Extinction in the 11.2 micron 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 micron 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 micron PAH bands is independent on the optical
depth, with small dispersion of ~5% indicative of a nearly constant intrinsic
flux ratio R_int = (f_12.7/f_11.2)_int = 0.377 +/- 0.020. Conversely, the
observed flux ratio, R_obs = (f_12.7/f_11.2)_obs strongly correlates with the
silicate strength (S_sil) confirming that differences in R_obs reflect
variation in the optical depth. 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 micron 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/L_sun =
10^9--10^13. These results consolidate L_11.2 as a robust tracer of star
formation in galaxies.
