(Proposal ID) S15B-025

(PI) Tanaka, Masayuki

(Proposal Title)
The Size-Mass Relation of Quiescent Galaxies at z~4

(Abstract)
Recent observations have revealed that galaxies undergo a factor of ~3 growth 
in size since z~2.  Quiescent galaxies at z~2 are surprisingly compact (r_{eff}
~1-2kpc) and dense and this has been a challenge to our framework of galaxy 
formation.  So far, the size-mass relation of galaxies up to z~2 has been the 
focus of the field, but one of the promising way to make a step forward is to 
go up to even higher redshifts.  We have identified a number of massive (~
10^{11}M_sun) quiescent galaxies at z=4 in UDS and COSMOS. Public HST/WFC3 
images have revealed their extremely small sizes;  r_{eff}~0.05 arcsec or 0.35 
kpc. However, this size is well below the pixel size of WFC3 and thus are not 
robust. Higher angular resolution observations are crucially needed for 
reliable size measurements. Here we propose to conduct an LGSAO-assisted IRCS 
observation to achieve FWHM~0.06 arcsec, which should be sufficient to 
resolve r_{eff}<~0.03 arcsec.  We aim to measure accurate sizes of 12 quiescent
 galaxies and robustly establish the size-mass relation at z~4 for the very 
first time. We will also measure morphologies and observationally test if the 
Hubble sequence already emerged at z~4.