Deep-sky survey detects an X-ray emitting pair of galaxies
Tomasz Nowakowski
astronomy writer
Sadie Harley
scientific editor
Robert Egan
associate editor
By conducting multiwavelength observations with various telescopes and space observatories, astronomers from Tsinghua University and Steward Observatory have detected a galaxy pair exhibiting significant X-ray emission. The finding was in a research paper published July 31 on the pre-print server arXiv.
The Great Observatories Origins Deep Survey (GOODS) is a deep-sky survey conducted by multiple observatories to study the formation and evolution of galaxies. It combines multiwavelength data from space observatories like the Hubble Space Telescope (HST), Chandra X-ray Observatory, Spitzer spacecraft, XMM-Newton satellite, and the largest ground-based facilities, such as the Very Large Telescope (VLT), Keck telescopes, Gemini Observatory or the Very Large Array (VLA).
Recently, a team of astronomers led by Tsinghua University's Sijia Cai conducted a search for Chandra X-ray detected star-forming galaxies in the Southern field of the GOODS survey (GOODS-S). For this purpose, they combined observations from VLA and the Atacama Large Millimeter/submillimeter Array (ALMA), spectroscopic data from the James Webb Space Telescope (JWST) and VLT, as well as photometry from HST and JWST.
The observational campaign carried out by Cai's team resulted in the detection of a pair of galaxies at a redshift of 2.54. The two galaxies received designations UDF3 and UDF3-2.
According to the study, the stellar age of UDF3 and UDF3-2 was estimated to be 15 and 206 million years, while their stellar mass was calculated to be approximately 5.8 and 3.3 billion solar masses, respectively. UDF3 exhibits about 15.5 times higher star-formation rate than its companion—at a level of 529 solar masses per year.
The astronomers underline that the two galaxies likely interact gravitationally, with UDF3 exhibiting observable clumpy structures and UDF3-2 showing an elongated morphology due to tidal forces.
The observations found that both UDF3 and UDF3-2 showcase significant X-ray emission. Based on the collected data, the authors of the paper assume that this emission is dominated by high-mass X-ray binaries (XRBs) rather than active galactic nuclei (AGN).
"From the emission line diagnostics, full-spectra and multiple-wavelength SED [spectral energy distribution] fitting from restframe ultraviolet to mid-infrared, as well as morphological analysis, we confirm that the X-ray emission from both member galaxies in this pair are contributed by X-ray binaries and should not be dominated by AGN," the researchers conclude.
If the hypothesis is confirmed by further studies, the newfound galaxy system could be the highest-redshift star-forming pair with X-ray detected in an individual member galaxy. Moreover, this would also mean that X-ray luminosity in high-redshift galaxies may not completely come from AGN activities.
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More information: Sijia Cai et al, Discovery of a Pair of Galaxies with Both Hosting X-ray Binary Candidates at z=2.544, arXiv (2025).
Journal information: arXiv
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