The Structure of Dwarf Galaxies in Extreme Environments

Abstract

The environment in which a galaxy resides plays a key role in how both the stellar component and the dark matter halo evolve. Satellites of massive halos provide important clues as to how accreted halos develop in a dense environment, and are subjected to unique effects such as tidal disruption and gas stripping. Isolated galaxies are shielded from effects induced by other galaxies, and so represent a `clean' test of how galaxies grow in the context of the $\Lambda$CDM paradigm. In this work, I examine the structure of a number of dwarf galaxies in a range of environments, from the most isolated members of the Local Group to satellites of the Milky Way, and consider what this suggests about the evolution of the galaxy halo. I first present new spectroscopy of the isolated dwarf spheroidal galaxy Tucana. Tucana displays a high velocity dispersion consistent with residing in a dense subhalo. I perform Jeans modelling of the density profile of Tucana, using the line--of--sight velocities of its member stars. I find that it favours a high central density consistent with pristine $\Lambda$CDM subhalos, and a massive dark matter halo consistent with expectations from abundance matching. Tucana appears to be significantly more centrally dense than other isolated Local Group dwarfs, and it is plausible that the dearth of recent star formation has allowed it to retain its central mass. I then present new chemo--kinematics of Hercules, a Milky Way satellite which is purported to be tidally disrupting on the basis of its elongated shape and apparent debris surrounding the central galaxy. I find little evidence for tidal disruption in the kinematics of Hercules, with no significant velocity gradient across the galaxy. The proper motion of Hercules is shown to be misaligned with the position angle of the elongation, contrary to expectations from simulations, although this may be resolved by future astrometry. With current data, the orbital path of Hercules is too uncertain to determine the extent to which Hercules has been affected by the tidal influence of the Milky Way. In the final chapter I introduce a new survey of isolated, low mass dwarf galaxies in the Local Group. Under $\Lambda$CDM, galaxies at all mass scales are believed to grow through hierarchical merging of smaller bodies. Traces of this accretion history should be observable as substructures in the virial halo of galaxies. Using photometry of isolated dwarfs across a range of low halo masses, this survey aims to quantify whether such structured halos are observed at all scales, or whether there is a mass scale at which galaxies no longer accrete smaller systems.