(Proposal ID) S15B-040

(PI) Morokuma, Tomoki

(Proposal Title)
High-z Low-Mass Active BHs Selected via Hour-Scale Variability

(Abstract)
We propose optical spectroscopic observation with FOCAS for short-time (a few 
hours to a day)  variability-selected AGN at high redshifts possibly with 
supermassive black holes (SMBHs) as small as 10^{5-7} M_sun.  Small SMBHs are 
important building blocks for more massive SMBH formation in quasars and 
elliptical galaxies.  Considering the small size of the system, small SMBHs 
are expected to show rapid variability,  which can be an effective method to 
select them as an alternative method to deep X-ray  or deep optical 
spectroscopic observations if we utilize the unique wide-field deep imaging 
capability of Hyper Suprime-Cam (HSC) on the Subaru telescope. The targets in 
this proposal were selected using the {deepest} high-cadence (1-hour interval) 
HSC data  taken in Novermber, 2014 (S14B-048).  Among rapidly variable sources 
located at the center of the host galaxies,  we take optical (FOCAS) spectra 
of 4 good candidates  indicating that they are AGN with small SMBHs at high 
redshifts.  The aims of this proposal are to (1) measure the redshifts,  
(2) measure the CIV, MgII, or Hbeta emission line widths to estimate the SMBH 
masses, and (3) construct a sample of low-mass active SMBHs at higher 
redshifts than our accepted proposal (S15A-138).