Rybak, M.Van Marrewijk, J.Hodge, J.A.Andreani, P.Calistro Rivera, G.Graziani, L.McKean, J.P.Viti, S.Van der Werf, P.P.2024-04-162024-04-162023-11Rybak, M., Van Marrewijk, J., Hodge, J.A. et al. 2023, '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', Astronomy and Astrophysics, vol. 679, A119, pp. 1-10. https://DOI.org/10.1051/0004-6361/202347315.2329-1273 (print)2329-1265 (online)10.1051/0004-6361/202347315http://hdl.handle.net/2263/95559This paper makes use of the following ALMA data: #2016.1.00663.S, #2017.1.01694.S, and #2018.1.00747.S.We present deep ALMA Band 3 observations of the HCN, HCO+, and HNC(4–3) emission in SDP.81, a well-studied z = 3:042; strongly lensed galaxy. These lines trace the high-density gas, which remains almost entirely unexplored in z 1 galaxies. Additionally, these dense-gas tracers are potentially powerful diagnostics of the mechanical heating of the interstellar medium. While the HCN(4–3) and HNC(4–3) lines are not detected, the HCO+(4–3) emission is clearly detected and resolved. This is the third detection of this line in a high-redshift star-forming galaxy. We find an unusually high HCO+/HCN intensity ratio of 2:2. Based on the modelling of the photodissociation region, the most likely explanation for the elevated HCO+/HCN ratio is that SDP.81 has low mechanical heating, making up less than 10% of the total energy budget, along with a sub-solar metallicity of Z 0:5 Z . While such conditions might not be representative of the general population of high-redshift dusty galaxies, a lower-than-solar metallicity might significantly impact gas masses inferred from CO observations. In addition, we report the detection of CO(0–1) absorption from the foreground lensing galaxy and CO(1–0) emission from a massive companion to the lensing galaxy, approximately 50 kpc to the south-east.en© The Authors 2023. under the terms of the Creative Commons Attribution License.GalaxiesHigh-redshift – galaxiesIndividualSDP.81 – galaxiesISM – submillimeterISMArticle