Secular Evolution in the Green Valley
Thiago Goncalves (Universidade Federal do Rio de Janeiro)
The bimodality in galaxy properties has been observed at low and high redshift, with a clear distinction between star-forming galaxies in the blue cloud and passively evolving objects in the red sequence. The absence of galaxies with intermediate properties indicates that the quenching of star formation and subsequent transition between populations must happen rapidly. By using very deep spectroscopy with the DEIMOS instrument at the Keck telescope we are able to infer the star formation histories of so-called "green valley" galaxies at intermediate redshifts (z~0.8), when the universe was half its current age. We measure the stellar mass flux density of green valley galaxies transiting from the blue cloud to the red sequence and find that this transition happens more rapidly in the past and that at z~0.8 this process happens more rapidly for more massive galaxies. This suggests a top-down scenario in which the massive end of the red sequence forms first, representing another aspect of downsizing, with the mass flux density moving towards smaller galaxies in recent times. It remains an open question, however, which physical mechanisms are responsible for quenching star formation and how they may be more efficient at z~0.8 than at lower redshifts. To tackle this we have recently initiated a project to detect the presence of bars at low and high redshift, and correlate their strength with the quenching timescales. This will allow us to infer the influence of secular evolution in galaxies at different epochs.