IRAM 30m-EMIR Redshift Search of z = 3-4 Lensed Dusty Starbursts
  selected from the HerBS sample
  Using the EMIR instrument on the IRAM 30m telescope, we conducted a
spectroscopic redshift search of seven z$_{\rm phot}$ $\sim$ 4 sub-millimetre
bright galaxies selected from the Herschel Bright Sources (HerBS) sample with
fluxes at 500 $\mu$m greater than 80 mJy. For four sources, we obtained
spectroscopic redshifts between 3.4 < z < 4.1 through the detection of multiple
CO-spectral lines with J $\leq$ 3. Later, we detected low-J transitions for two
of these sources with the GBT including the CO(1-0) transition. For the
remaining three sources, more data are needed to determine the spectroscopic
redshift unambiguously. The measured CO luminosities and line widths suggest
that all these sources are gravitationally lensed. These observations
demonstrate that the 2 mm window is indispensable to confirm robust
spectroscopic redshifts for z < 4 sources. Finally, we present an efficient
graphical method to correctly identify spectroscopic redshifts.
