observations of Photon Dominated Regions (PDRs) with an unprecedented sensitivity, angular and spectral resolution. We will use these opportunities to probe low-UV PDRs in the Galaxy and thus the emission from the bulk of the material in 'normal' galaxies. This includes clouds in the halo of the Galaxy with a reduced metallicity, which will complement the surveys of the metal-poor ISM in Magellanic Clouds and other dwarf galaxies conducted in project C1. In addition, we will provide the PDR modelling support for these observations.
Nearby diffuse and translucent clouds as test cases for low-UV PDRs:
Most of the molecular gas in normal (=non-star burst) galaxies is exposed to the relatively
weak interstellar radiation field (ISRF). In contrast to the 'classical' PDRs, where molecular
clouds are exposed to strong FUV fields, the spectral line emission is only weak in clouds
exposed to the ISRF. Nonetheless, the emission from these low-UV PDRs can constitute a
significant fraction of the global emission in normal galaxies. An understanding of the line
formation in low-UV PDRs is therefore essential for the modelling of the integrated emission
from external galaxies. Moreover, with their low gas column densities, diffuse (Av<1) and
translucent clouds (Av=2-5) might represent early stages in the formation of dense, starforming
clouds and therefore might provide us with the opportunity to study the processes
relevant for formation of the dense molecular gas in the diffuse, ionized ISM.
The next three years will see the commissioning of several new sub-mm radio telescopes (for
example, NANTEN2, APEX) as well as the first light for SOFIA and Herschel. Together,
they will provide a complete coverage of the sub-mm and FIR domain. Most of the cooling
lines of PDRs radiate at these wavelengths. Therefore, the new telescopes will provide us with
a wealth of information on the chemical and physical processes in these regions. With their
increased sensitivity and expanded wavelength coverage, we can probe tracers and gas
cooling lines which are currently either difficult or not accessible to observations. These are,
for example, the fine-structure transition of C+ and C0, and the CO lines blocked by the
Earth's atmosphere. They are major coolants for low-UV PDRs, but not very well probed
observationally. Complemented low-J CO transition made with ground-based telescopes, this
data set will provide important information on the (photo-)chemistry and energy balance in
these regions, and allows for a check of our existing PDR models. In addition, we will probe
the water in translucent clouds throughout the inner Galaxy, and thereby constraining the
abundance of a key molecule in the chemical network of the ISM.
Nearby examples of metal-poor PDRs:
Galactic halo clouds and the Magellanic System
The Magellanic Clouds are the nearest examples of metal-poor dwarf galaxies, which are
thought to be building blocks of normal galaxies. Survey observations of PDR tracers are
planned for the Magellanic Clouds in the framework of the present SFB (C1, in collaboration
KOSMA and the University of Nagoya and in the framework of the NANTEN 2 project). The
project section C4 will provide the modelling support for these observations. The
observations are complemented by a study of Galactic halo clouds, objects with low visual
extinction and a reduced metallicity. They provide a link between the low-UV clouds in the
Galactic plane and the low-metallicity clouds in the Magellanic system. Both studies will
allow us to investigate the impact of the cloud geometry on the line emission in metal-poor
environments. This is important for future models of more distant galaxies, where a large
number of clouds are sampled within the telescope beam and assumptions regarding the cloud
structure have to be made.
See the home page of Prof. Bertoldi for more information.
