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Astron. Astrophys. 326, 537-553 (1997)

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1. Introduction

Collisions cause major perturbations in the stellar and gaseous contents of galaxies. The optical disks of spirals may be torn into elongated structures to form the tails, bridges, and shells which feature prominently in catalogues of interacting galaxies ( Vorontsov-Velyaminov (1959) ; Arp (1966) ; Arp & Madore (1987) ); in addition, the distribution of the different gas components becomes highly perturbed. Whereas unusually high concentrations of molecular gas have been detected in the core of mergers ( Sargent & Scoville (1991) ; Scoville et al. (1991) ), HI gas clouds are found in the outskirts of interacting systems, along and mostly at the end of optical tails emanating from the colliding bodies ( van der Hulst (1979) ; Hibbard (1995) ). All these phenomena may be explained by gravitational disturbances, although some other environmental effects, like ram pressure by intracluster gas ( Gunn & Gott (1972) ), cannot be excluded. Numerical simulations ( Toomre & Toomre (1972) ; Barnes & Hernquist (1992) ; Elmegreen et al. (1993) ; Hibbard & Mihos (1995) ) have played a crucial role in demonstrating how tidal forces shape the stellar and gaseous structures in interacting systems. Moreover, interactions are capable of triggering star formation. Vigorous starbursts are found associated with the molecular clumps in the central regions ( Sargent & Scoville (1991) ), whereas material pulled into extragalactic space may form new objects known as tidal dwarf galaxies ( Schweizer (1978) ; Mirabel et al. (1992) ; Duc (1995) ).

One of the clearest objects where both these phenomena can be observed is the interacting system Arp 105, studied in detail in Duc & Mirabel (1994) (hereafter Paper I). Arp 105, which is situated at a distance of 115 Mpc1 , is an example of a close encounter between a gas rich spiral, NGC 3561A, and an elliptical galaxy, NGC 3561B, near the centre of the galaxy cluster Abell 1185. Some general properties of both galaxies are summarized in Table 1. Two tidal tails emanate from NGC 3561A. The northern, 100 kpc long tail, hosts at its tip an irregular galaxy of Magellanic type, hereafter referred to as A105N. The southern filament, which appears at its base as a weak, diffuse counter-tail, crosses the elliptical and ends in a blue compact dwarf galaxy, referred to as A105S. North of the latter object, the tail also hosts several compact HII regions. The two small galaxies A105S and A105N contain active star-forming regions. In Paper I, we have reported on optical photometry and spectroscopy, as well as single dish HI and CO observations of this system.

In this article, detailed maps of the HI and CO gas distributions, derived from interferometric radio data, are presented. The observations were carried out with the NRAO2 -Very Large Array (VLA) in two configurations to map the HI 21-cm line and with the IRAM Plateau de Bure (PdB) interferometer to measure the distribution of the 12 CO(1-0) emission.


Table 1. Arp 105: miscellaneous data

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© European Southern Observatory (ESO) 1997

Online publication: October 15, 1997