  We investigate the use of MIR 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) emission-line galaxies. The MIR spectra were obtained with the Spitzer
Space Telescope Infrared Spectrograph (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 continuum and features were
extracted using PAHFIT (Smith et al. 2007), a decomposition code which we find
to yield PAH equivalent widths 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 PAH, continuum and emission line properties and systematic trends
between these MIR properties and optically derived physical properties such as
age, metallicity and radiation field hardness. We revisit the diagnostic
diagram relating PAH equivalent widths and [Ne II]12.8micrometers/[O
IV]25.9micrometers line ratios and find it to be in much better agreement with
the standard optical star-formation/AGN classification than when spline
decompositions are used, while also potentially revealing obscured AGNs. The
luminosity of individual PAH components, of the continuum, and with poorer
statistics, of the neon emission lines and molecular hydrogen lines, are found
to be tightly correlated to the total IR luminosity, making individual MIR
components good gauges of the total dust emission in SF galaxies. Like the
total IR luminosity, these individual components can be used to estimate dust
attenuation in the UV and in Halpha lines based on energy balance arguments. We
also propose average scaling relations between these components and dust
corrected, Halpha derived star-formation rates.
