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

## Abstract

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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## 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 (https://cda.harvard.edu/chaser/). The Hubble data used in this work is available at the Mikulski Archive for Space Telescopes (MAST; https://archive.stsci.edu) 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 (https://dataverse.harvard.edu/dataset.xhtml?persistentId=doi:10.7910/DVN/JCFRLB).

## Code availability

The data reduction pipelines used in this work are all publicly available. Chandra data were reduced using the CIAO package (http://cxc.harvard.edu/ciao/), Hubble data were reduced using Drizzlepac (http://drizzlepac.stsci.edu/), and FIRE data were reduced using the FIREHOSE package (http://web.mit.edu/rsimcoe/www/FIRE/ob_data.htm). The modelling of the foreground galaxy cluster potential was done using the publicly available Lenstool code (https://projets.lam.fr/projects/lenstool/wiki). Analysis of the FIRE spectra was performed using the IDL Astronomy User’s Library (https://idlastro.gsfc.nasa.gov/).

## References

1. 1.

Coe, D. et al. CLASH: three strongly lensed images of a candidate z ~ 11 galaxy. Astrophys. J. 762, 32 (2013).

2. 2.

Vieira, J. D. et al. Dusty starburst galaxies in the early Universe as revealed by gravitational lensing. Nature 495, 344–347 (2013).

3. 3.

Ishigaki, M. et al. Hubble Frontier Fields first complete cluster data: faint galaxies at z ~ 5–10 for UV luminosity functions and cosmic reionization. Astrophys. J. 799, 12 (2015).

4. 4.

Livermore, R. C., Finkelstein, S. L. & Lotz, J. M. Directly observing the galaxies likely responsible for reionization. Astrophys. J. 835, 113 (2017).

5. 5.

Laporte, N. et al. Dust in the reionization era: ALMA observations of a z = 8.38 gravitationally lensed galaxy. Astrophys. J. Lett. 837, L21 (2017).

6. 6.

Lotz, J. M. et al. The Frontier Fields: survey design and initial results. Astrophys. J. 837, 97 (2017).

7. 7.

Planck Collaboration Planck 2018 results. VI. Cosmological parameters. Preprint at https://arxiv.org/abs/1807.06209 (2018).

8. 8.

Carlstrom, J. E. et al. The 10 meter South Pole Telescope. Publ. Astron. Soc. Pac. 123, 568–581 (2011).

9. 9.

McDonald, M. et al. A massive, cooling-flow-induced starburst in the core of a luminous cluster of galaxies. Nature 488, 349–352 (2012).

10. 10.

Baldwin, A., Phillips, M. M. & Terlevich, R. Classification parameters for the emission-line spectra of extragalactic objects. Publ. Astron. Soc. Pac. 93, 5–19 (1981).

11. 11.

Kewley, L. J. et al. The cosmic BPT diagram: confronting theory with observations. Astrophys. J. Lett. 774, L10 (2013).

12. 12.

Fragos, T. et al. X-ray binary evolution across cosmic time. Astrophys. J. 764, 41 (2013).

13. 13.

Colbert, E. J. M., Heckman, T. M., Ptak, A. F., Strickland, D. K. & Weaver, K. A. Old and young X-ray point source populations in nearby galaxies. Astrophys. J. 602, 231–248 (2004).

14. 14.

Basu-Zych, A. R. et al. Evidence for elevated X-ray emission in local Lyman break galaxy analogs. Astrophys. J. 774, 152 (2013).

15. 15.

Brorby, M., Kaaret, P., Prestwich, A. & Mirabel, I. F. Enhanced X-ray emission from Lyman break analogues and a possible L X-SFR-metallicity plane. Mon. Not. R. Astron. Soc. 457, 4081–4088 (2016).

16. 16.

Kaaret, P., Brorby, M., Casella, L. & Prestwich, A. H. Resolving the X-ray emission from the Lyman-continuum emitting galaxy Tol 1247–232. Mon. Not. R. Astron. Soc. 471, 4234–4238 (2017).

17. 17.

Svoboda, J., Douna, V., Orlitová, I. & Ehle, M. Green Peas in X-rays. Astrophys. J. 880, 144–159 (2019).

18. 18.

Lehmer, B. D. et al. The evolution of normal galaxy X-ray emission through cosmic history: constraints from the 6 ms Chandra Deep Field-South. Astrophys. J. 825, 7 (2016).

19. 19.

Xue, Y. Q. et al. The 2 ms Chandra Deep Field-North Survey and the 250 ks extended Chandra Deep Field-South Survey: improved point-source catalogs. Astrophys. J. Suppl. 224, 15 (2016).

20. 20.

Luo, B. et al. The Chandra Deep Field-South Survey: 7 ms source catalogs. Astrophys. J. Suppl. 228, 2 (2017).

21. 21.

Basu-Zych, A. R. et al. The X-ray star formation story as told by Lyman break galaxies in the 4 ms CDF-S. Astrophys. J. 762, 45 (2013).

22. 22.

Aird, J., Coil, A. L. & Georgakakis, A. X-rays across the galaxy population—I. Tracing the main sequence of star formation. Mon. Not. R. Astron. Soc. 465, 3390–3415 (2017).

23. 23.

Mineo, S., Gilfanov, M. & Sunyaev, R. X-ray emission from star-forming galaxies—I. High-mass X-ray binaries. Mon. Not. R. Astron. Soc. 419, 2095–2115 (2012).

24. 24.

Ranalli, P., Comastri, A. & Setti, G. The 2–10 keV luminosity as a star formation rate indicator. Astron. Astrophys. 399, 39–50 (2003).

25. 25.

Prestwich, A. H. et al. Ultra-luminous X-ray sources in HARO II and the role of X-ray binaries in feedback in Lyα emitting galaxies. Astrophys. J. 812, 166 (2015).

26. 26.

Mineo, S., Gilfanov, M., Lehmer, B. D., Morrison, G. E. & Sunyaev, R. X-ray emission from star-forming galaxies—III. Calibration of the L X-SFR relation up to redshift z ~ 1.3. Mon. Not. R. Astron. Soc. 437, 1698–1707 (2014).

27. 27.

Basu-Zych, A. R. et al. Exploring the overabundance of ULXs in metal- and dust-poor local Lyman break analogs. Astrophys. J. 818, 140 (2016).

28. 28.

Mesinger, A., Ferrara, A. & Spiegel, D. S. Signatures of X-rays in the early Universe. Mon. Not. R. Astron. Soc. 431, 621–637 (2013).

29. 29.

Fialkov, A., Barkana, R. & Visbal, E. The observable signature of late heating of the Universe during cosmic reionization. Nature 506, 197–199 (2014).

30. 30.

Das, A., Mesinger, A., Pallottini, A., Ferrara, A. & Wise, J. H. High-mass X-ray binaries and the cosmic 21-cm signal: impact of host galaxy absorption. Mon. Not. R. Astron. Soc. 469, 1166–1174 (2017).

31. 31.

McDonald, M. et al. The growth of cool cores and evolution of cooling properties in a sample of 83 galaxy clusters at 0.3 < z < 1.2 selected from the SPT-SZ Survey. Astrophys. J. 774, 23 (2013).

32. 32.

McDonald, M. et al. Deep Chandra, HST-COS, and Megacam observations of the Phoenix cluster: extreme star formation and AGN feedback on hundred kiloparsec scales. Astrophys. J. 811, 111 (2015).

33. 33.

Ranalli, P. et al. X-ray properties of radio-selected star forming galaxies in the Chandra-COSMOS survey. Astron. Astrophys. 542, A16 (2012).

34. 34.

Simcoe, R. A. et al. FIRE: a near-infrared cross-dispersed echellette spectrometer for the Magellan telescopes. Proc. SPIE 7014, 70140U (2008).

35. 35.

Vacca, W. D., Cushing, M. C. & Rayner, J. T. A method of correcting near-infrared spectra for telluric absorption. Publ. Astron. Soc. Pac. 115, 389–409 (2003).

36. 36.

Cushing, M. C., Vacca, W. D. & Rayner, J. T. Spextool: a spectral extraction package for SpeX, a 0.8-5.5 micron cross-dispersed spectrograph. Publ. Astron. Soc. Pac. 116, 362–376 (2004).

37. 37.

Bayliss, M. B. Broadband photometry of 105 giant arcs: redshift constraints and implications for giant arc statistics. Astrophys. J. 744, 156 (2012).

38. 38.

Fazio, G. G. et al. The Infrared Array Camera (IRAC) for the Spitzer Space Telescope. Astrophys. J. Suppl. 154, 10–17 (2004).

39. 39.

Bleem, L. E. et al. Galaxy clusters discovered via the Sunyaev-Zel’dovich effect in the 2500-square-degree SPT-SZ survey. Astrophys. J. Suppl. 216, 27 (2015).

40. 40.

Ashby, M. L. N. et al. The Spitzer Deep, Wide-Field Survey. Astrophys. J. 701, 428–453 (2009).

41. 41.

Jullo, E. et al. A Bayesian approach to strong lensing modelling of galaxy clusters. New J. Phys. 9, 447 (2007).

42. 42.

Bayliss, M. B. et al. SPT-GMOS: a Gemini/GMOS-South spectroscopic survey of galaxy clusters in the SPT-SZ survey. Astrophys. J. Suppl. 227, 3 (2016).

43. 43.

Bertin, E. & Arnouts, S. SExtractor: software for source extraction. Astron. Astrophys. Suppl. 117, 393–404 (1996).

44. 44.

Gladders, M. D. & Yee, H. K. C. A new method for galaxy cluster detection. I. The algorithm. Astron. J. 120, 2148–2162 (2000).

45. 45.

Johnson, T. L. et al. Star formation at z = 2.481 in the lensed galaxy SDSS J1110 = 6459. I. Lens modeling and source reconstruction. Astrophys. J. 843, 78 (2017).

46. 46.

Sharma, S. et al. High-resolution spatial analysis of a z ~ 2 lensed galaxy using adaptive coadded source-plane reconstruction. Mon. Not. R. Astron. Soc. 481, 1427–1440 (2018).

47. 47.

Calzetti, D. The dust opacity of star-forming galaxies. Publ. Astron. Soc. Pac. 113, 1449–1485 (2001).

48. 48.

Osterbrock, D. E. Astrophysics of Gaseous Nebulae and Active Galactic Nuclei (University Science Books, 1989).

49. 49.

Denicoló, G., Terlevich, R. & Terlevich, E. New light on the search for low-metallicity galaxies—I. The N2 calibrator. Mon. Not. R. Astron. Soc. 330, 69–74 (2002).

50. 50.

Pettini, M. & Pagel, B. E. J. [O iii]/[N ii] as an abundance indicator at high redshift. Mon. Not. R. Astron. Soc. 348, L59–L63 (2004).

51. 51.

Asplund, M., Grevesse, N., Sauval, A. J. & Scott, P. The chemical composition of the Sun. Annu. Rev. Astron. Astrophys. 47, 481–522 (2009).

52. 52.

Kennicutt, R. C. & Evans, N. J. Star formation in the Milky Way and nearby galaxies. Annu. Rev. Astron. Astrophys. 50, 531–608 (2012).

53. 53.

Leitherer, C. et al. Starburst99: synthesis models for galaxies with active star formation. Astrophys. J. Suppl. 123, 3–40 (1999).

54. 54.

Salpeter, E. E. The luminosity function and stellar evolution. Astrophys. J. 121, 161 (1955).

55. 55.

Kroupa, P. On the variation of the initial mass function. Mon. Not. R. Astron. Soc. 322, 231–246 (2001).

56. 56.

Bell, E. F. & de Jong, R. S. Stellar mass-to-light ratios and the Tully-Fisher relation. Astrophys. J. 550, 212–229 (2001).

57. 57.

Thomas, D. et al. Stellar velocity dispersions and emission line properties of SDSS-III/BOSS galaxies. Mon. Not. R. Astron. Soc. 431, 1383–1397 (2013).

58. 58.

Terashima, Y., Iyomoto, N., Ho, L. C. & Ptak, A. F. X-Ray properties of LINERs and low-luminosity Seyfert galaxies observed with ASCA. I. Observations and results. Astrophys. J. Suppl. 139, 1–36 (2002).

59. 59.

Jones, M. L. et al. The intrinsic Eddington ratio distribution of active galactic nuclei in star-forming galaxies from the Sloan Digital Sky Survey. Astrophys. J. 826, 12 (2016).

60. 60.

She, R., Ho, L. C. & Feng, H. Chandra survey of nearby galaxies: a significant population of candidate central black holes in late-type galaxies. Astrophys. J. 842, 131 (2017).

61. 61.

Wang, J. X., Malhotra, S., Rhoads, J. E. & Norman, C. A. Identifying high-redshift active galactic nuclei using X-ray hardness. Astrophys. J. 612, L109–L112 (2004).

62. 62.

Kong, A. K. H., Yang, Y. J., Hsieh, P.-Y., Mak, D. S. Y. & Pun, C. S. J. The ultraluminous X-ray sources near the center of M82. Astrophys. J. 671, 349–357 (2007).

63. 63.

Reines, A. E., Sivakoff, G. R., Johnson, K. E. & Brogan, C. L. An actively accreting massive black hole in the dwarf starburst galaxy Henize2–10. Nature 470, 66–68 (2011).

64. 64.

Reines, A. E. et al. A candidate massive black hole in the low-metallicity dwarf galaxy pair Mrk 709. Astrophys. J. Lett. 787, L30 (2014).

65. 65.

Reines, A. E. et al. Deep Chandra observations of the compact starburst galaxy Henize 2–10: X-rays from the massive black hole. Astrophys. J. Lett. 830, L35 (2016).

## Acknowledgements

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.

## Ethics declarations

### Competing interests

The authors declare no competing interests.

Peer review information Nature Astronomy thanks Jean-Paul Kneib and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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## Supplementary information

### Supplementary Information

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

• ### Anatomy of a Cooling Flow: The Feedback Response to Pure Cooling in the Core of the Phoenix Cluster

• M. McDonald
• , B. R. McNamara
• , G. M. Voit
• , M. Bayliss
• , B. A. Benson
• , M. Brodwin
• , R. E. A. Canning
• , M. K. Florian
• , G. P. Garmire
• , M. Gaspari