PRUSSIC III -- ALMA and NOEMA survey of dense gas in high-redshift star-forming galaxies

TL;DR

This study uses ALMA and NOEMA to detect dense gas tracers in high-redshift star-forming galaxies, revealing higher excitation, shorter depletion times, and an increase in cosmic dense-gas density with redshift, advancing understanding of galaxy evolution.

Contribution

First extensive detection of dense gas tracers in high-redshift galaxies, providing new insights into dense gas properties and cosmic evolution.

Findings

Detected 34 dense-gas transitions in 10 of 11 galaxies

Dense gas depletion times are shorter (~23 Myr) than in local galaxies

Cosmic dense-gas density increases by a factor of 7±4 from z=0 to z=2.5

Abstract

Characterising the relationship between dense gas and star formation is critical for understanding the assembly of galaxies throughout cosmic history. However, due to the faintness of standard dense-gas tracers - HCN, HCO+, and HNC - dense gas in high-redshift galaxies remains largely unexplored. We present ALMA and NOEMA observations targeting HCN/HCO+/HNC (3-2) and (4-3) emission lines in eleven (mostly) gravitationally lensed dusty star-forming galaxies (DSFGs) at redshift z = 1.6--3.2. We detect at least one line in 10 out of 11 galaxies. Altogether, we detect 34 dense-gas transitions, more than quadrupling the number of extant high-redshift detections. Additionally, in two targets, we detect lower-abundance CO isotopologues 13^CO and C^18O, as well as CN emission. We derive excitation coefficients for HCN, HCO+ and HNC in DSFGs, finding them to be systematically higher than those in nearby luminous infrared galaxies. Assuming a canonical dense-mass conversion factor (alpha_HCN = 10), we find that DSFGs have shorter dense- gas depletion times (median 23 Myr) than nearby galaxies (~60 Myr), with a star-forming efficiency per free-fall time of 1-2%, a factor of a few higher than in local galaxies. We find a wide range of dense-gas fractions, with HCN/CO ratios ranging between 0.01 and 0.15. Finally, we put the first constraints on the redshift evolution of the cosmic dense-gas density, which increases by a factor of 7+/-4 between z = 0 and z = 2.5, consistent with the evolution of the cosmic molecular-gas density.