Issue |
A&A
Volume 378, Number 1, October IV 2001
|
|
---|---|---|
Page(s) | 51 - 69 | |
Section | Cosmology (including clusters of galaxies) | |
DOI | https://doi.org/10.1051/0004-6361:20011109 | |
Published online | 15 October 2001 |
Abundant molecular gas in tidal dwarf galaxies: On-going galaxy formation
1
Observatoire de Bordeaux, UMR 5804, CNRS/INSU, BP 89, 33270 Floirac, France
2
CNRS URA 2052 and CEA/DSM/DAPNIA, Service d'astrophysique, Saclay, 91191 Gif sur Yvette Cedex, France
3
Institut de Radioastronomie Millimétrique, Avenida Divina Pastora 7, NC 18012 Granada, Spain
4
Cornell University, Astronomy Department, Ithaca, NY 14853, USA
5
ASIAA, Academia Sinica, PO Box 1-87, Nanking, Taipei 115, Taiwan
6
Departamento de Astronomía, Universidad de Guanajuato, Apdo. Postal 144, Guanajuato, Mexico
Corresponding author: J. Braine, braine@observ.u-bordeaux.fr
Received:
15
February
2001
Accepted:
6
August
2001
We investigate the process of galaxy formation as can be observed in the only
currently forming galaxies -the so-called Tidal Dwarf Galaxies, hereafter
TDGs -through observations of the molecular gas detected via its CO
(Carbon Monoxide) emission.
These objects are formed of material torn off of the outer parts of a spiral
disk due to tidal forces in a collision between two massive galaxies.
Molecular gas is a key element in the galaxy formation process,
providing the link between a cloud of gas and a bona fide galaxy.
We have detected CO in 8 TDGs (two of them have already been published in
Braine et al. 2000, hereafter Paper I), with an overall detection rate
of 80% , showing that molecular gas is abundant in TDGs, up to a few
. The CO emission coincides both spatially and
kinematically with the HI emission, indicating that the molecular gas
forms from the atomic hydrogen where the HI column density is high.
A possible trend of more evolved TDGs having greater molecular gas
masses is observed, in accord with the transformation of HI into H2.
Although TDGs share many of the properties of small irregulars, their CO
luminosity is much greater (factor ~100) than that of standard
dwarf galaxies of comparable luminosity. This is most likely a
consequence of the higher metallicity (≳1/3 solar) of TDGs which
makes CO a good tracer of molecular gas. This allows us to study star
formation in environments ordinarily inaccessible due to the extreme
difficulty of measuring the molecular gas mass. The star formation efficiency,
measured by the CO luminosity per Hα flux, is the same in TDGs and
full-sized spirals.
CO is likely the best tracer of the dynamics of these objects
because some fraction of the HI near the TDGs may be part of the tidal
tail and not bound to the TDG.
Although uncertainties are large for individual objects, as the geometry
is unknown, our sample is now of eight detected objects and we find that
the "dynamical" masses of TDGs, estimated from the CO line widths,
seem not to be greater than the "visible"
masses (HI + H2 + a stellar component). Although higher spatial resolution
CO (and HI) observations would help reduce the uncertainties, we find that
TDGs require no dark matter, which would make them the only
galaxy-sized systems where this is the case. Dark matter in spirals
should then be in a halo and not a rotating disk. Most dwarf galaxies
are dark matter-rich, implying that they are not of tidal origin.
We provide strong evidence that TDGs are self-gravitating entities, implying
that we are witnessing the ensemble of processes
in galaxy formation: concentration of large amounts of gas in a bound object,
condensation of the gas, which is atomic at this point, to form molecular
gas and the subsequent star formation from the dense molecular component.
Key words: stars: formation / galaxies: evolution / galaxies: formation / galaxies: interactions / galaxies: ISM / cosmology: dark matter
© ESO, 2001
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