Search for Ultra-faint Milky Way Satellites

Abstract

The current standard cosmological model is a remarkable achievement in providing an excellent description of the formation of largest-scale structures in the observable universe. However, there have been persistent tensions between observations and the theoretical model on galactic and sub-galactic scales. One of the most pressing issues was the “plane of satellite” problem: the distribution of observed Milky Way satellite galaxies appears to form a thin polar plane, which is largely inconsistent with the prediction of the standard cosmological model, i.e. an isotropic distribution of dark matter sub-halos. This problem was often seen as a consequence of observational biases because many of previously known satellite galaxies were discovered by the Sloan Digital Sky Survey whose survey footprint mainly focused on the northern Galactic pole region. Subsequent surveys to map the southern sky were thus required for an unbiased observational assessment of the standard cosmological model.

This thesis presents the discovery of five new low-luminosity Milky Way companions i.e. dwarf spheroidal galaxies and star clusters from the Sloan Digital Sky Survey, Dark Energy Survey, and the Stromlo Milky Way Satellite survey project: PegasusIII, Horologium II, and Kim 1–3. Pegasus III is an ultra-faint outer halo dwarf satellite in the constellation of Pegasus. The large mass-to-light ratio inferred from photometric and spectroscopic analysis indicates a substantial amount of dark matter component in Pegasus III. Horologium II is one of the ultra-faint dwarf satellites in the vicinity of the Magellanic Clouds discovered in the Dark Energy Survey Year 1 data. Its proximity to a satellite dwarf Horologium I implies their possible companionship. Kim 1 is an extremely low-mass star cluster in the constellation of Pegasus with an unusually high ellipticity and low central surface brightness, which implies that it is being tidally disrupted. Kim 2 is a low-mass outer halo star cluster in the constellation of Indus. Although this stellar system falls in between the realms of star clusters and dwarf galaxies on the size-luminosity plane, evidence of pronounced mass segregation in the object argues in favour of star cluster-like nature. Kim 3 is another extremely low-luminosity star cluster in the constellation of Centaurus. With the lack of well-defined centre and evidence of mass segregation, Kim 3 is also classified as a star cluster in the final stage of tidal disruption.

Both Pegasus III and Horologium II, as well as the majority of new dwarf satellites discovered by other recent studies, turn out tightly aligned with the original plane of satellites defined by previously known objects. This result confirms the presence of the plane of satellites throughout the whole sky, formerly speculated based on the distribution of clas- sical satellite galaxies decades ago. These findings largely conflict with the prediction of the current standard cosmological model and thus request its reexamination or an alternative model to account for the observation. The discovery of Kim 1–3 implies the possible existence of a large population of such extremely low-mass star clusters. These objects are possibly stripped down versions of formerly more massive star clusters or born extremely low-mass clusters that have survived to the present time. Dedicated observational and theoretical follow-up studies are required to determine their origin, which will shed a light on the formation and evolution history of star clusters in general. Show more