The VANDELS Survey: New constraints on the high-mass X-ray binary populations in normal star-forming galaxies at 3 < z < 5.5. (arXiv:2104.02624v1 [astro-ph.GA])

The VANDELS Survey: New constraints on the high-mass X-ray binary populations in normal star-forming galaxies at 3 < z < 5.5. (arXiv:2104.02624v1 [astro-ph.GA]) <a href="http://arxiv.org/find/astro-ph/1/au:+Saxena_A/0/1/0/all/0/1">A. Saxena</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Ellis_R/0/1/0/all/0/1">R. S. Ellis</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Forster_P/0/1/0/all/0/1">P. U. Forster</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Calabro_A/0/1/0/all/0/1">A. Calabro</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Pentericci_L/0/1/0/all/0/1">L. Pentericci</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Carnall_A/0/1/0/all/0/1">A. C. Carnall</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Castellano_M/0/1/0/all/0/1">M. Castellano</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Cullen_F/0/1/0/all/0/1">F. Cullen</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Fontana_A/0/1/0/all/0/1">A. Fontana</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Franco_M/0/1/0/all/0/1">M. Franco</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Fynbo_J/0/1/0/all/0/1">J. P. U. Fynbo</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Gargiulo_A/0/1/0/all/0/1">A. Gargiulo</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Garilli_B/0/1/0/all/0/1">B. Garilli</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Hathi_N/0/1/0/all/0/1">N. P. Hathi</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+McLeod_D/0/1/0/all/0/1">D. J. McLeod</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Zamorani_G/0/1/0/all/0/1">G. Zamorani</a>

We use VANDELS spectroscopic data overlapping with the $simeq$7 Ms Chandra
Deep Field South survey to extend studies of high-mass X-ray binary systems
(XRBs) in 301 normal star-forming galaxies in the redshift range $3 < z < 5.5$.
Our analysis evaluates correlations between X-ray luminosities ($L_X$), star
formation rates (SFR) and stellar metallicities ($Z_star$) 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 $L_X$/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 $L_X$/SFR to $zsimeq5$ proceeds as $(1 +
z)^{1.03 pm 0.02}$. Using stellar metallicities derived from photospheric
absorption features in our spectroscopic data, we confirm indications at lower
redshifts that $L_X$/SFR is stronger for metal-poor galaxies. We use
semi-analytic models to show that metallicity dependence of $L_X$/SFR alone may
not be sufficient to fully explain the observed redshift evolution of X-ray
emission from high-mass XRBs, particularly for galaxies with SFR $<30$
$M_odot$ yr$^{-1}$. We speculate that the discrepancy may arise due to reduced
overall stellar ages in the early Universe leading to higher $L_X$/SFR for the
same metallicity. We use our data to define the redshift-dependent contribution
of XRBs to the integrated X-ray luminosity density and, in comparison with
models, find that the contribution of high-mass XRBs to the cosmic X-ray
background at $z>6$ may be $gtrsim 0.25$ dex higher than previously estimated.

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