A NuSTAR and XMM-Newton Study of the Two Most Actively Star-forming Green Pea Galaxies (SDSS J0749+3337 and SDSS J0822+2241)

Taiki Kawamuro, Yoshihiro Ueda, Kohei Ichikawa, Masatoshi Imanishi, Takuma Izumi, Atsushi Tanimoto, Kenta Matsuoka

Research output: Contribution to journalArticle

Abstract

We explore X-ray evidence for the presence of active galactic nuclei (AGNs) in the two most actively star-forming Green Pea galaxies (GPs), SDSS J0749+3337 and SDSS J0822+2241, which have star formation rates (SFRs) of 123 M o yr-1 and 78 M o yr-1, respectively. The GPs have red mid-infrared (MIR) spectral energy distributions and higher 22 μm luminosities than expected from a proxy of the SFR (Hα luminosity), consistent with hosting AGNs with 2-10 keV luminosities of ∼1044 erg s-1. We thus obtain and analyze the first hard (>10 keV) X-ray data observed with NuSTAR and archival XMM-Newton data below 10 keV. From the NuSTAR ≈20 ks data, however, we find no significant hard X-ray emission. By contrast, soft X-ray emission with 0.5-8 keV luminosities of ≈1042 erg s-1 is significantly detected in both targets, which can be explained only by star formation (SF). A possible reason for the lack of clear evidence is that a putative AGN torus absorbs most of the X-ray emission. Applying a smooth-density AGN torus model, we determine minimum hydrogen column densities along the equatorial plane (Neq H) consistent with the nondetection. The results indicate N eq H ≳ 2 × 1024 cm-2 for SDSS J0749+3337 and N eq H ≳ 5 × 10 24cm-2 for SDSS J0822+2241. Therefore, the GPs may host such heavily obscured AGNs. Otherwise, no AGN exists and the MIR emission is ascribed to SF. Active SF in low-mass galaxies is indeed suggested to reproduce red MIR colors. This would imply that diagnostics based on MIR photometry data alone may misidentify such galaxies as AGNs.

Original languageEnglish
Article number48
JournalAstrophysical Journal
Volume881
Issue number1
DOIs
Publication statusPublished - 2019 Aug 10

Keywords

  • X-rays: galaxies
  • galaxies: active
  • galaxies: individual (SDSS J074936.77+333716.3 and SDSS J082247.66 +224144.0)
  • infrared: galaxies

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

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