Line Survey of External Galaxies

line survey of galaxies
So far about 40 molecules have been detected in nearby external galaxies. As a result, it became possible to study molecular abundances and chemical reactions in external galaxies. Since various galaxies with different properties and activities exist such as those with small or large amount of gas, with starburst and/or active galactic nucleus (AGN), and with interaction, etc., molecular line observations of various types of galaxies give us unique opportunities to study the effect of these properties/activities to the molecular medium. To study the impact of AGN on the interstellar medium, further systematic observations of molecular lines are indispensable. Nevertheless, no systematic unbiased scans of millimeter/submillimeter molecular lines toward AGN has been published to date, although two nearby starburst galaxies have been observed.

We started a project of line survey in the 3-mm band with new 3-mm band 2SB receiver (Nakajima et al. 2008; 2010) in the 45-m telescope at Nobeyama Radio Observatory in Japan toward NGC 1068. The beam size of the 45-m telescope is smaller than the size of the circumnuclear starburst ring in NGC 1068, and is therefore essential for the study of the impact of AGN to surrounding molecules, because we can mitigate the contamination of molecular lines from the circumnuclear starburst region in NGC 1068. We are also observing NGC 253 as a comparison to study the effect of AGN on molecular abundance.

We have successfully detected C2H, cyclic-C3H2 and H13CN in NGC 1068, along with the first detection of C2H N = 1-0 transition in NGC 253 and other detections of previously reported lines. There is no precedent for the detection of carbon-chain and carbon-ring molecules in this galaxy. The column densities of C2H are determined to be 3.4 x 10^15 cm^-2 toward NGC 1068 and 1.8 x 10^15 cm^-2 toward NGC 253. The column densities of the cyclic-C3H2 are determined to be 1.7 x 10^13 cm^-2 toward NGC 1068 and 4.4 x 10^13 cm^-2 toward NGC 253. We calculated the relative abundances of C2H and cyclic-C3H2 with respect to CS in both of the galaxies, and no significant differences were found between NGC 1068 and NGC 253. Thus, these basic carbon-containing molecules are insusceptible to AGN and/or these molecules exist in a cold gas away from AGN.

Reference
Nakajima, T. et al., "Detections of C2H, cyclic-C3H2, and H13CN in NGC 1068", The Astrophysical Journal 728, L38 (2011)
Nakajima, T. et al., "A Molecular Line Survey toward the Nearby Galaxies NGC 1068, NGC 253, and IC 342 at 3 mm with the Nobeyama 45-m Radio Telescope: Impact of AGN on 1 kpc Scale Molecular Abundance", Publ. Astron. Soc. Japan, 70, 7, pp.1-27 (2018)
Takano, S. et al., "A Molecular Line Survey toward the Nearby Galaxies NGC 1068, NGC 253, and IC 342 at 3 mm with the Nobeyama 45-m Radio Telescope: The Data", Publ. Astron. Soc. Japan, 71, S20, pp.1-23 (2019)