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Using the VLA and ALMA, we have obtained CO(2-1), [C II], [N II] lineemission and multiple dust continuum measurements in a sample of "normal"galaxies at $z=5-6$. We report the highest redshift detection of low-$J$ COemission from a Lyman Break Galaxy, at $z\sim5.7$. The CO line luminosityimplies a massive molecular gas reservoir of $(1.3\pm0.3)(\alpha_{\rmCO}/4.5\,M_\odot$ (K km s$^{-1}$ pc$^2)^{-1})\times10^{11}\,M_\odot$,suggesting low star formation efficiency, with a gas depletion timescale oforder $\sim$1 Gyr. This efficiency is much lower than traditionally observed in$z\gtrsim5$ starbursts, indicating that star forming conditions in MainSequence galaxies at $z\sim6$ may be comparable to those of normal galaxiesprobed up to $z\sim3$ to-date, but with rising gas fractions across the entireredshift range. We also obtain a deep CO upper limit for a Main Sequence galaxyat $z\sim5.3$ with $\sim3$ times lower SFR, perhaps implying a high$\alpha_{\rm CO}$ conversion factor, as typically found in low metallicitygalaxies. For a sample including both CO targets, we also find faint [N II]205$\,\mu$m emission relative to [C II] in all but the most IR-luminous"normal" galaxies at $z=5-6$, implying more intense or harder radiation fieldsin the ionized gas relative to lower redshift. These radiation propertiessuggest that low metallicity may be common in typical $\sim$10$^{10}\,M_\odot$galaxies at $z=5-6$. While a fraction of Main Sequence star formation in thefirst billion years may take place in conditions not dissimilar to lowerredshift, lower metallicity may affect the remainder of the population.

Low star formation efficiency in typical galaxies at z=5-6