In the last few years, a number of extremely wide band receivers have been developed to measure the redshift of distant heavily obscured objects. These receiver cover wide frequency bands at a low spectral resolution. Even if this spectral resolution is similar or even wider than the tipical linewidth of local galaxies, it is still possible to study the brightest molecular transitions in relatively large samples of galaxies at a relatively low observing time expense. In this section we summarize some of the latest publications making use of these wide band instruments.
Redshift Search Receiver (RSR) at FCRAO 14m .- The RSR, design as a facility instrument for the 50m LMT, is a dual polarization dual beam instrument with 4 broadband receivers that covers the 37 GHz accross the 3mm atmospheric window, between 74 and 111 GHz. The spectral resolution of ~ 100 km s-1 provided is barely enough to resolve the typical linewidth of ~ 200 km s-1 in the central region of local galaxies. This spectrometer was used to survey the 3 mm atmospheric band towards a sample of 10 galaxies with different types of nuclear activity (see Table 1 in Snell et al. 2011). A total of 33 transition of 13 molecular species were detected. The low number of spectral features as compared with similar scans with the IRAM 30m or Nobeyama 45m is a consequence of the line smoothing due to the low spectra resolution. Still line ratios between the brighter spectral features could be extracted from this survey. For example the HCO+/HCN ratio was observed to be enhanced in AGN dominated galaxies when compared to starburst (SB) dominated galaxies (Snell et al. 2011). This result, in disagreement with the result from Krips et al. (2008), where the ratio HCO+/HCN is observed to be enhanced in SB galaxies, shows the difficulty of establishing activity templates in extragalactic sources. It is indeed difficult to disantangle the contribution from AGN and SB at the low resolution provided by single dish observations. As such, the highest HCO+/HCN ratios from the surveys from Snell et al. (2011) were derived from the LIRGs NGC 3690 and NGC 6240 where, even if known to host an active nucleus, the contribution from the star formation cannot be excluded.
Z-Spec at CSO 10.4m .- Z-Spec is a millimeter wave grating spectrometer dispersing the light to an array of 160 bolometers. Though covering an enormous bandwidth of almost 120 GHz from 190 to 307 GHz, the spectral resolution is very low. Velocity resolution ranges from 700 km s-1 at the lower frequencies to 1200 km s-1 at the higher end. This resolution is significantly coarser than the observed molecular linewidth even in distant galaxies. Still, this receiver has been used to survey large frequency bands in local galaxies such as the 3 positions observed towards the starburst galaxy M 82 (Naylor et al. 2010). With an integration of just 1 hour per position, 10 molecular species were detected within the band. Even though the resolution was heavily limited, it is possible to simultaneous observe three transitions of CS and therefore to constrain and compare the physical properties of the molecular gas in the three observed positions. Similarly, Kamenetzky et al. (2011) observed the central region of NGC 1068 with Z-spec, where they identified 12 molecular species.
Though not intended as a molecular line survey, the observations with Z-Spec towards teh Cloverleaf galaxy at z ~ 2.56 covered the rest frame wavelenghts between 272 and 444 µm (Bradford et al. 2009). At these wavelenghts, four transition of CO (from J = 6-5 up to 9-8) were clearly detected and [CI] at 370 µm blended to the CO 7-6 transition. Additionally, tentative detection of H2O in emission and CH+ and LiH+ in absorption were reported. These CO simultaneous measurements, together with previous lower-J detections with the IRAM PdBI, allow the detailed study of the CO spectral line energy distribution (SLED) in the cloverleaf (Figure 2 in Bradford et al. 2009). The studies at high-z with broad band spectrometers at ground based telescopes are similar to those carried out with Herschel space observatory toward nearby galaxies. For example, the observations with the SPIRE spectrometer covering the frequencies from 467 to 989 GHz measured the CO emission from J = 5-4 to 13-12 towards the ULIRG Mrk 231 (van der Werf et al. 2010). The resulting CO SLED has been modelled with a combination of dense gas exposed to a strong UV radiation and a significant X-ray contribution from the supermassive black hole to explain the line luminosity of the higher transitions. Similar SLED were modelled towards M 82 based on both SPIRE and HIFI spectrometers on board Herschel (Panuzzo et al. 2010, Loenen et al. 2010). The SPIRE spectrum towards Mrk 231 (van der Werf et al. 2010) also resulted in the detection rotational lines of H2O, OH+, H2O+, CH+, and HF, which shows the potential of molecular studies in the submillimeter to infrared wavelenghts.