Enhanced X-ray emission from Lyman break analogues and a possibleL_X–SFR–metallicity plane

Abstract

The source of energetic photons that heated and reionized the early Universe remains uncertain. Early galaxies had low metallicity and recent population synthesis calculations suggest that the number and luminosity of high-mass X-ray binaries are enhanced in star-forming galaxies with low metallicity, offering a potentially important and previously overlooked source of heating and reionization. Lyman break analogue (LBA) galaxies are local galaxies that strongly resemble the high-redshift, star-forming Lyman Break Galaxies and have been suggested as local analogues to these metal-deficient galaxies found in the early Universe. We studied a sample of ten LBAs in order to measure the relation between star formation rate and X-ray luminosity. We found that for LBAs with metallicities in the range 12 + log 10 (O/H) = 8.15 − 8.80, the L X −SFR relation was log 10 (L X /SFR [erg s −1 M −1 yr]) = 39.85(±0.10) in the 0.5 − 8 keV band with a dispersion of σ = 0.25 dex. This is an enhancement of nearly a factor of 2 in the L 0.5−8keV -SFR relation relative to results for nearby, near-solar metallicity galaxies. The enhancement is significant at the 98.2% level (2.4σ). Our enhanced L X /SFR relation is consistent with the metallicity-dependent predicted value from population synthesis models. We discuss the possibility of a L X -SFR-Metallicity plane for star-forming galaxies. These results are important to our understanding of reionization and the formation of early galaxies.

The properties of He IIλ1640 emitters at z ∼ 2.5–5 from the VANDELS survey

“…X-ray binaries (XRBs) have been suggested as one such mechanism for several decades now (see Garnett et al 1991, for example). Schaerer et al (2019) showed that XRB populations in low metallicity galaxies can account for the He ii line strength, and Fornasini et al (2019) recently showed that the contribution from XRBs indeed increases with decreasing metallicities at high redshifts, as was already shown for low redshift galaxies (Douna et al 2015;Brorby et al 2016). Therefore, it is necessary to explore the role of XRBs in low-metallicity, He ii emitting galaxies with significant enough statistics.…”

The formation of stellar black holes

“…However, the metallicity dependence of BH-HMXBs has been observationally confirmed for galaxies in the local universe 78,79,80 . Using a large set of data on the sizes and X-ray luminosities (mostly coming from accreting BHs) of HMXBs in nearby galaxies with known metallicities and star formation rates, it is found that HMXBs are typically ten times more numerous per unit star formation in low-metallicity galaxies (<20% solar) than in solar-metallicity galaxies 80 .…”

Blue Supergiant X-Ray Binaries in the Nearby Dwarf Galaxy IC 10

“…We conclude that the age of the IC 10 starburst is fortuitously such that massive binary interaction is approaching its peak. The picture painted in Prestwich et al (2015) and Brorby et al (2016) advances the hope that the observable parameters can be organized into a "fundamental plane" in SFR, XLF, and metalicity. Populating such a plane requires deep multiwavelength observations of star forming galaxies across age and metalicity space and IC 10 is the youngest and most accessible case study.…”