Systematic Variations in CO₂/H₂O Ice Abundance Ratios in Nearby Galaxies Found with AKARI Near-infrared Spectroscopy

Abstract

We report CO₂/H₂O ice abundance ratios in seven nearby star-forming galaxies based on the AKARI near-infrared (2.5-5.0 μm) spectra. The CO₂/H₂O ice abundance ratios show clear variations between 0.05 and 0.2 with the averaged value of 0.14 ± 0.01. The previous study on M82 revealed that the CO₂/H₂O ice abundance ratios strongly correlate with the intensity ratios of the hydrogen recombination Brα line to the polycyclic aromatic hydrocarbon (PAH) 3.3 μm feature. In the present study, however, we find no correlation for the seven galaxies as a whole due to systematic differences in the relation between CO₂/H₂O ice abundance and Brα/PAH 3.3 μm intensity ratios from galaxy to galaxy. This result suggests that there is another parameter that determines the CO₂/H₂O ice abundance ratios in a galaxy in addition to the Brα/PAH 3.3 μm ratios. We find that the CO₂/H₂O ice abundance ratios positively correlate with the specific star formation rates of the galaxies. From these results, we conclude that CO₂/H₂O ice abundance ratios tend to be high in young star-forming galaxies.