(Proposal ID) S16A-033

(PI) Fujimoto, Seiji

(Proposal Title)
Uncovering the New Class of ALMA Sources Assisted by Gravitational Lensing

(Abstract)
We propose MOIRCS HK500 spectroscopy for  a) the sample of 11 faint ALMA mm 
sources with optical counterparts at z_{phot}~ 2 and b) the control sample of 
21 sBzK and BX/BM galaxies that are undetected in the ALMA data but with 
optical luminosities/colors similar to a). The a) sample is newly obtained by 
the systematic deep ALMA survey (Fujimoto et al. 2015), and composed of 6 
field and 5 strongly-lensed ALMA sources whose intrinsic mm-band fluxes and 
stellar masses are 1 and 2 order(s) of magnitude, respectively, lower than the 
known submm galaxies. We will obtain MOIRCS spectra having strong Halpha and 
[O{III}]lambda5007 lines and covering weak [N{II}]lambda6583 and Hbeta lines, 
and accomplish the following two goals to uncover these new-class ALMA sources.
1) We will determine spectroscopic redshifts with Halpha and [O{III}] lines, 
and obtain, for the first time, a rough redshift distribution of the faint 
ALMA sources. 2) We will estimate gas-phase metallicity and dust extinction 
with N2 indices ([N{II}]/Halpha) and Balmer decrements (Halpha/Hbeta) in 
stacked and individual spectra for a) and b) samples  to understand the 
difference between the ALMA detected and undetected sources. This program 
will be the first step for statistical studies of new-class ALMA sources.