PhD ceremony Mr. K. George: Recent star formation in cluster early-type galaxies. Evidence from dynamics and stellar populations over the past 7 Gyr

PhD ceremony: Mr. K. George, 11.00 uur, Academiegebouw, Broerstraat 5, Groningen

Dissertation: Recent star formation in cluster early-type galaxies. Evidence from dynamics and stellar populations over the past 7 Gyr

Promotor(s): prof. S.C. Trager

Faculty: Mathematics and Natural Sciences

Unlike the Milky Way Galaxy, some galaxies in the local Universe are observed to be composed of very old, red stars. Popularly called early-type galaxies, these systems are hypothesized to have been fully formed when the Universe was less than half of its current age. Yet, our current understanding of galaxy formation expects galaxies to continue to form and evolve through to the present era. Indeed, the population of early-type galaxies is observed to have increased in stellar mass over the past eight billion years. This thesis addresses this apparent contradiction by studying these predominantly old stellar systems in large, dense clusters of galaxies, in search of evidence for recent star-formation episodes. Using images and spectra from the Hubble Space Telescope, ESO's Very Large Telescope in Chile and the W. M. Keck Telescopes in Hawaii, we studied galaxies residing in four clusters at lookback times as far back as seven billion years. Comparing their dynamical and stellar mass estimates to their luminosity, we find that the galaxies in our sample exhibit strong evidence for recent star formation, independent of the epoch at which they are observed. That is, we have found the recent spark of life in these presumably "dead" systems suggesting that their stars were formed over the course of cosmic history, not in a single burst when the Universe was very young. Therefore, the classic evolutionary picture of "red and dead" galaxies needs revision given the results presented in this thesis.