The origin of the X-ray luminosity of the green pea galaxies: X-ray binaries or active galactic nuclei?

Abstract

Context. Green pea galaxies (GPs) are renowned for their compact sizes, low masses, strong emission lines, high star formation rates (SFRs), and being analogs to high-z Lyα-emitting galaxies. Aims. This investigation focuses on a curated sample of six GPs with X-ray detections, sourced from XMM-Newton, Swift, Chandra and eROSITA, with the aim to elucidate the origin of their X-ray luminosity. Methods. We determined the GPs’ physical properties, including the SFRs, stellar masses, and metallicities, based on multiwavelength photometry and LAMOST spectra analysis. Results. Within the LX–SFR relation, GPs predominantly occupy the high specific SFR domain, where high-mass X-ray binaries (HMXBs) dominate, leading to an excess in X-ray luminosity compared to the sole contributions from HMXBs (LXHMXB). Moreover, GPs exhibit a noticeable excess in X-ray luminosity within the LX–SFR–metallicity relationship. The cumulative input from X-ray binaries, hot gas, hot interstellar medium, and young stellar objects falls short in accounting for the X-ray luminosity observed in GPs. The presence of active galactic nucleus (AGNs) surfaces is suggested based on mid-infrared color–color criteria. Furthermore, based on the MBH derived from LAMOST optical spectra, GPs conform to the MBH–M⋆ scaling relation. Conclusions. The origin of the X-ray excess likely stems from the combined contributions of HMXBs and AGNs, although further scrutiny via X-ray spectra and spatially resolved imaging using forthcoming facilities is needed to confirm this.