X-ray properties of He ii λ 1640 emitting galaxies in VANDELS

We explore X-ray emission from a sample of 18 He ii 1640 emitting star-forming galaxies at z 2:3 3:6 from the VANDELS survey in the Chandra Deep Field South, to set constraints on the role of X-ray sources in powering the He ii emission. We find that 4 He ii emitters have tentative detections with S/N 2 and have X-ray luminosities, LX = 1:5 4:9 1041 erg s1. The stacked luminosity of all 18 He ii emitters is 2:6 1041 erg s1, and that of a subset of 13 narrow He ii emitters (FHWM(He ii) < 1000 km s1) is 3:1 1041 erg s1. We also measure stacked LX for non-He ii emitters through bootstrapping of matched samples, and find LX = 2:5 1041 erg s1, which is not significantly different from LX measured for He ii emitters. The LX per star-formation rate for He ii emitters (log¹LXSFRº 40:0) and non-emitters (log¹LXSFRº 39:9) are also comparable and in line with the redshift evolution and metallicity dependence predicted by models. Due to the non-significant difference between the X-ray emission from galaxies with and without He ii, we conclude that X-ray binaries or weak or obscured AGNs are unlikely to be the dominant producers of He ii ionising photons in VANDELS star-forming galaxies at z 3. Given the comparable physical properties of both He ii emitters and non-emitters reported previously, alternative He ii ionising mechanisms such as localised low-metallicity stellar populations, Pop-III stars, etc. may need to be explored.