Extinction in the 11.2 mu m PAH band and the low L_11.2/L_IR in ULIRGs

We present a method for recovering the intrinsic (extinction-corrected) luminosity of the 11.2 μm PAH band in galaxy spectra. Using 105 high S/N Spitzer/IRS spectra of star-forming galaxies, we show that the equivalent width ratio of the 12.7 and 11.2 μm PAH bands is independent on the optical depth (τ), with small dispersion (∼5 per cent) indicative of a nearly constant intrinsic flux ratio R_int = (f12.7/f11.2)int = 0.377 ± 0.020. Conversely, the observed flux ratio, R_obs = (f12.7/f11.2)obs, strongly correlates with the silicate strength (Ssil) confirming that differences in R_obs reflect variation in τ. The relation between R_obs and S_sil reproduces predictions for the Galactic Centre extinction law but disagrees with other laws. We calibrate the total extinction affecting the 11.2 μm PAH from R_obs, which we apply to another sample of 215 galaxies with accurate measurements of the total infrared luminosity (L_IR) to investigate the impact of extinction on L_11.2/L_IR. Correlation between L_11.2/L_IR and R_obs independently on L_IR suggests that increased extinction explains the well-known decrease in the average L_11.2/L_IR at high L_IR. The extinction-corrected L_11.2 is proportional to L_IR in the range L_IR = 109–1013 L⊙. These results consolidate L_11.2 as a robust tracer of star formation in galaxies.