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arXiv · 2009.00676

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

Antonio Hernan-Caballero·Henrik W. W. Spoon·Almudena Alonso-Herrero·Evanthia Hatziminaoglou·Georgios E. Magdis·Pablo G. Perez-Gonzalez·Miguel Pereira-Santaella·Santiago Arribas·Isabella Cortzen·Alvaro Labiano·Javier Piqueras·Dimitra Rigopoulou
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Abstract

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.

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Antonio Hernan-Caballero, Henrik W. W. Spoon, Almudena Alonso-Herrero, Evanthia Hatziminaoglou, Georgios E. Magdis, Pablo G. Perez-Gonzalez, Miguel Pereira-Santaella, Santiago Arribas, Isabella Cortzen, Alvaro Labiano, Javier Piqueras, Dimitra Rigopoulou. 2020-09-01. Extinction in the 11.2 micron PAH band and the low L_11.2/L_IR in ULIRGs. https://doi.org/10.1093/mnras%2Fstaa2282

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