# Large turbulent reservoirs of cold molecular gas around high-redshift starburst galaxies

## Abstract

Starburst galaxies at the peak of cosmic star formation1 are among the most extreme star-forming engines in the Universe, producing stars over about 100 million years (ref. 2). The star-formation rates of these galaxies, which exceed 100 solar masses per year, require large reservoirs of cold molecular gas3 to be delivered to their cores, despite strong feedback from stars or active galactic nuclei4,5. Consequently, starburst galaxies are ideal for studying the interplay between this feedback and the growth of a galaxy6. The methylidyne cation, CH+, is a most useful molecule for such studies because it cannot form in cold gas without suprathermal energy input, so its presence indicates dissipation of mechanical energy7,8,9 or strong ultraviolet irradiation10,11. Here we report the detection of CH+ (J = 1–0) emission and absorption lines in the spectra of six lensed starburst galaxies12,13,14,15 at redshifts near 2.5. This line has such a high critical density for excitation that it is emitted only in very dense gas, and is absorbed in low-density gas10. We find that the CH+ emission lines, which are broader than 1,000 kilometres per second, originate in dense shock waves powered by hot galactic winds. The CH+ absorption lines reveal highly turbulent reservoirs of cool (about 100 kelvin), low-density gas, extending far (more than 10 kiloparsecs) outside the starburst galaxies (which have radii of less than 1 kiloparsec). We show that the galactic winds sustain turbulence in the 10-kiloparsec-scale environments of the galaxies, processing these environments into multiphase, gravitationally bound reservoirs. However, the mass outflow rates are found to be insufficient to balance the star-formation rates. Another mass input is therefore required for these reservoirs, which could be provided by ongoing mergers16 or cold-stream accretion17,18. Our results suggest that galactic feedback, coupled jointly to turbulence and gravity, extends the starburst phase of a galaxy instead of quenching it.

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E.F. and B.G. acknowledge support from the national CNRS programme Physique et Chimie du Milieu Interstellaire (PCMI). ## Author information E.F., E.B., F.B. and D.E. conceived the initial scientific argument and wrote the ALMA proposal with B.G., M.A.Z., P.M.A., A.O. and R.S.B. M.A.Z. reduced the ALMA data. B.G. and E.F. analysed the spectra. B.G. provided the results of the shock models. R.J.I., I.O. and F.W. were invited to join the team at a later stage to provide the results of the lens models (I.O.) and to contribute to a year-long debate on the data interpretation. E.F. wrote the paper with contributions from all authors. Correspondence to E. Falgarone. ## Ethics declarations ### Competing interests The authors declare no competing financial interests. ## Additional information Publisher's note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. ## Extended data figures and tables ### Extended Data Figure 1 Position–velocity diagrams of CH+emission and absorption along selected cuts across the sources. The cuts are made along the east–west direction for G09v1.40, NAv1.56 and NAv1.144, along the long axis of the lensed images for the Eyelash, and along a northeast–southwest direction for SDP17b. CH+ emission appears in white (blue contours) and absorption in black (green contours). The first contour level and steps are 2σ. A velocity gradient is seen in the absorption of the Eyelash that is two times smaller than that detected in CO (ref. 24). ### Extended Data Figure 2 CH+ emission and absorption overlaid on dust continuum emission for the Eyelash, SDP17b and G09v1.40. The integrated emission (blue contours) and absorption (red contours) of the CH+ lines, with contour levels in steps of 2σ, are overlaid on continuum emission (grey scale). All of the images are lensed and so the differences between the distribution of dust continuum and CH+ line emission are affected by differential lensing. ### Extended Data Figure 3 As in Extended Data Fig. 2, but for NAv1.56, NAv1.144 and SDP130. Only emission is detected for SDP130. ## PowerPoint slides ### PowerPoint slide for Fig. 1 ### PowerPoint slide for Fig. 2 ## Rights and permissions Reprints and Permissions ## About this article ### Cite this article Falgarone, E., Zwaan, M., Godard, B. et al. 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