We present the results of interferometric spectral line observations of Arp 220 at 3.5 mm and 1.2 mm from the Plateau de Bure Interferometer (PdBI), imaging the two nuclear disks in H 13 CN(1 − 0) and (3 − 2), H 13 CO + (1 − 0) and (3 − 2), and HN 13 C(3 − 2) as well as SiO(2 − 1) and (6 − 5), HC 15 N(3 − 2), and SO(6 6 − 5 5 ). The gas traced by SiO(6 − 5) has a complex and extended kinematic signature including a prominent P Cygni profile, almost identical to previous observations of HCO + (3 − 2). Spatial offsets 0.1 north and south of the continuum centre in the emission and absorption of the SiO(6 − 5) P Cygni profile in the western nucleus (WN) imply a bipolar outflow, delineating the northern and southern edges of its disk and suggesting a disk radius of ∼ 40 pc, consistent with that found by ALMA observations of Arp 220. We address the blending of SiO(6−5) and H 13 CO + (3−2) by considering two limiting cases with regards to the H 13 CO + emission throughout our analysis. Large velocity gradient (LVG) modelling is used to constrain the physical conditions of the gas and to infer abundance ratios in the two nuclei. Our most conservative lower limit on the [H 13 CN]/[H 13 CO + ] abundance ratio is 11 in the WN, cf. 0.10 in the eastern nucleus (EN). Comparing these ratios to the literature we argue on chemical grounds for an energetically significant AGN in the WN driving either X-ray or shock chemistry, and a dominant starburst in the EN.

We present ALMA and VLA detections of the dense molecular gas tracers HCN, HCO + and HNC in two lensed, high-redshift starbursts selected from the Herschel-ATLAS survey: H-ATLAS J090740.0−004200 (SDP.9, z ∼ 1.6) and H-ATLAS J091043.1−000321 (SDP.11, z ∼ 1.8). ALMA observed the J = 3 − 2 transitions in both sources, while the VLA observed the J = 1 − 0 transitions in SDP.9. We have detected all observed HCN and HCO + lines in SDP.9 and SDP.11, and also HNC(3-2) in SDP.9. The amplification factors for both galaxies have been determined from sub-arcsec resolution CO and dust emission observations carried out with NOEMA and the SMA. The HNC(1-0)/HCN(1-0) line ratio in SDP.9 suggests the presence of photon-dominated regions, as it happens to most local (U)LIRGs. The CO, HCN and HCO + SLEDs of SDP.9 are compatible to those found for many local, infrared (IR) bright galaxies, indicating that the molecular gas in local and high-redshift dusty starbursts can have similar excitation conditions. We obtain that the correlation between total IR (L IR ) and dense line (L dense ) luminosity in SDP.9 and SDP.11 and local star-forming galaxies can be represented by a single relation. The scatter of the L IR − L dense correlation, together with the lack of sensitive dense molecular gas tracer observations for a homogeneous sample of highredshift galaxies, prevents us from distinguishing differential trends with redshift. Our results suggest that the intense star formation found in some high-redshift dusty, luminous starbursts is associated with more massive dense molecular gas reservoirs and higher dense molecular gas fractions.

We explore the recoverability of gas physical conditions with the Large Velocity Gradient (LVG) model, using the public code RADEX and the molecules HCN and CO. Examining a wide parameter range with a series of models of increasing complexity, we use both grid and Monte Carlo Markov Chain methods to recover the input conditions, and quantify the inherent and noise-induced uncertainties in the model results. We find that even with the benefit of generous assumptions, the LVG models struggle to recover any parameter better than to within half a dex, although we find no evidence of systemic offsets. Examining isotopologue lines, we demonstrate that it is always preferable to model the isotopologue abundance ratio as a free parameterdue to large biases introduced in all other parameters when an incorrect ratio is assumed. Finally, we explore the effects of the background radiation temperature on CO and HCN line ratios, with an emphasis on the effect of the CMB at > z 4, and show that while the effect on the line ratios is minor, the effect on the spectral line energy distribution peak is significant and that the CO(1-0) line luminosity to H 2 mass conversion factor (α CO ) needs to be altered to account for the loss of contrast against the hotter CMB as redshift increases.

We present the first observations of H 13 CN 1 0 ( ) -, H 13 CO + 1 0 ( ) -, and SiO 2 1 ( ) -in NGC 6240, obtained with the IRAM Plateau de Bure Interferometer. Combining a Markov Chain Monte Carlo code with Large Velocity Gradient (LVG) modeling, and with additional data from the literature, we simultaneously fit three gas phases and six molecular species to constrain the physical condition of the molecular gas, including mass−luminosity conversion factors. We find M 10 10  of dense molecular gas in cold, dense clouds (T 10 k~K , n 10 ) which we attribute to isotope fractionation in the cold, dense clouds.

A strong correlation between the far-IR and HCN(1−0) line luminosities, known as the Gao-Solomon relation, has been observed to hold over more than 10 orders of magnitude in the local universe. Departures from this relation at redshifts 1.5 have been interpreted as evidence for increased dense gas star formation efficiency in luminous galaxies during the period of peak of star formation in the history of the universe. We examine whether the offsets from the relation can be explained by the hotter Cosmic Microwave Background (CMB) at high redshift, which, due to a loss of contrast against the hotter background, reduces the observable molecular-line flux far more significantly than the far-IR continuum bands. Simple line-of-sight modeling argues for highly significant departures from the Gao-Solomon relation at high redshift for kinetic temperatures ∼15 K, while more complex toy-galaxy models based on NGC 1068 suggest a much weaker effect with the galaxy integrated HCN line flux falling by only 10% at z=3, within the intrinsic scatter of the relation. We conclude that, while the CMB is unlikely to explain the deviations reported in the literature, it may introduce a second-order effect on the relation by raising the low-luminosity end of the Gao-Solomon relation in cooler galaxies. A similar examination of the CO-IR relation finds tantalizing signs of the CMB having a measurable effect on the integrated CO emission in highredshift galaxies, but these signs cannot be confirmed with the current data.