Marie Treyer (Caltech)

Title: Star-Formation and AGN activity in normal galaxies from mid-infrared spectra


We investigate the use of mid-infrared Polycyclic Aromatic Hydrocarbon
(PAH) bands, continuum and emission lines as probes of star-formation
and AGN activity in a sample of 100 `normal' and local (z~0.1) galaxies.
The MIR spectra were obtained with the Spitzer Space Telescope Infrared
Spectrograph (IRS). The continuum and features were extracted using PAHFIT
(Smith et al. 2007), a decomposition code which we find to yield PAH
strengths up to 30 times larger than the commonly used spline methods.
Despite the lack of extreme objects in our sample (such as strong AGNs,
low metallicity galaxies or ULIRGs), we find significant variations in
MIR spectral properties and systematic trends between these properties
and optically derived physical properties such as age, metallicity and
radiation field hardness. Based on the strongest of these trends, we
revisit the diagnostic diagram relating PAH equivalent widths and
[NeII]/[OIV], and find it very efficient at distinguishing weak AGNs from
SF galaxies in a region of the parameter space previously quite blurred
when spline interpolations were used to fit the continuum.
The luminosity of individual MIR components (PAH, continuum, neon lines)
are found to be tightly correlated to the total IR luminosity and can be
used to estimate dust attenuation in the UV and in Halpha lines based on
energy balance arguments.