Program: GN-2017A-Q-31

Submillimeter Galaxies (SMGs) are the most likely progenitors of modern massive elliptical galaxies. Their strong clustering strength at z ~ 2.5 indicates that they live in massive (∼ 10^13M⊙) dark matter halos. Current models predict that the gas accreted by such halos should be shock-heated to the virial temperature (T ~ 10^7 K), and the cooling rate of this hot plasma is too low to sustain the observed extreme star formation rates of SMGs. Models also indicate that gas-rich major mergers are not likely to be a significant source of cold gas for these SMGs. Nevertheless, a massive reservoir of cold molecular gas must be present if SMGs have lifetimes of more than 100 Myr, and this proposal is part of a program to detect and characterize it. We propose to use extremely rare QSO sightlines to probe cold gas in the circumgalactic medium of foreground SMGs. Such a unique sample of projected SMG-QSO pairs can only be provided by combining the recently completed wide-area Herschel surveys with existing SDSS spectroscopic surveys of QSOs. However, finding projected SMG-QSO pairs is only the first step. Most critically, a spectroscopic redshift of the SMG is needed to (1) confirm that it is at a lower redshift than the projected QSO, and (2) to identify which of the numerous expected absorption line systems in the QSO spectrum is associated with the SMG. The wide spectral coverage of GNIRS makes it currently the instrument of choice for obtaining the SMG redshifts. Here we request 22.4 GNIRS hours to obtain spectroscopic redshifts of 16 SMGs with excellent ALMA positions along QSO sightlines. As the first study targeting the source and characteristics of the inflowing gas in SMGs, this program is in a unique position to constrain galaxy formation models.