The 2nd Generation z(Redshift) and Early Universe Spectrometer Part I: First-light observation of a highly lensed local-ULIRG analog at high-z

We report first science results from our new spectrometer, the 2nd generation z(Redshift) and Early Universe Spectrometer (ZEUS-2), recently commissioned on the Atacama Pathfinder Experiment telescope (APEX). ZEUS-2 is a submillimeter grating spectrometer optimized for detecting the faint and broad lines from distant galaxies that are redshifted into the telluric windows from 200 to 850 microns. It utilizes a focal plane array of transition-edge sensed bolometers, the first use of these arrays for astrophysical spectroscopy. ZEUS-2 promises to be an important tool for studying galaxies in the years to come due to its synergy with ALMA and its capabilities in the short submillimeter windows that are unique in the post Herschel era. Here we report on our first detection of the [CII] 158 $mu m$ line with ZEUS-2. We detect the line at z ~ 1.8 from H-ATLAS J091043.1-000322 with a line flux of $(6.44 pm 0.42) times 10^{-18} W m^{-2}$. Combined with its far-infrared luminosity and a new Herschel-PACS detection of the [OI] 63 $mu m $ line we model the line emission as coming from a photo-dissociation region with far-ultraviolet radiation field, $G approx 2 times 10^{4} G_{0}$, gas density, $n approx 1 times 10^{3} cm^{-3}$ and size between ~ 0.4 and 1 kpc. Based on this model, we conclude that H-ATLAS J091043.1-000322 is a high redshift analogue of a local ultra-luminous infrared galaxy, i.e. it is likely the site of a compact starburst due to a major merger. Further identification of these merging systems is important for constraining galaxy formation and evolution models.