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Astron. Astrophys. 363, 93-107 (2000)

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1. Introduction

Bars are regarded as an important transport mechanism of material toward the central regions of galaxies, fuelling nuclear starbursts and driving galaxy evolution through the concentration of mass close to the nucleus (e.g. Sakamoto et al. 1999, Combes 1990and references therein). To accomplish this, gas has to flow inward through the bar; however, long bars that are filled with dense gas that can be traced in CO are rare and thus likely transient. Among the few specimens known are NGC 1530 (Reynaud & Downes 1998), M 100 (Sempere & García-Burillo 1997), UGC 2855 (Hüttemeister et al. 1999), NGC 2903 and NGC 3627 (Regan et al. 1999). These bars differ significantly in their properties, i.e. velocity field, linewidth and derived shock structure. This is somewhat surprising, since the gas should in all cases be responding to a strong bar potential in a similar way. The dynamics of this response should then have consequences for the properties of the gas in the bar and the nucleus, e.g. produce a diffuse component unbound from clouds (e.g. Das & Jog 1995). The evolutionary state of the bar and the degree of central mass concentration may be instrumental in regulating the conditions of the gas in the bar (see models by Athanassoula 1992). More specifically, the gas is funnelled from outer [FORMULA]-orbits to inner [FORMULA]-orbits as the bar evolves in time (see e.g. the simulations by Friedli & Benz 1993).

To better understand the gas flow, the star formation efficiency along the bar and the feeding of a starburst, it is essential to know the physical state of the gas both along the bar and in the central structure. An analysis based on more molecules than 12CO, the most abundant species after H2, is necessary to investigate gas properties and to go beyond the study of the morphological and kinematic structure of a system. Achieving this goal requires the detection and high-resolution mapping of very faint lines, observations which are at the sensitivity limit of today's millimeter interferometers.

In this paper, we present such a study, carried out in one of the best known gas-rich barred galaxies. NGC 7479, classified a (mild) starburst SBb galaxy containing a LINER nucleus, has a gaseous bar of [FORMULA] kpc projected length (for an assumed distance of [FORMULA] Mpc, see Table 1), which has been repeatedly mapped in 12CO: Sempere et al. (1995) present single dish data in the [FORMULA] and [FORMULA] transition. Interferometric [FORMULA] maps have been obtained by Quillen et al. (1995) and recently by Laine et al. (1999). Our results include the most sensitive 12CO map to date, but we focus on the relation of the 12CO emission to the first interferometric 13CO[FORMULA] and HCN([FORMULA]) data obtained in NGC 7479, and the physical state of the gas we can infer from the comparison of these molecules to the 12CO distribution. Preliminary results of this study were given in Aalto et al. (1997a).


[TABLE]

Table 1. Adopted properties of NGC 7479.
Notes:
a) Radio continuum peak position (Neff & Hutchings 1992, VLA A-Array at 1.490 MHz)
b) from the UGC catalogue (Nilson 1973)
c) from Groosbol (1985) i: angle of inclination
d) from Sempere et al. (1995)
e) Position angle (north-to-east) of the disk major axis (see e.g. the UGC catalogue) and the gaseous bar. The position angle of the stellar bar is even closer to that of the disk major axis (Sempere et al. 1995).


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Online publication: December 5, 2000
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