Dwarf galaxies in nearby Southern groups

Abstract

This thesis describes a successful survey to find dwarf irregular galaxies in the two most nearby southern groups of galaxies, the Sculptor group at 2.5 Mpc and the Centaurus A group at 3.5 Mpc. Candidates were selected from a visual search on ESO SRC J films, then observed in neutral hydrogen (HI) at Parkes, as well as in Ha. In the Sculptor group 12 dwarfs are detected, and in Centaurus A 20 (from which 21 were already known members), yielding a dwarf-to-giant ratio of roughly 3. Some of the detected dwarfs in Sculptor are amongst the faintest dwarf irregulars ever found. The spatial and velocity distributions of these dwarfs are studied, and their radial velocities used to deduce the dynamical state of each group. Surface photometry in B,R,I of these dwarfs is presented. The structural properties of the dwarfs of each group are compared. The dwarfs are shown to follow a surface-brightness-magnitude relation, and their effective radii is shown to decrease with magnitude over the whole range of 7 magnitudes. The luminosity function for the two groups is found to be still rising much fainter than previously believed, down to Mb — —12.5. But despite the large number of dwarfs detected the slope is a = —1.1. For a subsample of 8 dwarfs, high resolution HI imaging with the VLA and the AT is presented. . Two objects are found to have peculiar kinematics (NGC 625, ESO 245-G05), possibly due to interactions. For the remaining objects (UGCA 442, SDIG, ESO 381-G20, DDO 161, ESO 444-G84, ESO 325-Gll), rotation is clearly supporting these systems. Multi-component mass-models are calculated, revealing dark matter accounts for between 50% and up to more than 90% of the total mass in these galaxies. Scaling laws for the dark halo parameters are inspected, and it is shown that the total mass-to-light ratio is increasing for fainter bluer dwarfs, reaching up to (A//LB)dyn = 90. Show more