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Astron. Astrophys. 346, 45-57 (1999)

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1. Introduction

Barred galaxies may be the rule rather than the exception: It seems possible that as many as two thirds of all galaxies have bars (e.g. Noguchi 1996). However, most of these stellar bars are almost devoid of molecular gas. Long, continuous (i.e. non-nuclear) gas-rich bars that are roughly aligned with the stellar bar seem to be rare phenomena (e.g. Friedli 1998). Most prominent among the few known examples are the bars of NGC 7479 (total length [FORMULA] kpc, Sempere et al. 1995b, Quillen et al. 1995, Aalto et al. 1998), NGC 1530 (total length more than 20 kpc, Downes et al. 1996) and NGC 2903 (total length [FORMULA] kpc, Jackson et al. 1991). Centrally concentrated distributions of molecular gas are far more common in barred galaxies; a great variety of different morphologies can be found, ranging from circumnuclear rings and `twin peak' distributions to central peaks (e.g. Kenney et al. 1992, Kenney 1996).

While the necessity of bars in general in triggering nuclear activity or enhanced star formation, up to starbursts, is somewhat controversial (Hawarden et al. 1996, Ho et al. 1997), they definitely provide a mechanism of funnelling gas from the outer parts of a galaxy to its central region (Athanassoula 1992). As a result of tidal strain, shocks and cloud-cloud collisions that should occur in the non-axisymmetric potential of a bar, gaseous bars also are a laboratory to study diffuse and possibly unbound gas (e.g. Das & Jog 1995). This is a type of interstellar medium (ISM) likely to be important in the bulge and central regions of many galaxies including the Milky Way (Dahmen et al. 1998), the properties of which are largely unexplored.

Since the triggering of bars is thought to be due to galaxy interactions in many, though not all, cases, good candidates for gaseous, `active' bars are galaxies in the early stages of interaction, when the gas flow along the bar has just started. We therefore launched a project in which we search distant pairs of galaxies known to contain molecular gas and a stellar bar for gas along the bar.

In this paper, we report the discovery of one of the longest continuous molecular bars known, extending over [FORMULA] kpc, in the galaxy UGC 2855. UGC 2855 and its companion UGC 2866, at a projected distance of 10´, have never been studied individually but only as parts of surveys (in molecular gas by Sofue et al. 1997, Elfhag et al. 1996, Aalto et al. 1995, Young et al. 1995). Both of them are, however, FIR-bright IRAS galaxies containing a large amount of molecular gas (Aalto et al. 1995). We sum up their few known properties in Table 1. Following Condon et al. (1996), a distance of 20 Mpc to both galaxies is adopted throughout this paper. Thus, an angular size of [FORMULA] corresponds to 480 pc on a linear scale, and UGC 2855 and UGC 2866 have a (projected) distance to each other of [FORMULA] kpc. Sofue et al. (1997), who included UGC 2855 in their sample of galaxies for which they obtained rotation curves from CO single dish observations, derive a distance of 28.77 Mpc from the Tully-Fisher relation. If this distance is correct, the length of the bar in UGC 2855 rises to [FORMULA] 10 kpc.


Table 1. Properties of UGC 2855 and UGC 2866.
a) Radio continuum peak position (Condon et al. (1996)
b) calculated from the axes ratio
c) from the RC3 catalogue (de Vaucouleurs et al. 1991)
d) from the UGC catalogue (Nilson 1973)
e) from Condon et al. (1996)
f) from Cohen & Volk 1989
g) from Zenner & Lenzen 1993

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© European Southern Observatory (ESO) 1999

Online publication: May 6, 1999