X-ray emission from He II 1640 emitting galaxies in VANDELS

A. Saxena, L. Pentericci, D. Schaerer, R. Schneider, R. Amorin, A. Bongiorno, A. Calabrò, M. Castellano, A. Cimatti, F. Cullen, A. Fontana, J. P. U. Fynbo, N. Hathi, D. J. McLeod, M. Talia, G. Zamorani

Published 2020-03-30Version 1

We present X-ray measurements for 18 star-forming galaxies showing He II $\lambda 1640$ emission at $z\sim2.2-5$ in the Chandra Deep Field South, to test whether they show any X-ray excess compared to galaxies with no He II emission. Using aperture photometry on the 7 Ms Chandra image, we find that even though He II emitting galaxies in our sample (especially those with FWHM(He II) < 1000 km~s$^{-1}$) have marginally higher X-ray emission than galaxies with no He II, the difference is not statistically significant. The X-ray luminosity per star formation rate ($L_X$/SFR) for He II emitters and non-emitters are comparable, which rules out any enhanced contribution from young X-ray sources. The $L_X$/SFR at $z\sim3$ for both He II emitters and non-emitters is consistent with observations at lower redshifts, and in line with the redshift evolution predicted by models. We also find that our $L_X$/SFR measurements are consistent with the metallicity dependence predicted and observed in the literature. Therefore, we conclude that there is no significant difference between the X-ray emission from galaxies with and without He II emission, which rules out enhanced contribution from XRBs or weak or obscured AGN in galaxies with strong He II at $z\sim3$. Given the other similarities in the physical properties of both He II emitters and non-emitters reported earlier, alternative sources of He II ionising photon production, such as localised low-metallicity stellar populations, Pop-III stars, etc. may need to be explored.