Issue |
A&A
Volume 446, Number 3, February II 2006
|
|
---|---|---|
Page(s) | 877 - 896 | |
Section | Extragalactic astronomy | |
DOI | https://doi.org/10.1051/0004-6361:20053890 | |
Published online | 20 January 2006 |
ISM properties in low-metallicity environments
I. Mid-infrared spectra of dwarf galaxies
1
Service d'Astrophysique, CEA/Saclay, l'Orme des Merisiers, 91191 Gif-sur-Yvette, France e-mail: smadden@cea.fr
2
Observational Cosmology Lab., Code 665, NASA Goddard Space Flight Center, Greenbelt MD 20771, USA
3
Institut d'Astrophysique Spatiale (CNRS), Université de Paris XI, 91405 Orsay, France
Received:
22
July
2005
Accepted:
27
September
2005
We present new ISOCAM mid-infrared spectra of three starbursting nearby dwarf
galaxies, NGC1569, NGC
1140 and II
Zw
40 and the 30
Dor region of the LMC and explore the properties
of the ISM in low-metallicity environments, also using additional sources from
the literature.
We analyse the various components of the ISM probed by the mid-infrared
observations and compare them with other Galactic and extragalactic objects.
The MIR spectra of the low-metallicity starburst sources are dominated by the
[ Ne iii]
and [ S iv]
lines, as well as a steeply rising dust continuum.
PAH bands are generaly faint, both locally and averaged over the full galaxy,
in stark contrast to dustier starburst galaxies, where the PAH features are
very prominant and even dominate on global scales.
The hardness of the modeled interstellar radiation fields for the dwarf
galaxies increases as the presence of PAH band emission becomes less
pronounced.
The [ Ne iii] /[ Ne ii] ratios averaged over the full galaxy are strikingly high,
often >10.
Thus, the hard radiation fields are pronounced and pervasive.
We find a prominent correlation between the PAHs/VSGs and the [ Ne iii] /[ Ne ii]
ratios for a wide range of objects, including the low metallicity galaxies as
well as Galactic H ii regions and other metal-rich galaxies.
This effect is consistent with the hardness of the interstellar radiation
field playing a major role in the destruction of PAHs in the low metallicity
ISM.
We see a PAHs/VSGs and metallicity correlation, also found by
Engelbracht et al. (2005, ApJ, 628, 29) for a larger survey.
Combined effects of metallicity and radiation field seem to be playing
important roles in the observed behavior of PAHs in the low metallicity
systems.
Key words: galaxies: dwarf / galaxies: starburst / ISM: general / galaxies: ISM
© ESO, 2006
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