The Arecibo ARP 220 Spectral Census. I. Discovery of the Pre-Biotic Molecule Methanimine and New Cm-Wavelength Transitions of Other Molecules

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 (CH₂NH) in emission, three v ₂ = 1 direct l-type absorption lines of HCN, and an absorption feature likely to be from either ¹⁸OH or formic acid (HCOOH). In addition, we report the detection of two, possibly three, transitions of λ4 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.