NuSTAR Observations of Four Mid-IR Selected Dual AGN Candidates in Galaxy Mergers

Abstract

Mergers of galaxies are a ubiquitous phenomenon in the Universe and represent a natural consequence of the ``bottom-up'' mass accumulation and galaxy evolution cosmological paradigm. It is generally accepted that the peak of AGN accretion activity occurs at nuclear separations of $\lesssim10$ kpc for major mergers. Here we present new NuSTAR and XMM-Newton observations for a subsample of mid-IR preselected dual AGN candidates in an effort to better constrain the column densities along the line-of-sight for each system. Only one dual AGN candidate, J0841+0101, is detected as a single, unresolved source in the XMM-Newton and NuSTAR imaging, while the remaining three dual AGN candidates, J0122+0100, J1221+1137, and J1306+0735, are not detected with NuSTAR; if these non-detections are due to obscuration alone, these systems are consistent with being absorbed by column densities of log($N_{\rm{H}}/\rm{cm}^{-2}$) $\geq$ 24.9, 24.8, and 24.6, which are roughly consistent with previously inferred column densities in these merging systems. In the case of J0841+0101, the analysis of the 0.3-30 keV spectra reveal a line-of-sight column density of $N_{\rm{H}}\gtrsim10^{24}$ cm$^{-2}$, significantly larger than the column densities previously reported for this system and demonstrating the importance of the higher signal-to-noise XMM-Newton spectra and access to the $>10$ keV energies via NuSTAR. Though it is unclear if J0841+0101 truly hosts a dual AGN, these results are in agreement with the high obscuring columns expected in AGNs in late-stage mergers.