An X-ray detection of star formation in a highly magnified giant arc

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In the past decade, our understanding of how stars and galaxies formed during the first 5 billion years after the Big Bang has been revolutionized by observations that leverage gravitational lensing by intervening masses, which act as natural cosmic telescopes to magnify background sources. Previous studies have harnessed this effect to probe the distant Universe at ultraviolet, optical, infrared and millimetre wavelengths1,2,3,4,5,6. However, strong-lensing studies of young, star-forming galaxies have never extended into X-ray wavelengths, which uniquely trace high-energy phenomena. Here, we report an X-ray detection of star formation in a highly magnified, strongly lensed galaxy. This lensed galaxy, seen during the first third of the history of the Universe, is a low-mass, low-metallicity starburst with elevated X-ray emission, and is a likely analogue to the first generation of galaxies. Our measurements yield insight into the role that X-ray emission from stellar populations in the first generation of galaxies may play in reionizing the Universe. This observation paves the way for future strong-lensing-assisted X-ray studies of distant galaxies reaching orders of magnitude below the detection limits of current deep fields, and previews the depths that will be attainable with future X-ray observatories.

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Fig. 1: False-colour and X-ray images of the giant arc in SPT-CLJ2344-4243.
Fig. 2: Samples of star-forming galaxies with measured X-ray luminosities.
Fig. 3: Relationship between the X-ray luminosity and the SFR.

Data availability

The data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request. This paper makes use of Chandra data from observation IDs 13401, 16135, 16545, 19581, 19582, 19583, 20630, 20631, 20634, 20635, 20636 and 20797. All raw Chandra data are available for download from the Chandra X-ray Center ( The Hubble data used in this work is available at the Mikulski Archive for Space Telescopes (MAST; under proposal ID 15315. The full raw and reduced FIRE spectroscopy used in this work is freely available upon request. The reduced spectrum is publicly available for download at the Harvard Dataverse (

Code availability

The data reduction pipelines used in this work are all publicly available. Chandra data were reduced using the CIAO package (, Hubble data were reduced using Drizzlepac (, and FIRE data were reduced using the FIREHOSE package ( The modelling of the foreground galaxy cluster potential was done using the publicly available Lenstool code ( Analysis of the FIRE spectra was performed using the IDL Astronomy User’s Library (


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Support for this work was provided by NASA through Chandra award number GO7-18124, issued by the Chandra X-ray Observatory Center, which is operated by the Smithsonian Astrophysical Observatory for and on behalf of the National Aeronautics Space Administration under contract NAS8-03060. Additional support was provided by NASA through the Space Telescope Science Institute (HST-GO-15315), which is operated by the Association of Universities for Research in Astronomy, Incorporated, under NASA contract NAS5-26555.

Author information

M.B.B. performed the analysis of the FIRE spectroscopy, Chandra, Hubble and Spitzer data, and wrote the article text with input and contributions from all authors. M.M. acquired the Chandra and Hubble data. M.B.B. and M.M. reduced the Chandra X-ray data. M.B.B. reduced the FIRE NIR spectroscopy, which was obtained from observations performed by M.B.B. and M.D.G. M.B. acquired the Spitzer data. M.F. reduced the Hubble data. K.S. computed the strong lensing model of the foreground cluster lens and produced the reconstruction of the galaxy in the source plane. The authors are ordered in two alphabetical tiers after M.F.

Correspondence to M. B. Bayliss.

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Supplementary Figs. 1–6, Tables 1 and 2 and refs. 1–3.

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Bayliss, M.B., McDonald, M., Sharon, K. et al. An X-ray detection of star formation in a highly magnified giant arc. Nat Astron (2019) doi:10.1038/s41550-019-0888-7

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