Astronomy & Astrophysics

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Issue		A&A Volume 397, Number 2, January II 2003
Page(s)		545 - 555
Section		Galactic structure and dynamics
DOI		10.1051/0004-6361:20021522

A&A 397, 545-555 (2003)
DOI: 10.1051/0004-6361:20021522

Tidal dwarf candidates in a sample of interacting galaxies

II. Properties and kinematics of the ionized gas ^,

P. M. Weilbacher¹, P.-A. Duc² and U. Fritze-v. Alvensleben¹

¹ Universitäts-Sternwarte, Geismarlandstr. 11, 37083 Göttingen, Germany,
e-mail: {weilbach,ufritze}@uni-sw.gwdg.de
² CNRS URA 2052 and CEA, DSM, DAPNIA, Service d'Astrophysique, Centre d'Études de Saclay, 91191 Gif-sur-Yvette Cedex, France
e-mail: paduc@cea.fr

(Received 3 May 2002 / Accepted 16 October 2002)

Abstract
We present low-resolution spectroscopy of the ionized gas in a sample of optical knots located along the tidal features of 14 interacting galaxies previously selected as candidate Tidal Dwarf Galaxies (TDGs). From redshift measurements, we are able to confirm their physical association with the interacting system in almost all cases. For most knots, the oxygen abundance does not depend on the blue luminosity. The average, $12+\log({\rm O}/{\rm H})$ $=8.34\pm0.20$ , is typical of TDGs and is comparable to that measured in the outer stellar disk of spirals from which they were formed. A few knots showing low metallicities are probably pre-existing low-mass companions. The estimated H $\alpha$ luminosity of the TDG candidates is higher than that of typical individual $\ion{H}{ii}$ regions in spiral disks and is comparable to the global H $\alpha$ luminosity of dwarf galaxies. We find several instances of velocity gradients with amplitudes apparently larger than 100 km s ^-1 in the ionized gas in the tidal knots and discuss various possible origins for the large velocity amplitudes. While we can exclude tidal streaming motions and outflows, we cannot rule out projection effects with the current resolution. The velocity gradients could be indicative of the internal kinematic characteristic of self-gravitating objects. Higher resolution spectra are required to confirm whether the tidal knots in our sample have already acquired their dynamical independence and are therefore genuine Tidal Dwarf Galaxies.

Key words: galaxies: formation -- galaxies: interactions -- techniques: spectroscopic -- galaxies: photometry

Offprint request: P. Weilbacher, weilbach@uni-sw.gwdg.de

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Tidal dwarf candidates in a sample of interacting galaxies

II. Properties and kinematics of the ionized gas ,

II. Properties and kinematics of the ionized gas ^,