Difference in the spatial distribution between H_2O and CO_2 ices in M82 found with AKARI

TL;DR

This study uses AKARI's near-infrared spectra to map the distribution of H2O and CO2 ices in galaxy M82, revealing significant spatial differences and variations in their abundance ratios across the galaxy.

Contribution

First spatially-resolved mapping of interstellar ice distributions in a galaxy, showing distinct patterns for H2O and CO2 ices and their abundance variations.

Findings

H2O ice is widely distributed in M82

CO2 ice is concentrated near the galactic center

Variations in CO2/H2O ice abundance ratio are observed across the galaxy

Abstract

With AKARI, we obtain the spatially-resolved near-infrared (2.5 - 5.0 um) spectra for the nearby starburst galaxy M82. These spectra clearly show the absorption features due to interstellar ices. Based on the spectra, we created the column density maps of H_2O and CO_2 ices. As a result, we find that the spatial distribution of H_2O ice is significantly different from that of CO_2 ice; H_2O ice is widely distributed, while CO_2 ice is concentrated near the galactic center. Our result for the first time reveals variations in CO_2/H_2O ice abundance ratio on a galactic scale, suggesting that the ice-forming interstellar environment changes within a galaxy. We discuss the cause of the spatial variations in the ice abundance ratio, utilizing spectral information on the hydrogen recombination Br{\alpha} and Br{\beta} lines and the polycyclic aromatic hydrocarbon 3.3 um emission appearing in the AKARI near-infrared spectra.