PRUSSIC I - a JVLA survey of HCN/HCO+/HNC (1-0) emission in z$\sim$3 dusty galaxies: Low dense-gas fractions in high-redshift star-forming galaxies

Abstract

Dusty star-forming galaxies (DSFGs) at redshift z$\ge$1 are among the most vigorously star-forming galaxies in the Universe. However, their dense ($\ge$10${}^5$ cm${}^{-3}$ ) gas phase - typically traced by HCN(1-0) - remains almost entirely unexplored: only two DSFGs have been detected in HCN(1-0) to date. We present results of a JVLA survey of the J=1-0 transition of HCN, HCO+, and HNC(1-0) in six strongly lensed DSFGs at z = 2.5 - 3.3, effectively doubling the number of DSFGs with deep observations of these lines. We detect HCN(1-0) emission in one source (J1202+5354, 4.4$\sigma$), with a tentative HCO+ (1-0) detection in another (J1609+6045, 3.3$\sigma$). Spectral stacking yields strict upper limits on the HCN/FIR ($\le$3.6$\times$10${}^{-4}$) and HCN/CO(1-0) ratios ($\le$0.045). The inferred HCN/FIR ratios (a proxy for the star-formation efficiency) are consistent with those in z$\sim$0 FIR-luminous starbursts. However, the HCN/CO ratios - a proxy for the dense-gas fraction - are a factor of a few lower than suggested by the two previous DSFG detections. Our results imply that most DSFGs have low dense-gas fractions. A comparison with Krumholz & Thompson (2007) models of star-forming galaxies indicates that the bulk of gas in DSFGs is at lower densities ($\approx$10${}^2$ cm${}^{-3}$ ), similar to "normal" star-forming galaxies, rather than ultraluminous starbursts.

Submitted to A&A 28th April 2022, accepted 19th July 2022

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• z-GAL -- A NOEMA spectroscopic redshift survey of bright Herschel galaxies: [III] Physical properties
• Dense gas scaling relations at kiloparsec scales across nearby galaxies with the ALMA ALMOND and IRAM 30m EMPIRE surveys
• PRUSSIC II -- ALMA imaging of dense-gas tracers in SDP.81: Evidence for low mechanical heating and a sub-solar metallicity in a z=3.04 dusty galaxy