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

¹ Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, People’s Republic of China
² CAS Key Laboratory of Optical Astronomy, National Astronomical Observatories, Beijing 100101, China
³ School of Astronomy and Space Science, University of Chinese Academy of Sciences, Beijing 101408, China
⁴ University of Chinese Academy of Sciences, Nanjing, Jiangsu 211135, China
⁵ Key Laboratory for Research in Galaxies and Cosmology, Shanghai Astronomical Observatory, Chinese Academy of Sciences, 80 Nandan Road, Shanghai 200030, China
⁶ School of Astronomy and Space Sciences, University of Chinese Academy of Sciences, Beijing 100049, China
⁷ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
⁸ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The Netherlands

Received: 30 January 2024
Accepted: 10 June 2024

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 L_X–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 (L_X^HMXB). Moreover, GPs exhibit a noticeable excess in X-ray luminosity within the L_X–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 M_BH derived from LAMOST optical spectra, GPs conform to the M_BH–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.