Abstract
We present ALMA, NOEMA, and IRAM-30 m/EMIR observations of the high-density tracer molecules HCN, HCO+, and HNC in three of the brightest lensed dusty star-forming galaxies at z ≃ 3–3.5, part of the Planck’s Dusty Gravitationally Enhanced subMillimetre Sources (GEMS), with the aim of probing the gas reservoirs closely associated with their exceptional levels of star formation. We obtained robust detections of ten emission lines between Jup = 4 and 6, as well as several additional upper flux limits. In PLCK_G244.8+54.9, the brightest source at z = 3.0, the HNC(5–4) line emission at 0.1″ resolution, together with other spatially-integrated line profiles, suggests comparable distributions of dense and more diffuse gas reservoirs, at least over the most strongly magnified regions. This rules out any major effect from differential lensing. This line is blended with CN(4–3) and in this source, we measure a HNC(5–4)/CN(4–3) flux ratio of 1.76 ±0. 86. Dense-gas line profiles generally match those of mid-J CO lines, except in PLCK_G145.2+50.8, which also has dense-gas line fluxes that are relatively lower, perhaps due to fewer dense cores and more segregated dense and diffuse gas phases in this source. The HCO+/HCN ≳ 1 and HNC/HCN ∼ 1 line ratios in our sample are similar to those of nearby ultraluminous infrared galaxies (ULIRGs) and consistent with photon-dominated regions without any indication of important mechanical heating or active galactic nuclei feedback. We characterize the dense-gas excitation in PLCK_G244.8+54.9 using radiative transfer models assuming pure collisional excitation and find that mid-J HCN, HCO+, and HNC lines arise from a high-density phase with an H2 density of n ∼ 105–106 cm−3, although important degeneracies hinder a determination of the exact conditions. The three GEMS are consistent with extrapolations of dense-gas star-formation laws derived in the nearby Universe, adding further evidence that the extreme star-formation rates observed in the most active galaxies at z ∼ 3 are a consequence of their important dense-gas contents. The dense-gas-mass fractions traced by HCN/[CI] and HCO+/[CI] line ratios are elevated, but not exceptional as compared to other lensed dusty star-forming galaxies at z > 2, and they fall near the upper envelope of local ULIRGs. Despite the higher overall gas fractions and local gas-mass surface densities observed at high redshift, the dense-gas budget of rapidly star-forming galaxies seems to have evolved little between z ∼ 3 and z ∼ 0. Our results favor constant dense-gas depletion times in these populations, which is in agreement with theoretical models of star formation.
Highlights
Observational studies of the bright emission lines of 12CO have greatly improved our understanding of the global molecular gas content and relationship to star formation in galaxies from low (e.g., Kennicutt 1998) to high redshifts (e.g., Daddi et al 2010b; Genzel et al 2010; Carilli & Walter 2013)
Emission lines detected with NOrthern Extended Millimeter Array (NOEMA) and Atacama Large Millimeter/submillimeter Array (ALMA) have higher S/N and better resolution than those observed with Eight MIxer Receiver (EMIR), and most of them are well fitted by single Gaussians
This demonstrates that at least part of the high-excitation CO gas emission in PLCK_G244.8+54.9 arises from environments dense enough to be detected in HCN, HCO+, and HNC. This could correspond to a multiphase interstellar medium (ISM), with one or multiple very dense (n > 105 cm−3) and spatially-concentrated nuclear cores emitting HCN, HCO+, and HNC lines and hosting the Eddington-limited starburst identified in C17a, which is distributed over an extended, gas-rich and lower surface brightness disk-like component dominating the low- to mid-J CO emission. Such ISM structures are common in low-redshift ultraluminous infrared galaxies (ULIRGs) and a similar scenario has been recently proposed by Yang et al (2020) for a z = 3.6 sub-millimeter galaxies (SMGs) with dense-gas conditions probed by H2O lines
Summary
Observational studies of the bright emission lines of 12CO have greatly improved our understanding of the global molecular gas content and relationship to star formation in galaxies from low (e.g., Kennicutt 1998) to high redshifts (e.g., Daddi et al 2010b; Genzel et al 2010; Carilli & Walter 2013). Rotational transitions of HCN, HCO+, and HNC are usually collisionally excited by H2 in environments with densities above 105–106 cm−3, about three orders of magnitude higher than 12CO for a given J level, and have been extensively used to study the dense cores of molecular clouds in the Milky Way (e.g., Helfer & Blitz 1997) Despite their low brightness (with fluxes about one order of magnitude lower than for 12CO), these emission lines hold a great potential for tightly constraining. Subsequent studies discussed the behavior of this linear correlation for various highdensity tracer molecules, such as HCO+ (e.g., Papadopoulos 2007), for higher J levels (e.g., Graciá-Carpio et al 2008; Zhang et al 2014), and for high-redshift galaxies (e.g., Gao et al 2007) Most of these extragalactic studies have relied on galaxyintegrated measurements while focusing on central nuclear regions that host most of the dense gas under extreme ISM conditions. The rms values marked with an asterisk are given in mJy rather than mK
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