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

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 $123M_{\odot }$ yr${}^{-1}$ and $78M_{\odot }$ yr${}^{-1}$, respectively. The GPs have red mid-infrared (MIR) spectral energy distributions and higher 22 $\mu$m luminosities than expected from a proxy of the SFR (H$\alpha$ luminosity), consistent with hosting AGNs with 2-10 keV luminosities of $\sim 10^{44}$ 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 $\approx$20 ksec data, however, we find no significant hard X-ray emission. By contrast, soft X-ray emission with 0.5--8 keV luminosities of $\approx 10^{42}$ 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 ($N_{\mathrm{H}}^{\mathrm{eq}}$) consistent with the non-detection. The results indicate $N_{\mathrm{H}}^{\mathrm{eq}}\gtrsim 2\times 10^{24}$ cm${}^{-2}$ for SDSS J0749+3337 and $N_{\mathrm{H}}^{\mathrm{eq}}\gtrsim 5\times 10^{24}$ 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.

12 pages, 3 tables, 5 figures, accepted for publication in ApJ

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