S19B0118abst
S19B0118
We propose a 1-night MOSFIRE observation of star-forming galaxies located at z=4.6β5.2, the highest redshift interval where rest-frame optical lines ([Oii] and [Neiii]) remain observable at K-band with ground-based telescopes. This timely program will for the first time uncover redshift evolution of physical conditions of galaxy interstellar medium (ISM) beyond z=5, in advance of the upcoming James Webb Space Telescope to be launched no earlier than 2021. At this epoch, (1) UV-luminous objects have two orders of magnitude higher SFR surface densities than local, (2) stellar opacities should be exceptionally low due to a lack of iron, (3) ISM heating by the cosmic microwave background and cosmic ray needs to be taken into account. All these factors can change ISM to be more extreme. With this motivation, we compare ionization states of galaxies by the same, well-calibrated method (i.e., [Neiii]-to-[Oii] line ratio) from the local all the way to the early Universe combining our sample with literature. Such a fair evaluation can provide us with direct insights into extreme ISM properties at z>4 suggested by studies based on emission/absorption lines at the rest-frame FIR and UV which are getting a lot of attention as new diagnostics determining ISM conditions at high redshifts in recent times.
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