Cold accretion in early galaxy formation and its Lyman-alpha signatures

The Lya emission has played an important role in detecting high-redshift galaxies, including currently the most distant one at redshift z=8.6. It may also contain important information on 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 calculations using the newly improved ART^2 code. Our cosmological simulation uses the Aquila initial condition which zooms in onto a Milky Way-like halo with high resolutions, and our radiative transfer couples multi-wavelength continuum, Lya line, and ionization of hydrogen. We find that the modeled galaxy starts to form at redshift z ~ 24 through efficient accretion of cold gas, which produces a strong Lya line with a luminosity of L(Lya) ~ 10^42 erg/s as early as z ~ 14. The Lya 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 Lya luminosity by excitation cooling increases with redshift, and it becomes dominant at z >= 6. We predict that L* galaxies such as the modeled one may be detected at z <= 8 by JWST and ALMA with a reasonable integration time. Beyond redshift 12, however, only Lya line may be observable by narrow band surveys. Our results suggest that Lya line is one of the most powerful tools to detect the first generation of galaxies, and to decipher their formation mechanism.