In this colloquium, I will present some of our recent works on two major topics.
First is the formation of first galaxies at z>6 and their radiative properties. We perform radiative transfer (RT) calculations (post-processing) of simulated galaxies in cosmological hydrodynamic simulations using GADGET-3 with star formation and feedback, and show the variable nature of multi-wavelength appearance of first galaxies, as observed recently by ALMA via far-IR lines at z>7.
Second topic is the direct collapse (DC) scenario of supermassive black hole (SMBH) seed formation. We have performed, for the first time, on-the-fly RT simulation (with diffuse radiation) of DC scenario in a realistic cosmological context using Enzo AMR code. We have succeeded in simulating the optically-thick phase of initial collapse using FLD approximation, and evolved the system for about 40 yrs, where a thick disk with some rotation formed. We discuss the implication of our results on success/failure of DC scenario.