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# The gravitationally unstable gas disk of a starburst galaxy 12 billion years ago

## Abstract

Galaxies in the early Universe that are bright at submillimetre wavelengths (submillimetre-bright galaxies) are forming stars at a rate roughly 1,000 times higher than the Milky Way. A large fraction of the new stars form in the central kiloparsec of the galaxy1,2,3, a region that is comparable in size to the massive, quiescent galaxies found at the peak of cosmic star-formation history4 and the cores of present-day giant elliptical galaxies. The physical and kinematic properties inside these compact starburst cores are poorly understood because probing them at relevant spatial scales requires extremely high angular resolution. Here we report observations with a linear resolution of 550 parsecs of gas and dust in an unlensed, submillimetre-bright galaxy at a redshift of z = 4.3, when the Universe was less than two billion years old. We resolve the spatial and kinematic structure of the molecular gas inside the heavily dust-obscured core and show that the underlying gas disk is clumpy and rotationally supported (that is, its rotation velocity is larger than the velocity dispersion). Our analysis of the molecular gas mass per unit area suggests that the starburst disk is gravitationally unstable, which implies that the self-gravity of the gas is stronger than the differential rotation of the disk and the internal pressure due to stellar-radiation feedback. As a result of the gravitational instability in the disk, the molecular gas would be consumed by star formation on a timescale of 100 million years, which is comparable to gas depletion times in merging starburst galaxies5.

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## Acknowledgements

We thank J. Baba for discussions about a gravitational instability in SMGs. This work was supported by JSPS KAKENHI JP17J04449. We thank the ALMA staff and in particular the EA-ARC staff for their support. This research has made use of data from ALMA and HerMES project (http://hermes.sussex.ac.uk/). 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 (South Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and NAOJ. HerMES is a Herschel Key Programme utilizing Guaranteed Time from the SPIRE instrument team, ESAC scientists and a mission scientist. Data analysis was in part carried out on the common-use data analysis computer system at the Astronomy Data Center (ADC) of the National Astronomical Observatory of Japan.

### Reviewer information

Nature thanks F. Bournaud and the other anonymous reviewer(s) for their contribution to the peer review of this work.

## Author information

Authors

### Contributions

K.T. led the project and reduced the ALMA data. K.T. and D.I. wrote the manuscript. M.S.Y. reduced the Large Millimeter Telescope data and edited the final manuscript. Other authors contributed to the interpretation and commented on the ALMA proposal and the paper.

## Ethics declarations

### Competing interests

The authors declare no competing interests.

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 Fig. 1 Galaxy-integrated CO (4–3), CO (1–0) and C i (2–1) spectra of AzTEC-1.

The CO (4–3) spectrum is extracted using an 0.8″-diameter aperture in the natural-weighted map cube. The C i (1–0) and C i (2–1) spectra are extracted from the peak positions in map cubes with 1.7″ × 1.1″ and 0.8″ × 0.7″ resolution, respectively. Yellow shaded regions show the velocity range v = −315 km s−1 to v = +315 km s−1, in which the velocity-integrated line fluxes are measured.

### Extended Data Fig. 2 Galaxy-integrated SED of AzTEC-1.

Red circles show the photometric data from Subaru (r′, i′, z′)37, VISTA (Ks)37, Spitzer (3.6 μm, 4.4 μm)37, Herschel (250 μm, 350 μm, 500 μm)38,39, ALMA (860 μm, 2.1 mm, 3.2 mm) and JVLA (10 cm)40. The black line shows the best-fitting SED model from MAGPHYS41,42.

### Extended Data Fig. 3 CO spectra along the kinematic major axis.

Spectra are extracted at a position angle of PA = −64°. The spatial offset x from the galactic centre is shown at the upper left of each panel. Red lines indicate the spectra of the best-fitting dynamical model produced by GalPaK3D.

### Extended Data Fig. 4 Full MCMC chain for 20,000 iterations.

Red solid lines and black dashed lines indicate the median and 95% confidence interval of the last 60% of the MCMC chain.

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Tadaki, K., Iono, D., Yun, M.S. et al. The gravitationally unstable gas disk of a starburst galaxy 12 billion years ago. Nature 560, 613–616 (2018). https://doi.org/10.1038/s41586-018-0443-1

• Accepted:

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• Issue Date:

• DOI: https://doi.org/10.1038/s41586-018-0443-1

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