Abstract
Gas clouds in present-day galaxies are inefficient at forming stars. Low star-formation efficiency is a critical parameter in galaxy evolution: it is why stars are still forming nearly 14 billion years after the Big Bang1 and why star clusters generally do not survive their births, instead dispersing to form galactic disks or bulges2. Yet the existence of ancient massive bound star clusters (globular clusters) in the Milky Way suggests that efficiencies were higher when they formed ten billion years ago. A local dwarf galaxy, NGC 5253, has a young star cluster that provides an example of highly efficient star formation3. Here we report the detection of the J = 3→2 rotational transition of CO at the location of the massive cluster. The gas cloud is hot, dense, quiescent and extremely dusty. Its gas-to-dust ratio is lower than the Galactic value, which we attribute to dust enrichment by the embedded star cluster. Its star-formation efficiency exceeds 50 per cent, tenfold that of clouds in the Milky Way. We suggest that high efficiency results from the force-feeding of star formation by a streamer of gas falling into the galaxy.
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Acknowledgements
We thank J. Carpenter, S. Goodwin, M. Heyer, L. Hunt, R. Hurt, M. Jura, C. Lada, C. Leitherer and S. Van Dyk for assistance with the analysis. The Submillimeter Array is a joint project between the Smithsonian Astrophysical Observatory and the Academia Sinica Institute of Astronomy and Astrophysics and is funded by the Smithsonian Institution and the Academica Sinica.
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J.L.T., S.C.B., D.J.B., A.K. and D.S.M. performed the observations. J.L.T., S.C.B. and P.T.P.H. conceived the project and wrote the observing proposal. J.-H.Z. reduced and imaged the Submillimeter Array 870-μm data; A.K. reduced, imaged and analysed the SHARC 350-μm data. J.L.T. and S.M.C. obtained derived quantities and performed data analysis. J.L.T. wrote the first draft and constructed figures. All authors read, discussed and commented on the draft.
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Extended data figures and tables
Extended Data Figure 1 Channel maps of CO(3→2) emission in NGC 5253.
Positions are relative to a reference position 13 h 39 m 56.249 s, −31° 38′ 29″ (J2000). Channels are 10 km s−1 wide; the heliocentric velocity is noted on the individual maps. The colour bar range maximum is 1 Jy per beam for each 10 km s−1 channel. The beam is 4″ × 2″, p.a. 0°.
Extended Data Figure 2 SMA image of CO(3→2) with CO(2→1).
CO(3→2) emission is shown in colour, with an image2 from the Owens Valley Millimeter Array of CO(2→1) in contours. The SMA CO(3→2) image has been smoothed from its original 4″ × 2″, p.a. 0° resolution to match the 9.7″ × 5″, p.a. −84°, beam of the CO(2→1) image. The colour image flux range is 3–40 Jy km s−1 per beam. Contours are linear multiples of 4 Jy km s−1 per beam.
Extended Data Figure 3 RADEX modelling of Cloud D and streamer.
Escape probability transfer modelling of the CO(3→2) to CO(2→1) line ratio. Models were run using a black-body radiation field, spherical escape probability and NCO = 1016 cm−2. a, Cloud D. Magenta is for the value I32/I21 = 2.25, the optically thin limit, and indigo is for the 1σ lower limit to the measured value of 2.6. There is no solution for I32/I21 = 2.6. b, Streamer. Magenta is for I32/I21 = 0.98, and indigo and green are solutions for the −1σ and +1σ values, respectively.
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Turner, J., Beck, S., Benford, D. et al. Highly efficient star formation in NGC 5253 possibly from stream-fed accretion. Nature 519, 331–333 (2015). https://doi.org/10.1038/nature14218
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DOI: https://doi.org/10.1038/nature14218
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