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