2019ApJ...882..168P

Using the Very Large Array and ALMA, we have obtained CO(2-1), [C II], and [N II] line emission and multiple dust continuum measurements in a sample of "normal" galaxies at z = 5-6. We report the highest-redshift detection of low-J CO emission from a Lyman break galaxy, at z ∼ 5.7. The CO line luminosity implies a massive molecular gas reservoir of (1.3 ± 0.3)(α_CO/4.5 M_☉ (K km s^–1 pc²)^–1) x 10¹¹ M_☉, suggesting low star formation efficiency with a gas depletion timescale of order ∼1 Gyr. This efficiency is much lower than traditionally observed in z >= 5 starbursts, indicating that star-forming conditions in main-sequence galaxies at z ∼ 6 may be comparable to those of normal galaxies probed up to z ∼ 3 to date but with rising gas fractions across the entire redshift range. We also obtain a deep CO upper limit for a main-sequence galaxy at z ∼ 5.3 with an approximately three times lower star formation rate, perhaps implying a high α_CO conversion factor, as typically found in low-metallicity galaxies. For a sample including both CO targets, we also find faint [N II] 205 µm emission relative to [C II] in all but the most IR-luminous "normal" galaxies at z = 5-6, implying more intense or harder radiation fields in the ionized gas relative to lower redshift. These radiation properties suggest that low metallicity may be common in typical ∼10¹⁰ M_☉ galaxies at z = 5-6. While a fraction of main-sequence star formation in the first billion yr may take place in conditions not dissimilar to lower redshift, lower metallicity may affect the remainder of the population.