Cold accretion in early galaxy formation and its Lyα Signatures

Lyα emission has played an important role in detecting high-redshift galaxies, including recently distant ones at redshifts. It may also contain important information concerning the origin of these galaxies. Here, we investigate the formation of a typical L^∗ galaxy and its observational signatures at the earliest stage by combining a cosmological hydrodynamic simulation with three-dimensional radiative transfer (RT) calculations using the newly improved code. Our cosmological simulation uses the Aquila initial condition, which zooms in on a Milky-Way-like halo with high resolutions, and our RT couples multi-wavelength continuum, Lyα line, and ionization of hydrogen. We find that the modeled galaxy starts to form at redshift z ∼ 24 through the efficient accretion of cold gas, which produces a strong Lyα line with a luminosity of as early as z ∼ 14. The Lyα emission appears to trace the cold, dense gas. The lines exhibit asymmetric, single-peak profiles, and are shifted to the blue wing, a characteristic feature of gas inflow. Moreover, the contribution to the total Lyα luminosity from excitation cooling increases with redshift and becomes dominant at z 6. We predict that L^∗ galaxies such as the modeled one may be detected at z ≲ 8 by the James Webb Space Telescopeand Atacama Large Millimeter Array with a reasonable integration time. Beyond redshift 12, however, the Lyα line may only be observable by spectroscopic surveys. Our results suggest that the Lyα line is one of the most powerful tools to detect the first generation of galaxies and decipher their formation mechanism.