Galaxy Halo Assembly Bias

The large scale structure observable today requires an additional influencing factor over the homogenous and isotropic paradigm given by the Friedmann-Lemaitre-Robertson-Walker metric. Quantum perturbations in the early Universe can become macroscopic through inflation, leaving regions of over-density in the matter field. A requirement of galaxy formation is that we need a far greater gravitational potential than can be given through the amount of 'baryonic' matter we observe today. This naturally leads to the suggestion that instead pressureless matter drives the Universe's expansion and structure growth. Over-densities in causal contact grow linearly through perturbation theory, however some regions can grow non-linearly leading to eventual collapse into halos.

Assumed to be composed of dark matter, these halos provide an excellent environment for structure growth, such as galaxies, since the gravitational collapse balances the random motions of the contained particles. Small halos form earlier and then can grow through a series of hierarchical merging. Halo growth is however deeply dependent on the larger geometric environment of the cosmic web which encompasses them. Regions of the cosmic web can be classified as a series of voids, filaments, sheets or knots (e.g. Eardley et al. 2015, Joachimi et al. 2016) depending on the number of dimensions seen to be in gravitational collapse. Knots, observable as galaxy clusters, are collapsing in three dimensions and produce strong tidal forces affecting the growth of smaller surrounding halos, for example.

The characteristic properties of observable galaxies therefore are thought to be inherently linked to both their surrounding halo environment and the geometric environment they reside in. Identifying galaxy features which imply the attributes and history of the halo, however, are often subtle and hence require careful statistical analysis of data-rich surveys. In particular assembly bias corresponds to the theory that for halos of the same mass; earlier forming halos are more strongly clustered than their later forming counterparts. This previously has been detected through finding that central galaxies with a lower specific star formation rate, and therefore are older, typically cluster more than those with active star formation for a given total stellar mass (e.g. Wang et al. 2013). Due to the obvious obstacle of dark matter being electromagnetically invisible, reconstructing a halo's past requires a combination of innovative approaches.

Current (MaNGA (SDSS-IV)) and future (DESI and Euclid) surveys promise the data-rich framework that such analyses require. The first aim of this project is to link the kinematic properties of individual galaxy components, such as stars and gas, to the kinematics of the surrounding dark matter halo. Due to the difference in post-merger relaxation timescales of gas and stars, their rotational misalignment may be able to act as a primer on halo age and therefore a further test of assembly bias.





Training Completed:
STFC Summer School
Tutoring and Demonstrating in the Sciences: An introduction
Assessment & Academic Misconduct (Science): An introduction
Postgraduate Researcher Induction (all disciplines)
Postgraduate Researcher Essentials
Diversity in the Workplace Online Training Module September 2016
Class Rep training for PG reps

Training in process / to be completed:
SUPA Induction
SUPA Graduate School (20 hours core skills / 40 hours physics learning including: Advanced Data Analysis (Semester 1), Observing Course (Semester 2))
MaNGA conference and new student school in Shanghai
Telescope training trip at Teide Observatory, Tenerife