tokens: luminosity an excellent proxy of the star formation rate. In Arp 220, the L ′ CS(7–6) -to- L ′ CS(4–3) ratio is equal to 0.76. Using the calibration of Zhang et al. ( 2014 ), this leads to an L IR estimate of 1.6 × 10 12 L ⊙ , consistent with the L IR determined by Rangwala et al. ( 2011 ) using Herschel /Spectral and Photometric Imaging Receiver (SPIRE) data (i.e. 1.8 × 10 12 L ⊙ ). The relation linking L CS(4–3) and L IR can now be calibrated with SEPIA Band 5. H 2 O – submillimetre water line emission is considered to be an important gas cooling mechanism in warm molecular clouds and the H 2 O line (ν rest = 183.3 GHz) only requires moderate densities ( n (H 2 ) = 10 4 –10 5 cm −3 ) and low temperatures ( T ∼ 40K) to be excited (Cernicharo et al. 1994 ). We confirm the detection of H 2 O(3 3,1 –2 2,0 ) emission in Arp 220 (Fig. 1 , middle), with a peak flux of 0.14 Jy and a width at half-power of 332±31 km s −1 . These values are consistent with the peak flux (0.17 Jy) and width at half-power (310 km s −1 ) obtained with IRAM data by C06 (see section 4.1 for further analysis). CH 3 OH – extragalactic methanol maser emission has been detected towards many objects (Henkel et al. 1987 ; Sjouwerman et al. 2010 ). In particular, millimetre methanol transitions are known to be sensitive (but sometimes degenerate) tracers of temperature and density. Millimetre CH 3 OH emission has been detected in Arp 220 by Martín et al. ( 2011 ), particularly the 5–4 group (241.8 GHz). Salter et al. ( 2008 ) also report an absorption feature from the 6.7 GHz methanol line. Our SEPIA observations give us an access to the 193.5 GHz methanol (4–3) group (Fig. 1 , bottom). The peak flux is 68 mJy. The water and methanol lines have similar widths (equal to 333 km s −1 for CH 3 OH). We note that this is at the lower end of the line width range (280–590 km s −1 ) estimated from thermal molecules in Arp 220 by G09 . HC 3 N – molecules from the cyanopolyyne family like HC 3 N are standard tracers of the dense gas in Galactic star-forming regions but are also detected in extragalactic environments (Aladro et al. 2011 ; Lindberg et al. 2011 ) in many transitions (from 9–8 to 28–27). Arp 220 is one of the very few ‘HC 3 N-luminous’ galaxies (i.e. that the HC 3 N(10–9) line intensity is at least 15 per cent of the HCN(1–0) line intensity; Aalto et al. 2002 ; Martín et al. 2011 ). The HC 3 N(20–19) line is narrow and detected at 16 per cent of the HNC(2–1) luminosity. Its profile looks symmetric but its peak is shifted. This is consistent with the asymmetry observed in the 10–9 transition by Aalto et al. ( 2002 ) and could suggest that most of the emission emerges from the eastern nucleus. C 34 S – we observe a strong and broad C 34 S(4–3) line at 192.8 GHz. The line is unfortunately at the edge of the velocity range observed. An HC 3 N vibrationally excited line resides in a similar frequency range (192.6 GHz; see Fig. 1 , bottom)). Vibrationally excited emission of this
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