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

Abstract 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) × 10 42 erg s −1 for UDF3 and (0.40 ± 0.12) × 10 42 erg s −1 for UDF3-2. Furthermore, we measure the star formation rate (SFR) of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>52</mml:mn> <mml:msubsup> <mml:mn>9</mml:mn> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>88</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>64</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>3</mml:mn> <mml:msubsup> <mml:mn>4</mml:mn> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>6</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>6</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> 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.