(Proposal ID) S17B-061

(PI) Onodera, Masato

(Proposal Title)
Direct metallicity measurement and ISM physical conditions at z>3

(Abstract)
Physical conditions of ionized gas of star-forming regions in galaxies is 
known to evolve toward high redshift, showing more than an order of magnitude 
elevation in ionization parameters at z>3 than those in the local Universe. 
In order to unveil the origin of the elevated ionization parameter at high 
redshift, extensive photoionization modeling is essential, but currently the 
number of free parameters is large and an inevitable systematic uncertainty is 
observationally introduced by the metallicity measured with locally defined 
calibrations. Here, we propose a systematic near-IR spectroscopic observation 
of Ly alpha  emitters at z=3.1 and 3.7 with Keck/MOSFIRE aiming at individually
detecting [O III] lambda4363 for ~50 such objects, which enables us an 
unbiased estimate of gas-phase metallicity via the electron temperature 
method. By fixing metallicity as well as the electron temperature and density
from the rest-frame emission line ratios, we will investigate which parameters,
i.e., amount of ionizing flux, hardness of stellar spectra, or geometry of H II
regions, are the dominant source(s) of the high ionization parameter. We will 
also provide a metallicty calibration tailored for z>3 star-forming galaxies 
and revisit the mass--metallicity relation and its dependence on star formation
rate at high redshift.