01 January 2015
Congratulations to Allison Wing Shan Man
Allison Wing Shan Man successfully defended her PhD thesis "Galaxy Evolution Over the Past Eleven Billion Years: Mergers and Quiescence" on Wednesday 17 December 2014. Allison has been working at DARK with supervisors Sune Toft and Andrew Zirm.
An intriguing population of distant red galaxies, only visible with deep near-infrared observations, was discovered a decade ago. They are the most massive and evolved galaxies at early epochs, which have undergone a rapid build-up of stellar mass followed by a short-lived, effective phase of star formation quenching. Curiously, their stellar mass surface densities are an order of magnitude higher than their local counterparts, containing 10^11 solar masses within a radius of only 1.5 kpc. Such dense galaxies are very rare in the local Universe, therefore they must have grown significantly in size over the past eleven billion years to become present-day giant ellipticals.
Despite the wealth of observations, the dominant physical mechanisms driving the evolution of quiescent galaxies are still fiercely debated. Is galaxy merging a primary channel in evolving the sizes and stellar masses of quiescent galaxies? Are quiescent galaxies truly passive, or their stellar populations are in fact not old but reddened by dust? I will discuss the findings from my PhD thesis during the defense. Firstly, I will present the latest galaxy merger rate measurements out to z=3. I conclude that galaxy merging can account for the stellar mass growth of massive quiescent galaxies, but can only account of half of the observed size evolution at most. Secondly, I will present a stacking analysis of the mid-, far-infrared and radio data for quiescent galaxies. I confirm that quiescent galaxies, which are selected to have modest dust-unobscured star formation rates, do not host significant dust-obscured star formation. This confirms the existence of a population of quiescent galaxies out to z=3, and corroborates the need for quenching mechanisms. Additionally, the stacked radio flux densities exceed the expectation from the total star formation rates, indicating a widespread presence of low-luminosity radio active galactic nuclei among massive quiescent galaxies.
Read the thesis here >>