Galaxies into the Dark Ages

We consider the capabilities of current and future large facilities operating at 2-3 mm wavelength to detect and image the [C II] 158 μm line from galaxies into the cosmic “dark ages” (z ˜ 10-20). The [C II] line may prove to be a powerful tool in determining spectroscopic redshifts, and galaxy dynamics, for the first galaxies. We emphasize that the nature, and even existence, of such extreme redshift galaxies, remains at the frontier of open questions in galaxy formation. In 40 hr, the Atacama Large Millimeter Array has the sensitivity to detect the integrated [C II] line emission from a moderate metallicity, active star-forming galaxy [{Z}A=0.2 {Z}⊙ ; star formation rate ({SFR})=5 {M}⊙ yr-1], at z = 10 at a significance of 6σ. The next-generation Very Large Array (ngVLA) will detect the integrated [C II] line emission from a Milky Way-like SFR galaxy ({Z}A=0.2 {Z}⊙ , {SFR}=1 {M}⊙ yr-1), at z = 15 at a significance of 6σ. Imaging simulations show that the ngVLA can determine rotation dynamics for active star-forming galaxies at z˜ 15, if they exist. Based on our very limited knowledge of the extreme redshift universe, we calculate the count rate in blind, volumetric surveys for [C II] emission at z˜ 10-20. The detection rates in blind surveys will be slow (of the order of unity per 40 hr pointing). However, the observations are well suited to commensal searches. We compare [C II] with the [O III] 88 μm line, and other ancillary information in high z galaxies that would aid these studies.