Understanding the 8 micron vs. Pa-alpha relationship on sub-arcsecond scales in Luminous Infrared Galaxies

TL;DR

This study examines the detailed relationship between 8 micron and Pa-alpha emissions in 122 HII regions within 10 low-redshift LIRGs, revealing how age and PAH features influence their correlation at high spatial resolution.

Contribution

It provides a high-resolution analysis of the 8 micron versus Pa-alpha emission relation in LIRGs, extending previous findings to higher luminosities and clarifying the roles of age and PAH features.

Findings

The 8 micron vs. Pa-alpha relation extends to higher luminosities in LIRGs.

The ratio varies by a factor of ten, influenced by age and PAH contributions.

A model with instantaneous star formation explains the observed ratio range.

Abstract

This work explores in detail the relation between the 8 micron and the Pa-alpha emissions for 122 HII regions identified in a sample of 10 low-z LIRGs with nearly constant metallicity (12 + log (O/H) ~ 8.8). We use Gemini/T-ReCS high-spatial resolution (<~ 0.4" ~ 120 pc for the average distance of 60 Mpc of our sample) mid-infrared imaging (at 8.7 micron or 10.3 micron) together with HST/NICMOS continuum and Pa-alpha images. The LIRG HII regions extend the L_8micron vs. L_Pa-alpha relation found for HII knots in the high-metallicity SINGS galaxies by about two orders of magnitude to higher luminosities. Since the metallicity of the LIRG sample is nearly constant, we can rule out this effect as a cause for the scatter seen in the relationship. In turn, it is attributed to two effects: age and PAH features. The L_8micron/L_Pa-alpha ratio, which varies by a factor of ten for the LIRG HII regions, is reproduced by a model with instantaneous star formation and ages ranging from ~ 4 to 7.5 Myr. The remaining dispersion around the model predictions for a given age is probably due to differential contributions of the PAH features (the 8.6 micron, in our case) to the 8 micron emission from galaxy to galaxy.