We use Spitzer 24Â \micron , 70Â \micron and ground based H \alpha data for a sample of 40 SINGS galaxies to establish a star formation rate ( SFR ) indicator using 70Â \micron emission for sub–galactic ( \sim 0.05 - 2 kpc ) line-emitting regions and to investigate limits in application .
A linear correlation between 70Â \micron and SFR is found and a star formation indicator SFR ( 70 ) is proposed for line-emitting sub-galactic regions as \Sigma ( SFR ) ( { M _ { \odot } \cdot yr ^ { -1 } \cdot kpc ^ { -2 } } ) = 9.4 \times 10 ^ { -44 } % \Sigma ( 70 ) { ( ergs \cdot s ^ { -1 } \cdot kpc ^ { -2 } ) } , for regions with 12 + log ( O / H ) \gtrsim 8.4 and \Sigma ( SFR ) \gtrsim 10 ^ { -3 } ( M _ { \odot } \cdot yr ^ { -1 } \cdot kpc ^ { -2 } ) , with a 1- \sigma dispersion around the calibration of \sim 0.16 Â dex .
We also discuss the influence of metallicity on the scatter of the data .
Comparing with the SFR indicator at 70Â \micron for integrated light from galaxies , we find that there is \sim 40 \% excess 70Â \micron emission in galaxies , which can be attributed to stellar populations not involved in the current star formation activity .