M. Hudson: "Downsizing" from the fossil record

This is the final talk of Symposium 235 on Galaxy Evolution, but there will be much more other things going on tomorrow that are worth writing about.

Downsizing was mentioned frequently during the last days and in the presented survey, red, emission-line-less cluster-galaxies are used to measure the "fossil record" of galaxies, i.e. the old stars. The thousands of spectra are sorted by velocity dispersion (a measure of the total mass) and stacked together to get high quality average spectra with many spectral features that can be analysed to get ages of the stellar population.

Tey find that the smaller galaxies have smaller ages, i.e. downsizing. The age-spread is much larger at low masses than at the high-mass end. There was no morphological selection but of course it is ellipical that dominate the sample. The S0-type galaxies are slightly younger than Es with the same sigma, but this trend is weaker than the trend with sigma itself.

Comparisons with the total dynamical mass (also using Sauron-data) there is little room for dark matter (25%).

The ages also correlate with environment, i.e. distance from cluster center (16% change). Again, this is not a strong trend. Metallicity does not show a trend, but alpha-enhancment does. The tilt of the distribution in a color-agnitude diagram comes half from ages, half from metallicity.

By calculating backward, how te CMD would have looked for these galaxies at some earlier time, they can be compared to CMDs at some redshift.