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we aim at determining the main source of ionizing radiation and star - formation rate of normal galaxies from mir spectroscopy .
the spitzer - sdss - galex spectroscopic survey is an irs survey of 100 local galaxies in the lockman hole .
the data include galex fuv photometry , sdss optical imaging and spectroscopy , spitzer irac and mips photometry .
the sample has a surface brightness limit of 0.75 mjy sr@xmath3 at 5.8  and a flux limit of 1.5mjy at 24 .
it was selected to cover the range of physical properties of ` normal ' galaxies ( e.g. @xmath4 , @xmath5 , @xmath6 ) .
the redshifts span @xmath7 with a mean of 0.1 similar to that of the full sdss spectroscopic sample .
galaxies are classified as star - forming ( black dots ) , composite ( pink stars ) or agn ( red triangles ) according to the boundaries of @xcite and @xcite on the 2 ha  versus 3hb   `` bpt '' diagram @xcite .
first results were reported by @xcite .
we used pahfit @xcite to decompose the spectra into a sum of dust attenuated thermal dust continuum , pah features and emission lines .
the left panel of fig .
1 shows an example decomposition for a typical sf galaxy .
the right panel shows the mean spectra of sf galaxies , composite galaxies and agns along with the average starburst spectrum of @xcite .
the transition from starburst to sf galaxy to agn is marked by a declining continuum slope , decreased  and   emission and enhanced  emission .
the agn spectrum , and to a lesser extent the starburst spectrum , show weaker pah emission at low wavelength than the sf spectrum , an effect attributed to the destruction of pahs in intense far - uv radiation fields .
@xmath8 we find systematic trends between mir spectral properties and optically derived physical properties , in particular between short wavelength pah ews and 2 ha  ( gas phase metallicity ) , and between /  versus 3hb(radiation field hardness ) ( fig .
2 , left panel ) ; + @xmath8 the genzel et al .
( 1998 ) diagram has better resolution using pahfit than spline decompositions .
it is very similar to the optical `` bpt '' diagram ( fig .
2 , right panel ) .
the mixed sf / composite region may be revealing obscured agns in a large fraction of optically defined `` pure '' sf galaxies .
+ @xmath8 the pah , continuum , ne and h@xmath2 luminosities are tightly and nearly linearly correlated to the total ir luminosity , less so to the dust corrected  luminosity ( sfr ) ( fig .
3 , left panel ) ; + @xmath8 following @xcite , the mir components can be used to estimate dust attenuation in   and uv based on energy balance arguments ( fig .
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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 ( @xmath0 ) 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 @xmath1 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 /  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 h@xmath2 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  lines based on energy balance arguments .