to IAU
Symposium 241

# Abstract details

Early Type Galaxies in the Mid Infrared
A. Bressan

Abstract
With the advent of the Spitzer satellite new frontiers have been opened in the study of stellar populations of early type galaxies. In fact the mid infrared spectral region allows one to trace the presence of intermediate age stellar populations as well as even tiny amounts of ongoing star formation. The presence of intermediate age stellar populations can be recognized by the detection of the 10$\mu$m silicate emission due to AGB stars. This spectral feature, recently detected with Spitzer in bright early type galaxies of the Virgo cluster, was proposed by Bressan, Granato & Silva 1998 as a powerful tool to disentangle age and metallicity effects. Ongoing star formation is revealed by the presence of prominent features (PAHs and emission lines) in the MIR. By combining MIR infrared spectroscopic observations with existing broad band fluxes it is possible to obtain a very clean picture of the activity in the nuclear region of early type galaxies. MIR nebular lines are a strong diagnostic to disentangle star formation and AGN activity. They also provide a direct estimate of the chemical abundance of the gas. The fit of the global SED allows a precise determination of the mass involved in the episode of star formation, the mass of the obscuring dust and the mass of the gas. Observations in this new spectral window will cast light on the strength of the rejuvenation episodes, whose quantification from optical narrow band indices remains often ambiguous.