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 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× {10}24 cm-2 for SDSS J0749+3337 and {N}{{H}}eq}≳ 5× {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.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- August 2019
- DOI:
- 10.3847/1538-4357/ab2bf6
- arXiv:
- arXiv:1906.09653
- Bibcode:
- 2019ApJ...881...48K
- Keywords:
-
- galaxies: active;
- galaxies: individual: SDSS J074936.77+333716.3 and SDSS J082247.66+224144.0;
- infrared: galaxies;
- X-rays: galaxies;
- Astrophysics - Astrophysics of Galaxies
- E-Print:
- 12 pages, 3 tables, 5 figures, accepted for publication in ApJ