R. Terlevich on starbursts and downsizing

How does the evolution of galaxies depend on their mass? It's known that small galaxies seem to evolve slower than large ones. This is called downsizing and may seem counter-intuitive if one thinks that galaxies form hierarchically, but it is not. It simply stems from the fact that regions of higher density (like clusters) evolve quicker than low-density regions, where mergers are much less frequent.

Terlevich reconstructs the evolution of H-beta equivalent width from observations and finds a large discrepancy with models which he attributes to the fact that the continuum contains not only the newly formed stars, but also the old stars and thereby the whole star formation history.

Generally, he also finds that more luminous galaxies are older and that more metal-rich ones are older, too. No surprise here, but the correlation seems to be quite tight.