Discovery of a Pair of Galaxies with Both Hosting X-Ray Binary Candidates at z = 2.544

Among high-redshift galaxies, aside from active galactic nuclei (AGNs), X-ray binaries (XRBs) can be significant sources of X-ray emission. XRBs play a crucial role in galaxy evolution, reflecting the stellar populations of galaxies and regulating star formation through feedback, thereby shaping galaxy structure. In this study, we report a spectroscopically confirmed X-ray emitting galaxy pair (UDF3 and UDF3-2) at z = 2.544. By combining multiwavelength observations from JWST/NIRSpec Micro-Shutter Assembly spectra, JWST/NIRCam and MIRI imaging, Chandra, Hubble Space Telescope, Very Large Telescope, Atacama Large Millimeter/submillimeter Array, and Very Large Array, we analyze the ionized emission lines, which are primarily driven by H II region-like processes. Additionally, we find that the mid-infrared radiation can be fully attributed to dust emission from the galaxies themselves. Our results indicate that the X-ray emission from these two galaxies is dominated by high-mass XRBs, with luminosities of L_X = (1.43 ± 0.40) × 10⁴² erg s⁻¹ for UDF3 and (0.40 ± 0.12) × 10⁴² erg s⁻¹ for UDF3-2. Furthermore, we measure the star formation rate (SFR) of 529₋₈₈^{+ 64} M_⊙ yr⁻¹ for UDF3, placing it ≈0.5 dex below the L_X/SFR-z relation. This offset reflects the redshift-dependent enhancement of L_X/SFR-z relation, which is influenced by metallicity and serves as a key observable for XRB evolution. In contrast, UDF3-2, with an SFR of 34₋₆⁺⁶ M_⊙ yr⁻¹, aligns well with the L_X/SFR-z relation. This galaxy pair represents the highest redshift non-AGN-dominated galaxies with individual X-ray detections reported to date. This finding suggests that the contribution of XRBs to galaxy X-ray emission at high redshift may be underestimated.