Neumann, LukasBigiel, FrankBarnes, Ashley T.Gallagher, Molly J.Leroy, AdamUsero, AntonioRosolowsky, ErikBešlić, IvanaBoquien, MédéricCao, YixianChevance, MélanieColombo, DarioDale, Daniel A.Eibensteiner, CosimaGrasha, KathrynHenshaw, Jonathan D.Jiménez-Donaire, María J.Meidt, SharonMenon, Shyam H.Murphy, Eric J.Pan, Hsi AnQuerejeta, MiguelSaito, ToshikiSchinnerer, EvaStuber, Sophia K.Teng, Yu HsuanWilliams, Thomas G.2025-05-232025-05-230004-6361ORCID:/0000-0002-3247-5321/work/184104078http://www.scopus.com/inward/record.url?scp=85209138348&partnerID=8YFLogxKhttps://hdl.handle.net/1885/733752501The property of star formation rate (SFR) is tightly connected to the amount of dense gas in molecular clouds. However, it is not fully understood how the relationship between dense molecular gas and star formation varies within galaxies and in different morphological environments. Most previous studies have typically been limited to kiloparsec-scale resolution such that different environments could not be resolved. In this work, we present new ALMA observations of HCN(1 0) at 260 pc scale to test how the amount of dense gas and its ability to form stars varies with environmental properties. Combined with existing CO(2 1) observations from ALMA and Hα from MUSE, we measured the HCN/CO line ratio, a proxy for the dense gas fraction, and SFR/HCN, a proxy for the star formation efficiency of the dense gas. We find a systematic > 1 dex increase (decreases) of HCN/CO (SFR/HCN) towards the centre of the galaxy, and roughly flat trends of these ratios (average variations < 0.3 dex) throughout the disc. While spiral arms, interarm regions, and bar ends show similar HCN/CO and SFR/HCN, on the bar, there is a significantly lower SFR/HCN at a similar HCN/CO. The strong environmental influence on dense gas and star formation in the centre of NGC 4321, suggests either that clouds couple strongly to the surrounding pressure or that HCN emission traces more of the bulk molecular gas that is less efficiently converted into stars. Across the disc, where the ISM pressure is typically low, SFR/HCN is more constant, indicating a decoupling of the clouds from their surrounding environment. The low SFR/HCN on the bar suggests that gas dynamics (e.g. shear and streaming motions) can have a large effect on the efficiency with which dense gas is converted into stars. In addition, we show that HCN/CO is a good predictor of the mean molecular gas surface density at 260 pc scales across environments and physical conditions.We would like to thank the referee Jonathan Braine for his insightful comments and constructive feedback that helped improve the quality of the paper. This work was carried out as part of the PHANGS Collaboration. LN acknowledges funding from the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) - 516405419. AKL gratefully acknowledges support by grants 1653300 and 2205628 from the National Science Foundation, by award JWST-GO-02107.009-A, and by a Humboldt Research Award from the Alexander von Humboldt Foundation. The work of AKL is partially supported by the National Science Foundation under Grants No. 1615105, 1615109, and 1653300. AU acknowledges support from the Spanish grants PID2019- 108765GB-I00, funded by MCIN/AEI/10.13039/501100011033, and PID2022- 138560NB-I00, funded by MCIN/AEI/10.13039/501100011033/FEDER, EU. ER acknowledges the support of the Natural Sciences and Engineering Research Council of Canada (NSERC), funding reference number RGPIN-2022-03499. MB gratefully acknowledges support from the ANID BASAL project FB210003 and from the FONDECYT regular grant 1211000. MC gratefully acknowledges funding from the DFG through an Emmy Noether Research Group (grant number CH2137/1-1). COOL Research DAO is a Decentralized Autonomous Organization supporting research in astrophysics aimed at uncovering our cosmic origins. KG is supported by the Australian Research Council through the Discovery Early Career Researcher Award (DECRA) Fellowship (project number DE220100766) funded by the Australian Government. KG is supported by the Australian Research Council Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D), through project number CE170100013. JDH gratefully acknowledges financial support from the Royal Society (University Research Fellowship; URF/R1/221620). HAP acknowledges support by the National Science and Technology Council of Taiwan under grant 110-2112- M-032-020-MY3. MQ acknowledges support from the Spanish grant PID2019- 106027GA-C44, funded by MCIN/AEI/10.13039/501100011033. TS acknowledges funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (grant agreement No. 694343). ES acknowledges funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (grant agreement No. 694343). SKS acknowledges financial support from the German Research Foundation (DFG) via Sino-German research grant SCHI 536/11-1. Y-HT acknowledges funding support from NRAO Student Observing Support Grant SOSPADA-012 and from the National Science Foundation (NSF) under grant No. 2108081. TGW acknowledges funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (grant agreement No. 694343). This paper makes use of the following ALMA data ADS/JAO.ALMA#2011.0.00004.SV, ADS/JAO.ALMA#2015.1.00956.S, ADS/JAO.ALMA#2017.1.00815.S. ALMA is a partnership of ESO (representing its member states), NSF (USA), and NINS (Japan), together with NRC (Canada), NSC and ASIAA (Taiwan), and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO, and NAOJ. The National Radio Astronomy Observatory (NRAO) is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. We would like to thank the referee Jonathan Braine for his insightful comments and constructive feedback that helped improve the quality of the paper. This work was carried out as part of the PHANGS Collaboration. LN acknowledges funding from the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) - 516405419. AKL gratefully acknowledges support by grants 1653300 and 2205628 from the National Science Foundation, by award JWST-GO-02107.009-A, and by a Humboldt Research Award from the Alexander von Humboldt Foundation. The work of AKL is partially supported by the National Science Foundation under Grants No. 1615105, 1615109, and 1653300. AU acknowledges support from the Spanish grants PID2019-108765GB-I00, funded by MCIN/AEI/10.13039/501100011033, and PID2022-138560NB-I00, funded by MCIN/AEI/10.13039/501100011033/FEDER, EU. ER acknowledges the support of the Natural Sciences and Engineering Research Council of Canada (NSERC), funding reference number RGPIN-2022-03499. MB gratefully acknowledges support from the ANID BASAL project FB210003 and from the FONDECYT regular grant 1211000. MC gratefully acknowledges funding from the DFG through an Emmy Noether Research Group (grant number CH2137/1-1). COOL Research DAO is a Decentralized Autonomous Organization supporting research in astrophysics aimed at uncovering our cosmic origins. KG is supported by the Australian Research Council through the Discovery Early Career Researcher Award (DECRA) Fellowship (project number DE220100766) funded by the Australian Government. KG is supported by the Australian Research Council Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D), through project number CE170100013. JDH gratefully acknowledges financial support from the Royal Society (University Research Fellowship; URF/R1/221620). HAP acknowledges support by the National Science and Technology Council of Taiwan under grant 110-2112-M-032-020-MY3. MQ acknowledges support from the Spanish grant PID2019-106027GA-C44, funded by MCIN/AEI/10.13039/501100011033. TS acknowledges funding from the European Research Council (ERC) under the European Union\u2019s Horizon 2020 research and innovation programme (grant agreement No. 694343). ES acknowledges funding from the European Research Council (ERC) under the European Union\u2019s Horizon 2020 research and innovation programme (grant agreement No. 694343). SKS acknowledges financial support from the German Research Foundation (DFG) via Sino-German research grant SCHI 536/11-1. Y-HT acknowledges funding support from NRAO Student Observing Support Grant SOSPADA-012 and from the National Science Foundation (NSF) under grant No. 2108081. TGW acknowledges funding from the European Research Council (ERC) under the European Union\u2019s Horizon 2020 research and innovation programme (grant agreement No. 694343). This paper makes use of the following ALMA data ADS/JAO.ALMA#2011.0.00004.SV, ADS/JAO.ALMA#2015.1.00956.S, ADS/JAO.ALMA#2017.1.00815.S. ALMA is a partnership of ESO (representing its member states), NSF (USA), and NINS (Japan), together with NRC (Canada), NSC and ASIAA (Taiwan), and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO, and NAOJ. The National Radio Astronomy Observatory (NRAO) is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.enPublisher Copyright: © The Authors 2024.Galaxies: individual: NGC 4321Galaxies: ISMGalaxies: star formationISM: molecules2024-11-0110.1051/0004-6361/20244949685209138348