At a redshift of z = 0.018, Arp 220 is the nearest ULIRG. With a star formation rate of SFR ~ 300 M yr-1, this galaxy could be considered as a scaled down version of the galaxies observed at high redshift, with SFR up to an order of magnitude higher than that in Arp 220. The 1.3 mm line survey carried out with the SMA (Sect. 3) aimed to establish the differences and similarities between the molecular composition in ULIRGs and that found in local nearby starburst galaxies. Moreover, it aimed to study whether the nuclear power source, AGN and/or SB, had an imprint in the chemical composition. Previous to this spectral scan, a number of molecular studies had been carried out towards this source. The paper by Greve et al. (2009) presented a complete compilation of all single dish observations of CO, HCN, HCO+, HNC and HNCO, as well as CO isotopologues in Arp 220. A detailed study of the molecular content was also carried out in absorption with the Infrared Space Observatory (ISO) between 25 to 1300 µm (González-Alfonso et al. 2004).
The overall chemical composition derived from the 1.3 mm band emission lines resemble that in starbursts such as NGC 253. However, the emission from vibrationally excited transitions of HC3N and CH3CN, never detected in local starbursts, was detected towards Arp 220 (Martín et al. 2011). Vibrational temperatures derived from these transitions are in the range of Tvib = 300-500 K. Within our Galaxy, vibrationally excited emission of HC3N is observed to be tracing the hottest gas around star forming cores (de Vicente et al. 1997, de Vicente et al. 2000). In the extragalactic ISM, vibrational emission of HCN and HC3N have also been recently detected towards the also heavily obscured nucleus of the LIRG NGC 4418 (Costagliola & Aalto 2010, Sakamoto et al. 2010), as well as v2 = 1 l-type absorption lines of HCN towards Arp 220 (Salter et al. 2008) and mid-infrared vibration-rotation bands of C2H2, HCN, and CO2 in a sample of (U)LIRGs (Lahuis et al. 2007). The high vibrational temperatures derived from these detections is suggested to be the consequence of the IR-pumping (Sakamoto et al. 2010). Though a hot component associated to a buried AGN cannot be excluded (Costagliola & Aalto 2010), the detected vibrationally excited emission has been claimed to be most likely tracing the warmest gas in the extremely compact star forming regions within these heavily obscured environments (Martín et al. 2011). This scenario is supported by the detection of the 18O isotopologue of water in the Arp 220 1.3mm line survey. The water abundance estimated from the H218O/C18O ratio of ~ 2 × 105 is similar to that measured in Galactic hot cores (Gensheimer et al. 1996, Cernicharo et al. 2006). Such water detection should imply an enormous star formation concentration of the order of a few 106 Sgr B2(N)-like hot cores in a ~ 700 pc region (Martín et al. 2011).