Astron. Astrophys. 360, 447-456 (2000)

1. Introduction

Barred galaxies constitute a major fraction of all disc galaxies classified in the optical, more than 50% including strong bars and intermediate morphologies (Sellwood & Wilkinson 1993). This fraction increases when also near-infrared images are used for classification, thus underlining the importance for the general understanding of the evolution of galaxies. The non-axisymmetric potential has a strong impact on the gas dynamics and the star formation in barred systems. Observations reveal a correlation between the radial abundance gradient and the strength of the bar (Martin & Roy 1994; Friedli et al. 1994; Martinet & Friedli 1997). This is interpreted as the result of two effects caused by the bar: a stronger radial gas flow and hence a stronger radial mixing of metals and the efficiency of star formation. The radial mass transfer concentrates gas near the galactic center and at the ends of the bar at corotation. Enhanced star formation is the consequence of gas accumulation. The rotating bar potential also heats up the outer disk parts which leads to larger stellar velocity dispersions and a radial diffusion of stars. (Sellwood & Wilkinson 1993).

Galactic bars have also been considered to support the central infall of gas to feed a central "monster"(e.g. Beck et al. 1999). Several authors have claimed that active galactic nuclei (AGN) are more likely in barred galaxies than in non-barred ones (e.g. Simkin et al. 1980; Arsenault 1989). Hummel et al. (1990) note that the fraction of central radio sources in barred spirals is by a factor of 5 higher than in non-barred spirals. Other authors doubt that there is a significantly higher number of bars in galaxies harboring an AGN (e.g. Balick & Heckman 1982; Ho et al. 1997). It appears that the concentration of gas on a scale of 1 kpc at the galactic center required to enhance the central star formation can easily be achieved by a bar potential. It seems much more difficult, however, to accumulate enough gas on a scale of a few pc to tens of pc in order to produce an AGN. Other effects depending on the environment of the galaxies (interaction: Elmegreen et al. 1990; H  I contents: Cayatte et al. 1990) play an important role in mass distribution, gas flow, and therefore in the formation and evolution of bars and the star formation history in these systems.

One of the most famous, closest and most widely studied barred galaxies is NGC 4303 (M61), member of the Virgo Cluster, which is observed at an inclination of 27^o (Guhathakurta et al. 1988). Optical spectra of this galaxy indicate that it consists of a nuclear starburst and a LINER or Seyfert 2 nucleus (Filippenko & Sargent 1986; Kennicutt et al. 1989; Colina et al. 1997; Colina & Arribas 1999, hereafter CA99). Indications for a high star formation rate (SFR) in NGC 4303 are given by the numerous H  II regions (Hodge & Kennicutt 1983; Martin & Roy 1992, hereafter MR92) and three observed supernovae (van Dyk 1992). It also shows strong radio emission distributed over the entire disk (Condon 1983). Colina et al. (1997) and CA99 discussed the question of a starburst-AGN connection in this barred galaxy, using optical spectroscopy and HST UV images. The data range from a nuclear spiral structure of massive star-forming regions with an outer radius of 225 pc down to the unresolved core of a size 8 pc. From the UV data it is not clear if the core is a massive stellar cluster or a pure AGN.

VLA observations (Cayatte et al. 1990) show that NGC 4303 is not highly H  I deficient, which can be explained by only slight environmental influences in the outermost region of the Virgo Cluster. The projected distance to M87 is (Warmels 1988). No significant difference of the abundance gradient in the disk of NGC 4303 compared to non-barred spiral galaxies has been observed (MR92). Martinet & Friedli (1997) discussed the abundance gradient slope in terms of bar age. According to them, a steep gradient in the bar and a flat one in the outer disk are typical for a young bar while a single flat gradient in bar and disk characterizes an old bar. MR92did not determine the gradient at large radii because of a small number of H  II regions. Martinet & Friedli (1997) also claimed that bars in late-type spirals with enhanced star formation like NGC 4303 are expected to be young.

Probable interaction companions are the nearby galaxies NGC 4303 A (Condon 1983) and NGC 4292 (Cayatte et al. 1990), at distances of northwest and 10´ northeast, respectively.

© European Southern Observatory (ESO) 2000

Online publication: August 17, 2000
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