EDP Sciences
Free access
Volume 491, Number 2, November IV 2008
Page(s) 455 - 464
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:200810432
Published online 01 October 2008

A&A 491, 455-464 (2008)
DOI: 10.1051/0004-6361:200810432

Ram-pressure stripped molecular gas in the Virgo spiral galaxy NGC 4522

B. Vollmer1, J. Braine2, C. Pappalardo1, and P. Hily-Blant3

1  CDS, Observatoire astronomique de Strasbourg, 11 rue de l'université, 67000 Strasbourg, France
    e-mail: bvollmer@astro.u-strasbg.fr
2  Laboratoire d'Astrophysique de Bordeaux (OASU), Université de Bordeaux, UMR 5804, CNRS/INSU, BP 89, 33270 Floirac, France
3  IRAM, Domaine Universitaire, 300 rue de la Piscine, 38406 Saint-Martin d'Hères, France

Received 20 June 2008 / Accepted 17 September 2008

IRAM 30 m 12CO(1–0) and 12CO(2–1) HERA observations are presented for the ram-pressure stripped Virgo spiral galaxy NGC 4522. The CO emission is detected in the galactic disk and the extraplanar gas. The extraplanar CO emission follows the morphology of the atomic gas closely but is less extended. The CO maxima do not appear to correspond to regions where there is peak massive star formation as probed by H$\alpha$ emission. The presence of molecular gas is a necessary but not sufficient condition for star formation. Compared to the disk gas, the molecular fraction of the extraplanar gas is 30% lower and the star formation efficiency of the extraplanar gas is about 3 times lower. The comparison with an existing dynamical model extended by a recipe for distinguishing between atomic and molecular gas shows that a significant part of the gas is stripped in the form of overdense arm-like structures. It is argued that the molecular fraction depends on the square root of the total large-scale density. Based on the combination of the CO/H$\alpha$ and an analytical model, the total gas density is estimated to be about 4 times lower than that of the galactic disk. Molecules and stars form within this dense gas according to the same laws as in the galactic disk, i.e. they mainly depend on the total large-scale gas density. Star formation proceeds where the local large-scale gas density is highest. Given the complex 3D morphology this does not correspond to the peaks in the surface density. In the absence of a confining gravitational potential, the stripped gas arms will most probably disperse; i.e. the density of the gas will decrease and star formation will cease.

Key words: galaxies: individual: NGC 4522 -- galaxies: interactions -- galaxies: ISM -- galaxies: kinematics and dynamics -- stars: formation -- radio lines: ISM

© ESO 2008