Bars within Bars, Spirals within Spirals

Summary

ABSTRACT

Using new high-resolution imaging at optical wavelengths, we study the morphology of the inner kpc of two barred spirals, NGC 4321 and NGC 4314. In the case of NGC 4321, we present evidence for the existence of a nested bar and a nested spiral structure. In the case of NGC 4314, the inner star formation activity is not organized in spiral arms, but has a ring-like appearance. The role of large scale bars and of possible nested bar structure in the fueling of nuclear starbursts is briefly discussed.

INTRODUCTION

The need to dissipate angular momentum of material flowing into the central region of a galaxy is one of the problems in the process of fueling active galactic nuclei (AGN). Non-axisymmetric potentials are capable of achieving an efficient dissipation of angular momentum, and bars may well trigger the nuclear activity of galaxies (see e.g. Simkin et al. 1980). Shlosman et al. (1989) proposed a mechanism where a set of nested bars can play an important role in this process. “Bars within bars”, possibly of a similar kind to those proposed by Shlosman et al., have been observed in a number of galaxies (see e.g. de Vaucouleurs 1974; Friedli and Pfenniger 1993 and references therein).

We show here preliminary results from an imaging study of two barred spiral galaxies which show a mild form of nuclear activity: a (circum-)nuclear starburst. These galaxies, NGC 4314 and NGC 4321, are well-known cases where strong formation of massive stars takes place in a region within about 1 kpc in radius from the center (see e.g. Pogge 1989).