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Astron. Astrophys. 361, 500-506 (2000)

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1. Introduction

Over the last decades it has become clear that tidal forces during close encounters of galaxies can redistribute large masses as long tails or bridges between the interacting galaxies. The idea that part of these newly formed structures could form self-gravitating entities was proposed by Zwicky (1956). Numerical simulations (e.g. Elmegreen et al. 1993; Barnes & Hernquist 1996) support this scenario.

Observationally, tidal systems are best traced by the neutral gas phase (by means of HI observations) since it is this extended component of a galaxy which is most easily disrupted by interactions. Also, much work has been done in studying the tidal arms of interacting galaxies at optical wavelengths to find regions of active star-formation (`tidal dwarfs'). Impressive examples of tidal arms with on-going star-formation are, e.g., the Antenna galaxy (NGC 4038/39, Mirabel et al. 1992) and the Superantenna system (Mirabel et al. 1991). Based on an optical study of 42 Hickson compact groups of galaxies Hunsberger et al. (1996) speculate that up to 50% of the dwarf galaxies in such compact groups might be created by tidal interaction among giant parent galaxies.

Surprisingly little is known about molecular gas in tidal arms around interacting galaxies. However, this is an important issue since molecular clouds are the places where stars are born. The distribution of molecular gas in quiescent extragalactic objects (such as tidal arms) therefore gives clues as where to expect star formation to commence in the future.

Combes et al. (1988) were the first to detect a CO cloud outside the optical body of a galaxy (NGC 4438). Another small molecular cloud in a tidally influenced environment was detected by Brouillet et al. (1992) in a torn-out spiral arm of M 81. Smith et al. (1999) discovered a molecule rich tail in Arp 215 and argue that its metal rich gas has been driven out from the inner disk of the parent galaxy. CO-emission has also been detected in the outskirts of Cen A which can be probably attributed to its merger history (Charmandaris et al. 2000). Braine et al. (2000) described two further molecular clouds associated tidal arms of interacting galaxies, where star-formation already is taking place.

The most extended molecular complex in tidal arms of interacting galaxies was recently discovered by us near NGC 3077 (Walter & Heithausen 1999), member of the M 81 triplet. This complex is of particular interest since, although it is huge, hardly any star formation seems to be associated with it.

In this paper we present a detailed follow-up study of this complex (Walter & Heithausen 1999). Sect. 2 briefly describes our new CO observations obtained with the IRAM 30 m and the KOSMA 3 m radio telescopes. Our results are presented in Sect. 3. In Sect. 4.1 we analyze the excitation condition of the CO gas using our multi-transition observations and a simple LVG-model to match the observed line ratios. Masses for the molecular complexes are estimated in Sect. 4.2. A critical parameter for molecular gas is the shielding column density against the destroying UV radiation; we analyze the relation between HI and CO in the tidal arms in Sect. 4.3. The origin and future of the complexes are discussed in Sect. 5. We summarize our results and conclusions in Sect. 6, discuss the implications for metal loss and the enrichment of the intergalactic medium (IGM) due to gravitational interactions and speculate that we may be witnessing the birth of a new dwarf galaxy.

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© European Southern Observatory (ESO) 2000

Online publication: October 2, 2000