Issue |
A&A
Volume 460, Number 2, December III 2006
|
|
---|---|---|
Page(s) | 467 - 485 | |
Section | Extragalactic astronomy | |
DOI | https://doi.org/10.1051/0004-6361:20053872 | |
Published online | 12 September 2006 |
A survey of submillimeter C and CO lines in nearby galaxies
1
Laboratoire de Radioastronomie (LRA), Observatoire de Paris and École Normale Supérieure (CNRS-UMR 8112), 24 rue Lhomond, 75005 Paris, France e-mail: estelle.bayet@lra.ens.fr
2
California Institute of Technology, Downs Laboratory of Physics 320-47, Pasadena, CA 91125, USA
3
Max Planck Institute fuer Extraterrestrische Physik, Giessenbachstrasse, 85748 Garching, Germany
Received:
21
July
2005
Accepted:
21
August
2006
Aims.While the search for molecular gas in distant galaxies is based on
the detection of submillimeter CO rotational lines, the current CO surveys of nearby galaxies are restricted to the millimeter CO lines. The submillimeter CO lines are formed in warm and dense
molecular gas and are therefore sensitive to the physical
conditions whereas the CO () line is a tracer of the total molecular gas
mass. In order to be able to compare the properties of molecular
gas in nearby and distant galaxies, we have observed C and CO submillimeter lines (including the 12CO(6-5) and
12CO(7-6) lines) in a sample of nearby galaxies using the
Caltech Submillimeter Observatory (CSO).
Methods.We have obtained a complete view of the CO cooling curve (also called CO spectral energy distribution)
by combining the submillimeter CSO data with previous observations
found in the literature. We
made use of Large Velocity Gradient (LVG) models to analyse the
observed CO cooling curve, predict CO line intensities from to
in the studied galaxies,
and derive the physical properties of the warm and dense molecular
gas : the kinetic temperature (TK); the gas density
(n(H2)); the CO column density divided by the line width
CO)/
. The predictions for the line intensities
and for the total CO cooling power, obtained from LVG modelling
have been compared with predictions from Photo Dissociation Regions
(PDR) models.
Results.We show how the CO SED varies according to the galaxy
star forming activity. For active nuclei, the peak is located near
the 12CO(6-5) or 12CO(7-6)
rotational lines, while, for normal nuclei, most of the energy is
carried by the 12CO(4-3) and 12CO(5-4) lines. Whatever the
spectral type of the nucleus, the observed C cooling rate is lower
than the observed CO cooling rate (by a factor of 4). The CO cooling curve of nearby starburst galaxies (e.g. NGC 253) has a
quite similar shape to the CO cooling curve of distant
galaxies. Therefore, the CO cooling curves are useful
diagnostics for the star forming activity in distant objects.
Key words: galaxies: starburst / galaxies: ISM / galaxies: nuclei / submillimeter / ISM: molecules
© ESO, 2006
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