ABSTRACT

The fact of starburst activity in a non-negligible fraction of galaxies implies a refuelling mechanism, since the star formation rates observed are sufficient to exhaust the gas in situ over relatively short timescales. A number of “gas bars” – elongated structures in which molecular gas is observed to flow in markedly non-circular orbits – has been observed in galaxies with circumnuclear star-forming activity. In this paper I discuss whether the observed properties of these flows fit the dynamical models which have been proposed for refuelling starbursts, concluding that they may well do so, but not according to some rather over-simplified previously postulated scenarios.

INTRODUCTION

Observations during the past five years of molecular gas in external galaxies on scales of order 100 pc, made possible by the commissioning of millimetre wave interferometers, have virtually put within our research the solution to the problem of how starbursts are refuelled. The need for refuelling is clear since in the majority of cases the observed star-formation rates, of order 0.1 to 1 M⊙ per year, would exhaust the molecular gas observed within 100 pc of the nucleus in times between 108 and 107 years. On the other hand the observed association of starbursts and also Seyferts with the presence of bars in spirals, implies a causal link which would be satisfied naturally by the property of a bar to brake the rotational motion of interstellar gas, and thus allow it to flow down the gravitational potential gradient towards the nucleus. These considerations have led the molecular observers to search for evidence of the phenomenology of molecular inflow, with some success as I describe below.