(Proposal ID) S13A-073

(PI) Onodera, Masato

(Proposal Title)
Robust measurement of structural parameters for a passive galaxy at z=3

(Abstract)
We have recently discovered, and spectroscopically confirmed with HST/WFC3 
G141 spectroscopy, a passively evolving galaxy at z=2.996, the most distant 
early-type galaxy (ETG) with spectroscopic identification to date.  Structural 
properties of this source would allow us to extend for the first time to a 
spectroscopic ETG at z=3 the measurement of ETG size evolution with redshift. 
Current high resolution imaging (HST/WFC3 F140W) does not allow us to sample 
the rest-frame optical light, which is crucial to properly study the 
morphological structure. We thus propose to obtain K'-band imaging of this 
source with IRCS+AO188 assisted by the laser guide star, allowing to 
investigate: (i) the size evolution for a spectroscopically confirmed ETG 
closer to the formation epoch than any previous study; (ii) color/stellar 
population gradients within the galaxy  (F140W - K' ~ rest-frame U-V), thus 
better probing the underlying stellar mass profile, and gaining new crucial 
insight on the early formation and evolution of this source; and (iii) the 
presence of faint satellites which may be later accreted contributing to its 
size growth. High-resolution imaging probing the rest-frame optical at z=3 is 
currently impossible from space, thus AO from the ground is the only way to 
carry out this project.