The VANDELS Survey: New constraints on the high-mass X-ray binary populations in normal star-forming galaxies at 3 <z <5.5

A. Saxena, R. S. Ellis, P. U. Forster, A. Calabro, L. Pentericci, A. C. Carnall, M. Castellano, F. Cullen, A. Fontana, M. Franco, J. P. U. Fynbo, A. Gargiulo, B. Garilli, N. P. Hathi, D. J. McLeod, G. Zamorani

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We use VANDELS spectroscopic data overlapping with the ≃7 Ms Chandra Deep Field South survey to extend studies of high-mass X-ray binary systems (HMXBs) in 301 normal star-forming galaxies in the redshift range 3 < z < 5.5. Our analysis evaluates correlations between X-ray luminosities (LX), star formation rates (SFRs), and stellar metallicities (Z∗) to higher redshifts and over a wider range in galaxy properties than hitherto. Using a stacking analysis performed in bins of both redshift and SFR for sources with robust spectroscopic redshifts without AGN signatures, we find convincing evolutionary trends in the ratio LX/SFR to the highest redshifts probed, with a stronger trend for galaxies with lower SFRs. Combining our data with published samples at lower redshift, the evolution of LX/SFR to z ≃ 5 proceeds as (1 + z)1.03 ± 0.02. Using stellar metallicities derived from photospheric absorption features in our spectroscopic data, we confirm indications at lower redshifts that LX/SFR is stronger for metal-poor galaxies. We use semi-analytic models to show that metallicity dependence of LX/SFR alone may not be sufficient to fully explain the observed redshift evolution of X-ray emission from HMXBs, particularly for galaxies with SFR < 30 M⊙ yr-1. We speculate that reduced overall stellar ages and 'burstier' star formation histories in the early Universe may lead to higher LX/SFR for the same metallicity. We then define the redshift-dependent contribution of HMXBs to the integrated X-ray luminosity density and, in comparison with models, find that the contribution of HMXBs to the cosmic X-ray background at z > 6 may be ≳0.25 dex higher than previously estimated.

Original languageEnglish
Pages (from-to)4798–4812
Number of pages15
JournalMonthly Notices of the Royal Astronomical Society
Issue number4
Early online date31 May 2021
Publication statusPublished - 1 Aug 2021


  • X-rays: binaries
  • galaxies: evolution
  • galaxies: high-redshift


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