The Arecibo Arp 220 Spectral Census I: Discovery of the Pre-Biotic Molecule Methanimine and New Cm-wavelength Transitions of Other Molecules

TL;DR

This study reports the first extragalactic detection of pre-biotic molecule methanimine and new cm-wavelength transitions of other molecules in Arp 220, revealing a rich molecular spectrum that enhances understanding of starburst galaxy chemistry.

Contribution

It presents the discovery of methanimine and new molecular transitions in Arp 220, expanding knowledge of extragalactic molecular spectra and pre-biotic molecules.

Findings

First detection of methanimine in a distant galaxy

Identification of new cm-wavelength molecular transitions

Potential first extragalactic detection of methanol beyond 10 Mpc

Abstract

An on-going Arecibo line search between 1.1 and 10 GHz of the prototypical starburst/megamaser galaxy, Arp 220, has revealed a spectrum rich in molecular transitions. These include the ``pre-biotic'' molecules: methanimine (CH$_{2}$NH) in emission, three $v_{2}=1$ direct l-type absorption lines of HCN, and an absorption feature likely to be from either $^{18}$OH or formic acid (HCOOH). In addition, we report the detection of two, possibly three, transitions of $\lambda$4-cm excited OH not previously detected in Arp~220 which are seen in absorption, and a possible absorption feature from the 6.668-GHz line of methanol. This marks the first distant extragalactic detection of methanimine, a pre-biotic molecule. Also, if confirmed, the possible methanol absorption line presented here would represent the first extragalactic detection of methanol at a distance further than 10 Mpc. In addition, the strong, previously undetected, cm-wave HCN $v_{2}=1$ direct l-type lines will aid the study of dense molecular gas and active star-forming regions in this starburst galaxy.