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 ma
ps 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.
We present the spatially-resolved near-infrared (2.5-5.0 um) spectra of the edge-on starburst galaxy NGC253 obtained with the Infrared Camera onboard AKARI. Near the center of the galaxy, we clearly detect the absorption features of interstellar ices
(H_2O: 3.05 um, CO_2: 4.27 um, and XCN: 4.62 um) and the emission of polycyclic aromatic hydrocarbons (PAHs) at 3.29 um and hydrogen recombination line Br alpha at 4.05 um. We find that the distributions of the ices differ from those of the PAH and gas. We calculate the column densities of the ices and derive the abundance ratios of N(CO_2)/N(H_2O) = 0.17 +- 0.05. They are similar to those obtained around the massive young stellar objects in our Galaxy (0.17 +- 0.03), although much stronger interstellar radiation field and higher dust temperature are expected near the center of NGC253.
