Mass outflow and its consequences in the circumnuclear zones of galaxies

Abstract

The distribution of hydrogen gas, atomic plus molecular, in the discs of spiral galaxies, takes two characteristic extreme alternative forms. In one, the density peaks at the nucleus, and falls radially monotonically and roughly exponentially with radius. In the other there is a ‘hole’ in the gas distribution in the circumnuclear region. In this paper we examine the distributions of gas, and the kinematics in the central zones of a number of spirals which have been observed both spectroscopically and with photometric mapping. We find in addition to a ring structure in the gas, there is often measurable expansion with higher radial velocities occurring near the nucleus. Associated with the more expanded of these ring structures there appear annuli of younger stars with enhanced metallicities; and inside the gaseous ring older, less metallic populations. A strong correlation exists between the absence of central gas and the size of the nuclear bulge of a galaxy: Sa's and Sb's have lower ratios of gas to stars than Sc's and Sd's. We show that radially progressive bursts of star formation can account for a wide range of these observed phenomena and could be related to the presence ofliners in the interstellar medium close to the nucleus. The energetics and dynamical balance within the burst are considered in terms of three sources of outflow: supernovae, stellar winds, and radiation from massive stars, with the probability that all these mechanisms contribute to the collective phenomenon.