THE HEATING OF MID-INFRARED DUST IN THE NEARBY GALAXY M33: A TESTBED FOR TRACING GALAXY EVOLUTION

Abstract

Infrared emission is an invaluable tool for quantifying star formation in galaxies. Because the 8 mu m polycyclic aromatic hydrocarbon (PAH) emission has been found to correlate with other well-known star formation tracers, it has widely been used as a star formation rate (SFR) tracer. There are, however, studies that challenge the accuracy and reliability of the 8 mu m emission as a SFR tracer. Our study, part of the Herschel (Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA) M33 Extended Survey (HERM33ES) open time key program, aims at addressing this issue by analyzing the infrared emission from the nearby spiral galaxy M33 at the high spatial scale of similar to 75 pc. Combining data from the Herschel Space Observatory and the Spitzer Space Telescope, we find that the 8 mu m emission is better correlated with the 250 mu m emission, which traces cold interstellar gas, than with the 24 mu m emission. Furthermore, the L(8)/L(250) ratio is more tightly correlated with the 3.6 mu m emission, a tracer of evolved stellar populations and stellar mass, than with a combination of Ha and 24 mu m emission, a tracer of SFR. The L(8)/L(24) ratio is highly depressed in 24 mu m luminous regions, which correlate with known H II regions. We also compare our results with the dust emission models by Draine & Li. We confirm that the depression of 8 mu m PAH emission near star-forming regions is higher than what is predicted by models; this is possibly an effect of increased stellar radiation from young stars destroying the dust grains responsible for the 8 mu m emission as already suggested by other authors. We find that the majority of the 8 mu m emission is fully consistent with heating by the diffuse interstellar medium, similar to what recently determined for the dust emission in M31 by Draine et al. We also find that the fraction of 8 mu m emission associated with the diffuse interstellar radiation field ranges between similar to 60% and 80% and is 40% larger than the diffuse fraction at 24 mu m.