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# Large molecular gas reservoirs in ancestors of Milky Way-mass galaxies nine billion years ago

## Abstract

The gas accretion and star formation histories of galaxies like the Milky Way remain an outstanding problem in astrophysics1,2. Observations show that 8 billion years ago, the progenitors to Milky Way-mass galaxies were forming stars 30 times faster than today and were predicted to be rich in molecular gas3, in contrast to the low present-day gas fractions (<10%)46. Here we show the detection of molecular gas from the CO (J = 3–2) emission (rest-frame 345.8 GHz) in galaxies at redshifts z = 1.2–1.3, selected to have the stellar mass and star formation rate of the progenitors of today’s Milky Way-mass galaxies. The CO emission reveals large molecular gas masses, comparable to or exceeding the galaxy stellar masses, and implying that most of the baryons are in cold gas, not stars. The total luminosities of the galaxies from star formation and CO luminosities yield long gas consumption timescales. Compared to local spiral galaxies, the star formation efficiency, estimated from the ratio of total infrared luminosity (LIR) to CO emission, has remained nearly constant since redshift z = 1.2, despite the order of magnitude decrease in gas fraction, consistent with the results for other galaxies at this epoch710. Therefore, the physical processes that determine the rate at which gas cools to form stars in distant galaxies appear to be similar to that in local galaxies.

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## Acknowledgements

The authors thank their colleagues on the CANDELS and ZFOURGE surveys for providing high quality data. The authors also thank the ALMA staff for facilitating the observations and aiding in the calibration and reduction process. The authors acknowledge generous support from the Mitchell Institute for Fundamental Physics and Astronomy at Texas A&M University. This paper makes use of the following ALMA data: ADS/JAO.ALMA#2011.0.01234.S. ALMA is a partnership of the European Southern Observatory (representing its member states), the National Science Foundation (USA) and the National Institutes of Natural Sciences (Japan), together with the National Research Council (Canada), the National Science Council and the Academia Sinica Institute of Astronomy and Astrophysics (Taiwan) and the Korea Astronomy and Space Science Institute (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by the European Southern Observatory, Associated Universities, Inc./National Radio Astronomy Observatory and the National Astronomical Observatory of Japan. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.

## Author information

Authors

### Contributions

C.P. led the ALMA observing programme, handled the data reduction and led the writing of the manuscript. I.L., K.G., R.Q., L.S., C.S. and K.-V.T. contributed extensively to the ZFOURGE data set, used in much of the analysis. S.L.F., D.F. and R.C.L. contributed to the design of the ALMA observing programme and assisted in the reduction and interpretation of the ALMA data. G.B. and K.G. assisted in the interpretation of the ALMA data. M.D. and H.I. carried out the data analysis of the Spitzer and Herschel imaging. All coauthors contributed to the writing of the manuscript and to the ALMA observing programme.

### Corresponding author

Correspondence to C. Papovich.

## Ethics declarations

### Competing interests

The authors declare no competing financial interests.

## Supplementary information

### Supplementary Information

Supplementary Tables 1-2, Supplementary Figures 1–4 (PDF 322 kb)

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Papovich, C., Labbé, I., Glazebrook, K. et al. Large molecular gas reservoirs in ancestors of Milky Way-mass galaxies nine billion years ago. Nat Astron 1, 0003 (2017). https://doi.org/10.1038/s41550-016-0003

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• DOI: https://doi.org/10.1038/s41550-016-0003