Introduction

Several authors have proposed that one of the mechanisms for fuelling active galactic nuclei (AGN) may involve the presence of a stellar bar in the galaxy (Shlosman et al., 1989 and references therein). The bar potential produces an overall instability that drives the ambient gas of the disk inwards, forming a gaseous disk sorrounding the active nucleus at a radius of several hundred parsecs. Under certain circumstances the gas can dissipate its angular momentum and fall into the centre. Numerical simulations show that the movement of the gas towards the nucleus would be preferentially along the stellar bar in two arms (Athanassoula 1992). The shocked material in the flow would be delineated by dust lanes that, in regions close to the nucleus might be seen as a ring–like structure or “inner spiral arms”. We present here evidence of such a structure surrounding the nucleus in the Seyfert 1 galaxy NGC4151.

Results

Broad–band images of the nucleus of NGC4151 were obtained in the U,B,V,I and Z filters with the 1m JKT telescope in La Palma. The images were calibrated in magnitudes and then subtracted in pairs to produce a series of colour maps. The B and V images are clearly contaminated by extended emission lines. However, a line–free colour map was obtained by combining a continuum image obtained by Pérez et al. (1989) in a filter centred at 5700Å, with the I-band image.