cold, neutral HI clouds are expected to contribute significantly to the total [CII] cooling. However, the relative contribution to the total observed emission [CII] as function of galacto-centric distances or latitudes is yet unclear.
  2. [CII] emission line profiles
    An open issue is the interpretation of [CII] line shapes. It may be possible to use the line shapes to deconvolve the contribution of the different phases, but it remains to be shown that this is possible. Model studies of Hennebelle & Perault (2000) may show the way. Complementary HI data is needed here.

Model predictions shall be compared with already available observational data of the two nearby galaxies M83 or M51. The data comprises ISO/LWS FIR data as well as others. It is planned to obtain complete maps with NANTEN2 in the lines of atomic carbon and mid-J transitions of carbon monoxyde. The thesis aim is to prepare as best as possible for the oncoming [CII] observations with GREAT/SOFIA and HIFI/Herschel. First data is expected for fall 2006.