Vigorous star formation in galactic nuclei

Abstract

This thesis presents a study of vigorous star formation in the nuclei of nearby galaxies. A variety of techniques have been employed to investigate several aspects of this phenomenon.Analysis of m i d - an d far-infrared data, taken by the Infrared Astronomical Satellite, suggests that am o n g s t nearby spiral galaxies the presence of a stellar bar is a necessary condition for the existence of vigorous star-formation activity. Such regions contribute, on average, one third of the luminosity of the host galaxy.Medium-resolution radio continuum observations s h o w that active star formation occurs in the central regions of galaxies, supporting the hypothesis that it is caused by a bar-driven inflow of disk material.Near-infrared spectrophotometry has been used to study several of these highly dust-obscured regions in greater detail. Detection of the hydrogen recombination line Brq, which arises in photoionized gas, is further evidence for the presence of a p o p u ­ lation of massive stars. M e asurements of molecular hydrogen lines demonstrate that this gas is also photo-excited, in contrast with previous studies. Models of the gas excitation in star-forming regions are constructed an d comparison of the predicted line intensities with observations yields information regarding the gas density an d structure of the complexes.The ionized gas in the archetypal starburst galaxy M 82 is examined in greater detail through measurements made in mm-wavelength recombination lines ( H 4 0 and H53a). These observations avoid the problems which occur when shorter-wavelength lines are employed since they are unaffected by dust attenuation. The ionization state of the gas an d the total luminosity are used to infer the range of stellar masses present in this galaxy. This range may be very narrow.