Water, methanol and dense gas tracers in the local ULIRG Arp 220: Results from the new SEPIA Band 5 Science Verification campaign

TL;DR

This study uses the SEPIA Band 5 instrument on APEX to survey molecular lines in Arp 220, revealing new dense gas tracers and providing insights into the galaxy's interstellar medium conditions.

Contribution

First detection of certain molecular transitions in Arp 220 using SEPIA Band 5, enhancing understanding of dense gas and ISM conditions in ULIRGs.

Findings

Detection of new dense gas tracers like HNC(2-1), HCO+(2-1), and methanol lines.

Absence of variability in water megamaser suggests a thermal origin, not AGN-related.

Refined constraints on physical conditions of the ISM in Arp 220.

Abstract

We present a line survey of the ultra-luminous infrared galaxy Arp 220, taken with the newly installed SEPIA Band 5 instrument on APEX. We illustrate the capacity of SEPIA to detect the 183.3 GHz H2O 31,3-22,0 line against the atmospheric H2O absorption feature. We confirm the previous detection of the HCN(2-1) line, and detect new transitions of standard dense gas tracers such as HNC(2-1), HCO+(2-1), CS(4-3), C34S(4-3), HC3N(20-19). We also detect HCN(2-1) v2=1 and the 193.5 GHz methanol (4-3) group for the first time. The absence of time variations in the megamaser water line compared to previous observations seems to rule out an AGN nuclear origin for the line. It could, on the contrary, favor a thermal origin instead, but also possibly be a sign that the megamaser emission is associated with star-forming cores washed-out in the beam. We finally discuss how the new transitions of HCN, HNC, HCO+ refine our knowledge of the ISM physical conditions in Arp 220.