The cold molecular gas in contemporary galaxies is structured in discrete cloud complexes. These giant molecular clouds (GMCs), with 104–107 solar masses (M⊙) and radii of 5–100 parsecs, are the seeds of star formation1. Highlighting the molecular gas structure at such small scales in distant galaxies is observationally challenging. Only a handful of molecular clouds were reported in two extreme submillimetre galaxies at high redshift2,3,4. Here we search for GMCs in a typical Milky Way progenitor at z = 1.036. Using the Atacama Large Millimeter/submillimeter Array (ALMA), we mapped the CO(4–3) emission of this gravitationally lensed galaxy at high resolution, reading down to 30 parsecs, which is comparable to the resolution of CO observations of nearby galaxies5. We identify 17 molecular clouds, characterized by masses, surface densities and supersonic turbulence all of which are 10–100 times higher than present-day analogues. These properties question the universality of GMCs6 and suggest that GMCs inherit their properties from ambient interstellar medium. The measured cloud gas masses are similar to the masses of stellar clumps seen in the galaxy in comparable numbers7. This corroborates the formation of molecular clouds by fragmentation of distant turbulent galactic gas disks8,9, which then turn into stellar clumps ubiquitously observed in galaxies at ‘cosmic noon’ (ref. 10).
Access optionsAccess options
Subscribe to Journal
Get full journal access for 1 year
only $8.67 per issue
All prices are NET prices.
VAT will be added later in the checkout.
Rent or Buy article
Get time limited or full article access on ReadCube.
All prices are NET prices.
The ALMA raw data of the Cosmic Snake arc are available through the ALMA archive under the project identification 2013.1.01330.S. The HST images of MACS J1206.2–0847 are part of the CLASH, available at https://archive.stsci.edu/prepds/clash/. The data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request.
The reduction of the ALMA data was performed with the CASA pipeline version 4.2.2, available at https://almascience.eso.org/processing/science-pipeline. The PdBI data were reduced using GILDAS software, available at http://www.iram.fr/IRAMFR/GILDAS. The lens model was obtained using Lenstool, publicly available at https://projets.lam.fr/projects/lenstool/wiki. The spectral energy distribution fitting was performed with a modified version of the Hyperz code, available in its original form at https://ascl.net/1108.010.
Bolatto, A. D., Leroy, A. K., Rosolowsky, E., Walter, F. & Blitz, L. The resolved properties of extragalactic giant molecular clouds. Astrophys. J. 686, 948–965 (2008).
Swinbank, A. M. et al. ALMA resolves the properties of star-forming regions in a dense gas disk at z ~ 3. Astrophys. J. 806, L17–L22 (2015).
Sharda, P., Federrath, C., da Cunha, E., Swinbank, A. M. & Dye, S. Testing star formation laws in a starburst galaxy at redshift 3 resolved with ALMA. Mon. Not. R. Astron. Soc. 477, 4380–4390 (2018).
Tadaki, K. et al. The gravitationally unstable gas disk of a starburst galaxy 12 billion years ago. Nature 560, 613–616 (2018).
Sun, J. et al. Cloud-scale molecular gas properties in 15 nearby galaxies. Astrophys. J. 860, 172–211 (2018).
Hughes, A. et al. A comparative study of giant molecular clouds in M51, M33, and the Large Magellanic Cloud. Astrophys. J. 779, 46–66 (2013).
Cava, A. et al. The nature of giant clumps in distant galaxies probed by the anatomy of the Cosmic Snake. Nat. Astron. 2, 76–82 (2018).
Tamburello, V., Mayer, L., Shen, S. & Wadsley, J. A lower fragmentation mass scale in high-redshift galaxies and its implications on giant clumps: a systematic numerical study. Mon. Not. R. Astron. Soc. 453, 2490–2514 (2015).
Mandelker, N. et al. Giant clumps in simulated high-z galaxies: properties, evolution and dependence on feedback. Mon. Not. R. Astron. Soc. 464, 635–665 (2017).
Guo, Y. et al. Clumpy galaxies in CANDELS. I. The definition of UV clumps and the fraction of clumpy galaxies at 0.5 < z < 3. Astrophys. J. 800, 39–60 (2015).
Behroozi, P. S., Wechsler, R. H. & Conroy, C. The average star formation histories of galaxies in dark matter halos from z = 0–8. Astrophys. J. 770, 57–93 (2013).
Rodighiero, G. et al. The lesser role of starbursts in star formation at z = 2. Astrophys. J. 739, L40–L46 (2011).
Patrício, V. et al. Kinematics, turbulence, and star formation of z ~ 1 strongly lensed galaxies seen with MUSE. Mon. Not. R. Astron. Soc. 477, 18–44 (2018).
Wisnioski, E. et al. The KMOS3D survey: design, first results, and the evolution of galaxy kinematics from 0.7 ≤ z ≤ 2.7. Astrophys. J. 799, 209–236 (2015).
Dekel, A. et al. Cold streams in early massive hot haloes as the main mode of galaxy formation. Nature 457, 451–454 (2009).
Ebeling, H. et al. A spectacular giant arc in the massive cluster lens MACS J1206.2-0847. Mon. Not. R. Astron. Soc. 395, 1213–1224 (2009).
Heyer, M., Krawczyk, C., Duval, J. & Jackson, J. M. Re-examining Larson’s scaling relationships in galactic molecular clouds. Astrophys. J. 699, 1092–1103 (2009).
Donovan Meyer, J. et al. Resolved giant molecular clouds in nearby spiral galaxies: insights from the CANON CO(1–0) survey. Astrophys. J. 772, 107–123 (2013).
Colombo, D. et al. The PdBI Arcsecond Whirlpool Survey (PAWS): environmental dependence of giant molecular cloud properties in M51. Astrophys. J. 784, 3–35 (2014).
Corbelli, E. et al. From molecules to young stellar clusters: the star formation cycle across the disk of M33. Astron. Astrophys. 601, 146–164 (2017).
Larson, R. B. Turbulence and star formation in molecular clouds. Mon. Not. R. Astron. Soc. 194, 809–826 (1981).
Wei, L. H., Keto, E. & Ho, L. C. Two populations of molecular clouds in the Antennae galaxies. Astrophys. J. 750, 136–154 (2012).
Leroy, A. K. et al. ALMA reveals the molecular medium fueling the nearest nuclear starburst. Astrophys. J. 801, 25–53 (2015).
Bolatto, A. D., Wolfire, M. & Leroy, A. K. The CO-to-H2 conversion factor. Annu. Rev. Astron. Astrophys. 51, 207–268 (2013).
McKee, C. F. & Ostriker, E. C. Theory of star formation. Annu. Rev. Astron. Astrophys. 45, 565–687 (2007).
Brunt, C. M., Heyer, M. H. & Mac Low, M.-M. Turbulent driving scales in molecular clouds. Astron. Astrophys. 504, 883–890 (2009).
Evans, N. J. II et al. The Spitzer c2d legacy results: star-formation rates and efficiencies; evolution and lifetimes. Astrophys. J. Suppl. 181, 321–350 (2009).
Grudić, M. Y. et al. When feedback fails: the scaling and saturation of star formation efficiency. Mon. Not. R. Astron. Soc. 475, 3511–3528 (2018).
Kruijssen, J. M. D. et al. What controls star formation in the central 500 pc of the Galaxy? Mon. Not. R. Astron. Soc. 440, 3370–3391 (2014).
Renaud, F., Boily, C. M., Fleck, J.-J., Naab, T. & Theis, Ch Star cluster survival and compressive tides in Antennae-like mergers. Mon. Not. R. Astron. Soc. 391, L98–L102 (2008).
Jullo, E. et al. A Bayesian approach to strong lensing modelling of galaxy clusters. New J. Phys. 9, 447 (2007).
McMullin, J. P., Waters, B., Schiebel, D., Young, W. & Golap, K. in Astronomical Data Analysis Software and Systems XVI, Vol. 376 (eds. Shaw, R. A. et al.) 127 (Astronomical Society of the Pacific, 2007).
Daddi, E. et al. CO excitation of normal star-forming galaxies out to z = 1.5 as regulated by the properties of their interstellar medium. Astron. Astrophys. 577, A46–A65 (2015).
Walter, F. et al. ALMA Spectroscopic Survey in the Hubble Ultra Deep Field: survey description. Astrophys. J. 833, 67–82 (2016).
Hodge, J. A. et al. Kiloparsec-scale dust disks in high-redshift luminous submillimeter galaxies. Astrophys. J. 833, 103–118 (2016).
Solomon, P. M., Downes, D., Radford, S. J. E. & Barrett, J. W. The molecular interstellar medium in ultraluminous infrared galaxies. Astrophys. J. 478, 144–161 (1997).
Solomon, P. M., Rivolo, A. R., Barrett, J. & Yahil, A. Mass, luminosity, and line width relations of Galactic molecular clouds. Astrophys. J. 319, 730–741 (1987).
Postman, M. et al. The cluster lensing and supernova survey with Hubble: an overview. Astrophys. J. Suppl. Ser. 199, 25–47 (2012).
Schaerer, D. & de Barros, S. On the physical properties of z ~ 6–8 galaxies. Astron. Astrophys. 515, A73–A88 (2010).
Schaerer, D., de Barros, S. & Sklias, P. Properties of z ~ 3–6 Lyman break galaxies. I. Testing star formation histories and the SFR–mass relation with ALMA and near-IR spectroscopy. Astron. Astrophys. 549, A4–A24 (2013).
Sklias, P. et al. Star formation histories, extinction, and dust properties of strongly lensed z ~ 1.5–3 star-forming galaxies from the Herschel Lensing Survey. Astron. Astrophys. 561, A149–A176 (2014).
Bruzual, G. & Charlot, S. Stellar population synthesis at the resolution of 2003. Mon. Not. R. Astron. Soc. 344, 1000–1028 (2003).
Salpeter, E. E. The luminosity function and stellar evolution. Astrophys. J. 121, 161–167 (1955).
Elmegreen, B. G. Theory of starbursts in nuclear rings. Rev. Mex. Astron. Astrofis. Conf. Ser. 6, 165 (1997).
Elmegreen, B. G. Molecular cloud formation by gravitational instabilities in a clumpy interstellar medium. Astrophys. J. 344, 306–310 (1989).
The work of M.D.-Z., D.S., L.M. and A.C. was supported by the STARFORM Sinergia Project funded by the Swiss National Science Foundation. J.R. acknowledges support from the European Research Council starting grant 336736-CALENDS. W.R. is supported by the Thailand Research Fund/Office of the Higher Education Commission grant no. MRG6280259 and Chulalongkorn University’s CUniverse. P.G.P.-G. acknowledges support from the Spanish Government grant AYA2015-63650-P. This paper makes use of the following ALMA data: ADS/JAO.ALMA#2013.1.01330.S. ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada), MOST 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. We also used PdBI observations. PdBI is run by the Institut de Radioastronomie Millimétrique (IRAM, France), a partnership of the French CNRS, the German MPG and the Spanish IGN. Part of the analysis presented herein is also based on observations made with the NASA/ESA Hubble Space Telescope, and obtained from the Hubble Legacy Archive, which is a collaboration between the Space Telescope Science Institute (STScI/NASA), the Space Telescope European Coordinating Facility (ST-ECF/ESA) and the Canadian Astronomy Data Centre (CADC/NRC/CSA). We thank E. Chapillon from the ALMA Regional Center node of IRAM for her help and training on the reduction of the ALMA data, V. Patricio for sharing the kinematic analysis of the [O ii] emission of the Cosmic Snake galaxy and C. Georgy for the presentation of the VisIt 3D visualization tool.
The authors declare no competing interests.
Publisher’s note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.