Small-scale cosmology with dwarf galaxies

Abstract

The universe has spawned millions of galaxies – from the largest elliptical galaxies containing billions of stars, to the smallest dwarf galaxies with only a few thousands of stars, just bright enough to not drown in the sea of atmospheric light. Dwarf galaxies are the most common galaxies in the universe and are the main focus of this PhD thesis. By studying the abundance and distribution of dwarf galaxies swarming larger galaxies, we can test our current model of structure formation. In this PhD thesis a thorough search for hitherto undetected dwarf galaxies in the local neighbourhood, using advanced optical telescopes, i.e. the Dark Energy Camera and the Sloan Digital Sky Survey instrument, is presented. In total we have found 108 new dwarf galaxy candidates, corresponding to 10% of the known galaxy population in the Local Volume – if confirmed. We have followed-up three newly detected objects with the Very Large Telescope to measure their distances by resolving the tip of the red giant branch stars and established the memberships of two. This pilot study will be extended in the future. Of special interest is the Centaurus group – a galaxy group similar to our own with two massive galaxies (Cen A and M83) – separated into two main aggregates. Around Cen A the dwarf galaxies tend to be arranged in two planar structures seen almost edge on, which allowed us to consider the positions of our newly detected dwarfs within these planes. Surprisingly, only one plane seems to be significant. We have furthermore determined that 14 out of 16 dwarf galaxy satellites follow a coherent movement within this plane – suggesting a co-rotating plane-of-satellite. Such structures are rare in cosmological simulations (< 0.5%) but quite frequent in observations (making it the third case after our own Milky Way system and the Andromeda galaxy) – posing a major challenge to the standard model and with that, to our understanding of the structure formation on the scale of groups of galaxies.