http://arxiv.org/abs/2003.13800
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.
A. Saxena, L. Pentericci, D. Schaerer, et. al.
Wed, 1 Apr 20
49/83
Comments: 10 pages, 4 figures, submitted to MNRAS. Comments welcome!
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