Abstract
Dusty star-forming galaxies (DSFGs) at redshiftz ≥ 1 are among the most vigorously star-forming galaxies in the Universe. However, their dense (≥105cm−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 the results of aKarl G. JanskyVery Large Array survey of theJ = 1–0 transition of HCN, HCO+, and HNC(1–0) in six strongly lensed DSFGs atz = 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, 4.6σ), with a tentative HCO+(1–0) detection in another (J1609+6045, 3.3σ). Spectral stacking yields strict upper limits on the HCN/FIR (≤3.6 × 10−4) and HCN/CO(1–0) ratios (≤0.045). The inferred HCN/FIR ratios (a proxy for the star-formation efficiency) are consistent with those inz ∼ 0 far-infrared-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 theoretical models of star-forming galaxies indicates that the bulk of gas in DSFGs is at lower densities (≈102cm−3), similar to ‘normal’ star-forming galaxies, rather than ultraluminous starbursts.
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