Properties of Dust in Various Environments of Nearby Galaxies

Abstract

We have performed systematic studies of the properties of dust in various environments of nearby galaxies with AKARI. The unique capabilities of AKARI, such as near-infrared (near-IR) spectroscopy combined with all-sky coverage in the mid-and far-IR, enable us to study processing of dust, particularly carbonaceous grains includings polycyclic aromatic hydrocarbons (PAHs), for unbiased samples of nearby galaxies. In this paper, we first review our recent results on individual galaxies, highlighting the uniqueness of AKARI data for studies of nearby galaxies. Then we present results of our systematic studies on nearby starburst and early-type galaxies. From the former study based on the near-IR spectroscopy and mid-IR all-sky survey data, we find that the properties of PAHs change systematically from IR galaxies to ultraluminous IR galaxies, depending on the IR luminosity of a galaxy or galaxy population. From the latter study based on the mid-and far-IR all-sky survey data, we find that there is a global correlation between the amounts of dust and old stars in early-type galaxies, giving an observational constraint on the origin of the dust.Key words: Infrared: ISM -Infrared: galaxies -ISM: dust, extinction -Galaxies: ISM UNIQUENESS OF AKARI DATAFor studies of nearby galaxies, AKARI has provided us with unique datasets. All-sky mid-and far-infrared (far-IR) surveys at wavelengths of 9, 18, 65, 90, 140 and 160 µm are very powerful to detect IR emission widely extended around galaxies and even into intergalactic regions. Figure 1 shows the mid-and far-IR images of the edge-on starburst galaxy, NGC 253, obtained by the all-sky surveys. They clearly exhibit IR emission widely extended to the galactic halo regions. The AKARI surveys also enable us to investigate mid-to far-IR spectral energy distributions (SEDs) for unbiased samples of nearby galaxies all over the sky.Near-IR spectroscopy for a wavelength range of 2.5−5 µm offers valuable information on the interstellar media in galaxies with dust emission/absorption bands due to http://pkas.kas.org hydrocarbon particles and ices as well as hydrogen recombination emission lines. For example, we detected galactic outflows of polycyclic aromatic hydrocarbons (PAHs) in the halos of M 82 and NGC 1569 (Yamagishi et al. 2012;Onaka et al. 2010). We found large variations in the intensity of the aliphatic emission relative to aromatic emission in the galactic superwind regions of M 82 (Yamagishi et al. 2012) and the inner galactic bar of NGC 1097 (Kondo et al. 2012), indicating systematic changes in the properties of hydrocarbon grains possibly due to large-scale shocks. We also revealed spatial variations of relative abundances of interstellar H 2 O and CO 2 ices in NGC 253 and M 82, and found that they depend on the hardness of interstellar radiation field, i.e., massive star-forming activity Yamagishi et al. 2013).Wide dynamic ranges of signal detection are also im-135