2019ApJ...881...48K

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_☉ yr^–1 and 78 M_☉ 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 (N_H^eq) consistent with the nondetection. The results indicate N_H^eq≳2×10²⁴ cm^–2 for SDSS J0749+3337 and N_H^eq≳5×10²⁴ cm^–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.