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Published May 2019 | Published + Accepted Version
Journal Article Open

Dense gas is not enough: environmental variations in the star formation efficiency of dense molecular gas at 100 pc scales in M 51

Abstract

It remains unclear what sets the efficiency with which molecular gas transforms into stars. Here we present a new VLA map of the spiral galaxy M 51 in 33 GHz radio continuum, an extinction-free tracer of star formation, at 3″ scales (∼100 pc). We combined this map with interferometric PdBI/NOEMA observations of CO(1–0) and HCN(1–0) at matched resolution for three regions in M 51 (central molecular ring, northern and southern spiral arm segments). While our measurements roughly fall on the well-known correlation between total infrared and HCN luminosity, bridging the gap between Galactic and extragalactic observations, we find systematic offsets from that relation for different dynamical environments probed in M 51; for example, the southern arm segment is more quiescent due to low star formation efficiency (SFE) of the dense gas, despite its high dense gas fraction. Combining our results with measurements from the literature at 100 pc scales, we find that the SFE of the dense gas and the dense gas fraction anti-correlate and correlate, respectively, with the local stellar mass surface density. This is consistent with previous kpc-scale studies. In addition, we find a significant anti-correlation between the SFE and velocity dispersion of the dense gas. Finally, we confirm that a correlation also holds between star formation rate surface density and the dense gas fraction, but it is not stronger than the correlation with dense gas surface density. Our results are hard to reconcile with models relying on a universal gas density threshold for star formation and suggest that turbulence and galactic dynamics play a major role in setting how efficiently dense gas converts into stars.

Additional Information

© 2019 ESO. Article published by EDP Sciences. Received 18 December 2018; Accepted 26 February 2019; Published online 01 May 2019. Based on observations carried out with the IRAM Interferometer NOEMA. IRAM is supported by INSU/CNRS (France), MPG (Germany), and IGN (Spain). The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. This work was carried out as part of the PHANGS collaboration. We would like to thank Kazimierz Sliwa, Gaëlle Dumas, Melanie Krips, and Jürgen Ott for helpful assistance regarding data reduction. We also thank Neven Tomičić for providing SFR measurements for the Andromeda galaxy, and Sean T. Linden, Eve C. Ostriker, and Sébastien Viaene for useful feedback. The authors would also like to thank the anonymous referee for constructive comments. MQ and SEM acknowledge (partial) funding from the Deutsche Forschungsgemeinschaft (DFG) via grant SCHI 536/7-2 as part of the priority programme SPP 1573 "ISM-SPP: Physics of the Interstellar Medium". ES and CMF acknowledge funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (grant agreement No. 694343). The work of AKL and DU is partially supported by the National Science Foundation under Grants No. 1615105, 1615109, and 1653300. FB acknowledges funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (grant agreement No. 726384). APSH is a fellow of the International Max Planck Research School for Astronomy and Cosmic Physics at the University of Heidelberg (IMPRS-HD). The work of MJG and DU is partially supported by the National Science Foundation under Grants No. 1615105, 1615109, and 1653300. JMDK and MC gratefully acknowledge funding from the Deutsche Forschungsgemeinschaft (DFG) in the form of an Emmy Noether Research Group (grant number KR4801/1-1). JMDK gratefully acknowledges funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme via the ERC Starting Grant MUSTANG (grant agreement number 714907). SGB acknowledges support from the Spanish MINECO grant AYA2016-76682-C3-2-P. JP acknowledges support from the Programme National "Physique et Chimie du Milieu Interstellaire" (PCMI) of CNRS/INSU with INC/INP co-funded by CEA and CNES. ER acknowledges the support of the Natural Sciences and Engineering Research Council of Canada (NSERC), funding reference number RGPIN-2017-03987. SCOG acknowledges support from the DFG via SFB 881 "The Milky Way System" (sub-projects B1, B2 and B8).

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August 19, 2023
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