X-ray properties of He II λ1640 emitting galaxies in VANDELS

Abstract

We explore X-ray emission from a sample of 18 He II λ1640 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 s−1. The stacked luminosity of all 18 He II emitters is 2.6 × 1041 erg s−1, and that of a subset of 13 narrow He II emitters (FHWM(He II) < 1000 km s−1) is 3.1 × 1041 erg s−1. We also measure stacked LX for non-He II emitters through bootstrapping of matched samples, and find LX = 2.5 × 1041 erg s−1, which is not significantly different from LX measured for He II emitters. The LX per star formation rate for He II emitters (log (LX/SFR) - 40.0) and non-emitters (log (LX/SFR) - 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 active galactic nuclei are unlikely to be the dominant producers of He II ionizing 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 ionizing mechanisms such as localized low-metallicity stellar populations, Pop-III stars, etc. may need to be explored.