Mid IR properties of early-type galaxies
With the advent of the Spitzer satellite new frontiers have opened in the study of early type galaxies (ETGs). The mid infrared (MIR) spectral range allows to trace the presence of intermediate age stellar populations, detecting even tiny amounts of ongoing star formation and probing the gas conditions within ETGs. Intermediate age stellar populations are recognized through the analysis of the 10 micron broad silicate emission due to mass loosing AGB stars. The existence of this spectral feature has been proposed by Bressan, Granato & Silva (1998) and recently detected with Spitzer in bright ETGs of the Virgo cluster (Bressan et al 2006). Ongoing star formation is revealed by prominent PAHs features. The gas conditions can be analysed through the diagnostic of atomic and molecular emission lines.
Results: A systematic MIR study of about hundred ETGs in cluster and in the field is underway with observations performed in Spitzer Cycle 1 (IRS, PI Bressan, 20 hr), Cycle 2 (IRS Peak-Up, PI Bressan, 49 hr) and Cycle 3 (IRS, PI Ramazzo, 49hr), and with archival data available at the Spitzer Science Center. UV (Galex), Optical and NIR follow-up is under completion. NGC 4382 in the figure, is an example of an old passive evolving system. About 80% of the 20 Virgo ETGs galaxies show this kind of feature (Bressan et al 2006). An example of a Virgo ETG with ongoing nuclear star formation is NGC 4435 (Panuzzo et al 2006) an early-type galaxy interacting with NGC 4438. With MIR spectroscopy we obtain a very clean picture of the activity in the nuclear region of this galaxy. MIR nebular lines constitute a strong diagnostic tool to disentangle star formation and AGN activity. MIR observations also allow a direct determination of the chemical abundance of the gas, the mass involved in the episode of star formation and the mass of the obscuring dust. The MIR emission in the field galaxy NGC 1553 and NGC 5044 (shown in the figure) is consistent with the presence of more luminous and dust enshrouded AGB stars, suggesting the presence of an intermediate age population. These data allow, for the first time, an unbiased quantification of rejuvenation episodes, that cannot be revealed in the optical and UV because of the age metallicity degeneracy and even tiny amount of attenuation. Surprisingly strong H2 molecular emission coupled with lack of ongoing star formation, is detected in three out of about sixty field ETGs analysed so far (NGC 5044 is the example shown here). These fields are strikingly similar in the optical (many are classified as LINERs and are consistent with shock excitation). Rotational line emission is a genuine probe of H2 (warm) gas whose nature is under analysis. Combining 16 µm photometry of 50 ETGs in Coma cluster (Spitzer IRS Peak-Up) with 2MASS data, we have obtained the first mid infrared C-M relation of cluster ETGs (Clemens et al. 2008).
People: A. Bressan, L. Buson, M. Clemens, R. Rampazzo
Collaboration: F. Annibali (STScI), L. Danese(SISSA), P. Panuzzo (CEA, France), G. L. Granato, L. Silva (INAF OA Trieste), J. R. Valdes, O. Vega (INAOE, Mex)
Recent Publications: Clemens et al. (2008), MNRAS, 392,982; Bressan et al. (2007), IAU Symposium 241,395; Panuzzo et al. (2007), ApJ 656,206