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

We present a line survey of the ultraluminous infrared galaxy Arp 220, taken with the newly installed SEPIA (Swedish-European Southern Observatory PI receiver for APEX) Band 5 instrument on APEX (Atacama Pathfinder Experiment). We illustrate the capacity of SEPIA to detect the 183.3 GHz H₂O 3_1,3-2_2,0 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) and 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, favour 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 and HCO⁺ refine our knowledge of the interstellar medium physical conditions in Arp 220.