Low Star Formation Efficiency in Typical Galaxies at z = 5–6

Abstract

Using the VLA and ALMA, we have obtained CO(2-1), [Cii], [Nii] 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 pc 2 ) −1 ) × 10 11 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 ∼ 3 times lower SFR, 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 [Nii] 205 µm emission relative to [Cii] 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 ∼10 10 M galaxies at z = 5 − 6. While a fraction of Main Sequence star formation in the first billion years may take place in conditions not dissimilar to lower redshift, lower metallicity may affect the remainder of the population.

The ALPINE–ALMA [C ii] Survey: Multiwavelength Ancillary Data and Basic Physical Measurements

“…[C II]λ158 µm or [N II]λ205 µm) that provide insights into the gas and dust properties of galaxies (De Looze et al 2014;Pavesi et al 2019).…”

The ALPINE-ALMA [CII] survey

“…While local studies find a good correlation between [CII] and SFR, this relation may change at the lower metallicities of high-z galaxies. There is also evidence that [CII] is often emitted from the diffuse CNM or HII regions in addition to PDRs (Herrera-Camus et al 2017;Pineda et al 2013;Vallini et al 2015), and it also traces the difuse ionized gas; see (Pavesi et al 2016(Pavesi et al , 2019. ALPINE allows calibrating this relation by comparing [CII] derived SFR to other indicators (FIR, UV, SED) over a large range of physical properties.…”

Predicting FIR lines from simulated galaxies

“…However, because of the limited resolution and sensitivity of current observations, such an analysis cannot be performed on many sources, and a clear consensus about their location relative to the KS relation is still missing (see, e.g. Pavesi et al 2019).…”