The Evolution of Gas Kinematics in Star-Forming Field and Cluster Galaxies Since z~1

Abstract

A fundamental pursuit of astronomy is to understand how galaxies form and evolve. What drives the decline in the cosmic star formation rate density? Why are high redshift galaxies clumpy and turbulent? How can we explain the emergence of the Hubble sequence? To answer these questions we must unravel a complex interplay of different processes, including gas accretion, star formation, feedback, and environmental effects. Studying the gas kinematics of galaxies can provide valuable insight. In this thesis we use integral field spectroscopy to probe the evolution of star-forming field and cluster galaxies over the past 8 billion years.

We first present a multi-wavelength analysis of 27 dusty starburst galaxies in a massive cluster at z~0.4. It is thought that starbursts represent an intermediate phase in the transition from spirals to S0s in dense environments. We combine H-alpha kinematics with far-infrared imaging and millimetre spectroscopy, and find that most galaxies are rotationally supported, with high angular momentum and large cold gas reservoirs. It appears that the starbursts have only recently been accreted to the cluster. To complete the transition to S0s, they must undergo a dynamical heating of the disk, increase in concentration, and reduce their angular momentum by ~40%. We conclude that the most likely way to achieve this is via multiple tidal interactions with other cluster members.

We next study the velocity dispersion properties of 472 galaxies observed as part of the KMOS Redshift One Spectroscopic Survey (KROSS). Most galaxies at this epoch are rotationally supported, but dynamically hot and highly turbulent. In order to make robust kinematic measurements, we model the effects of beam smearing using a series of mock KMOS data cubes. We then combine KROSS with data from the SAMI survey (z~0.05) and an intermediate redshift MUSE sample (z~0.5), and find that while there is a weak trend between velocity dispersion and stellar mass, at fixed mass there is a strong increase in velocity dispersion with redshift. At all redshifts, galaxies appear to follow the same weak trend of increasing velocity dispersion with star formation rate. We also test the predictions of two analytic models which suggest that turbulence in the ISM is driven by gravitational instabilities or stellar feedback. However we find that further observations are required to rule-out either model.

Finally, to understand the role of galaxy kinematics in “crystallising” the Hubble sequence, we study the HST images of 231 KROSS galaxies. We quantify differences in morphology using the asymmetry parameter. This metric correlates very well with our visual interpretation of “clumpiness”, however there are no strong trends as a function of galaxy kinematics. On average, the velocity dispersion of clumps is consistent with the underlying disk, and there is no evidence to suggest that these star-forming regions are preferentially located towards the outskirts of the galaxy. We propose that adaptive optics assisted IFU observations would provide further insight, allowing us to test clump evolution theories and to study the radial distribution of angular momentum.