Mid-Infrared Spectral Indicators of Star-Formation and AGN Activity in
  Normal Galaxies
  We investigate the use of mid-infrared 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 IRS
as part of the Spitzer-SDSS-GALEX Spectroscopic Survey (SSGSS) which includes
multi-wavelength photometry from the UV to the FIR and optical spectroscopy.
The spectra were decomposed using PAHFIT (Smith et al. 2007), which we find to
yield PAH equivalent widths (EW) up to ~30 times larger than the commonly used
spline methods. Based on correlations between PAH, continuum and emission line
properties and optically derived physical properties (gas phase metallicity,
radiation field hardness), we revisit the diagnostic diagram relating PAH EWs
and [NeII]/[OIV] and find it more efficient as distinguishing weak AGNs from
star-forming galaxies than when spline decompositions are used. The luminosity
of individual MIR component (PAH, continuum, Ne and molecular hydrogen 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 Ha lines based on energy balance
arguments.
