Astronomy Tea Talk

I review our current understanding of the star formation histories of early-type galaxies (ETGs), in the context of recent studies of their rest-frame ultraviolet (UV) properties. Contrary to the classical notion of ETGs being old, passively-evolving objects, new rest-frame UV data reveal widespread recent star formation in these galaxies since z~1, with luminous [-23 < M(V) < -21] ETGs forming ~20% of their stellar mass over the last 8 billion years. A strong correspondence between blue UV colours and the presence of morphological disturbances indicates that the star formation is merger-driven. However, the major-merger rate is several factors too low to satisfy the numbers of morphologically-disturbed ETGs, indicating that minor mergers (with mass ratios between 1:4 and 1:10) drive the evolution of ETGs (and massive galaxies in general) after z~1. Using early-release data from the Wide Field Camera 3 (WFC3), I demonstrate how spatially-resolved UV-optical imaging of individual galaxies offers a comprehensive picture of their recent stellar assembly that is far more detailed than what is possible using integrated photometry. I introduce a new Cycle 19 HST-WFC3 programme, which will use UV-optical observations of a representative sample of ETGs to study local star formation and its drivers in unprecedented detail and provide empirical constraints on the (poorly understood) minor-merger process in the local Universe.