Early-type galaxies: the last 8 billion years

Speaker:

Sugata Kaviraj

I review our current understanding of the evolution 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 restframe UV data (from GALEX at low redshift and deep, optical surveys at intermediate redshift) reveal widespread recent star formation in these galaxies since z~1. Luminous [-23 < M(V) < -21] ETGs form up to ~15% of their stellar mass over the last 8 billion years, with their less massive counterparts forming 30-60% of their stars over the same timescale. 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, suggesting that minor mergers (mass ratios between 1:4 and 1:10) are likely to drive the evolution of ETGs (and massive galaxies in general) after z~1. Using early-release data from the Wide Field Camera 3 (WFC3), we demonstrate how spatially-resolved UV-optical imaging of individual galaxies can offer a comprehensive picture of their recent stellar assembly that is far more detailed than what is possible using integrated photometry. We 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.