NGC 1705 and the evolution of blue compact dwarf galaxies

Abstract

This thesis presents a multi-waveband study of the amorphous galaxy NGC 1705. It is shown to be a nearby (D « 5 Mpc for Hq = 75 kms-1 Mpc-1) nucleated blue compact dwarf (BCD) galaxy. Its emission line spectrum is like that seen in typical emission line galaxies, with an oxygen abundance like that of the LMC. NGC 1705 contains two stellar populations: a central high surface brightness (HSB) population, a kpc in diameter, which is actively forming stars, and a lower surface brightness (LSB) population with an exponential distribution of light, which is also a star forming population around 3 Gyr old. The HSB population of NGC 1705 defines its BCD morphology. It is a region of relatively continuous star formation at least 50 Myr old; about a Gyr old if the star formation rate has remained constant. Embedded objects within the boundaries of the HSB population are manifestations of its recent star formation. These objects include star clusters, some of which are ionizing sources, and perhaps individual supergiant stars. The brightest object is the unresolved off-centre nucleus which is probably a young (13 Myr old) globular cluster with a mass of of ~ 1.5 x 106 Mq. In the light of Ha, NGC 1705 has a bipolar morphology, with the axis of the flow roughly along the continuum minor axis. The expulsive nature of this flow is confirmed by the emission line kinematics, which show velocity splits of around 100 kms-1 over much of the face of the galaxy. A comparison of the integrated Ha and HI velocity profiles indicates that the neutral material is probably entrained in this flow. A simple expansion model is constructed and found to yield an expansion time-scale on the order of the age of the nucleus for any likely set of its free parameters. A comparison of the estimated mechanical energy release of the nucleus, HSB and LSB populations, and that required for the flow confirms that the nucleus provides the largest fraction of the energy required for the flow. It is shown that velocities greater than the escape velocity have already been achieved in the flow, and thus NGC 1705 is losing mass in a galactic wind. This result holds for all mass to light ratios expected for BCD’s, and is not sensitive to details of the flow geometry. The possible evolutionary outcomes from this mass loss includes complete destruction of the galaxy, if the total mass to light ratio is low (MT / LB <~ 0.8); quick evolution into a nucleated dwarf elliptical galaxy, if the gas is removed by the wind but the remainder of the galaxy survives; and the retention of some gas for future star formation. Show more