Buried AGNs in Advanced Mergers:Mid-infrared color selection as a dual AGN finder (1707.03921v1)

Abstract: A direct consequence of hierarchical galaxy formation is the existence of dual supermassive black holes (SMBHs), which may be preferentially triggered as active galactic nuclei (AGN) during galaxy mergers. Despite decades of searching, however, dual AGNs are extremely rare, and most have been discovered serendipitously. Using the all-sky WISE survey, we identified a population of over 100 morphologically identified interacting galaxies or mergers that display red mid-infrared colors often associated in extragalactic sources with powerful AGNs. The vast majority of these advanced mergers are optically classified as star-forming galaxies suggesting that they may represent an obscured population of AGNs that cannot be found through optical studies. In this work, we present Chandra/ACIS observations and near-infrared spectra with the Large Binocular Telescope of six advanced mergers with projected pair separations less than ~ 10 kpc. The combined X-ray, near-infrared, and mid-infrared properties of these mergers provide confirmation that four out of the six mergers host at least one AGN, with four of the mergers possibly hosting dual AGNs with projected separations less than ~10 kpc, despite showing no firm evidence for AGNs based on optical spectroscopic studies. Our results demonstrate that 1) optical studies miss a significant fraction of single and dual AGNs in advanced mergers, and 2) mid-infrared pre-selection is extremely effective in identifying dual AGN candidates in late-stage mergers. Our multi-wavelength observations suggest that the buried AGNs in these mergers are highly absorbed, with intrinsic column densities in excess of N_H >10^24cm^-2, consistent with hydrodynamic simulations.

Buried AGNs in Advanced Mergers: Mid-infrared Color Selection as a Dual AGN Finder

The paper presented in this paper explores the advanced merger galaxies that host hidden Active Galactic Nuclei (AGNs), potentially leading to the identification of dual AGN systems. Using the Wide-field Infrared Survey Explorer ({\it WISE}) data, the research isolates over 100 morphologically identified interacting galaxies displaying specific mid-infrared colors associated with powerful AGNs. Despite their optical classification as star-forming galaxies, these mergers likely host obscured AGNs. Through {\it Chandra/ACIS} X-ray observations and near-infrared spectra from the Large Binocular Telescope, the researchers analyze six advanced mergers with pair separations less than 10 kpc.

Key Findings

1. Dual AGN Identification: The paper confirms AGN presence in four of the six merger candidates, with possible dual AGNs in at least four mergers, shown via X-ray, mid-infrared, and near-infrared characteristics. Such systems were missed by conventional optical spectroscopic methods in the past.
2. Mid-infrared Pre-selection Efficiency: The paper demonstrates how mid-infrared color selection, specifically using the {\it WISE} $W1-W2$ color selection criterion, efficiently identifies galaxies with potentially dual AGNs hidden from optical spectroscopic studies. This method pinpointed multiple AGNs that optical methods failed to identify due to substantial obscuration by dust and gas.
3. Implications of High Obscuration: The buried AGNs exhibit high absorption levels with intrinsic column densities surpassing $N_\mathrm{H} >10^{24}$ cm$^{-2}$z, aligning with predictions from hydrodynamic simulations about late-stage mergers. Thus, optical techniques miss significant AGN activity during these stages.

Implications and Future Research

The implications of these findings are substantial for understanding galaxy evolution, as mergers and their associated AGN activity are key processes driving this evolution. The discussion also touches on the relevance of identifying these dual AGNs for gravitational wave studies and constraints on SMBH (Supermassive Black Hole) formation histories.

Future research in AI could focus on automating the process of AGN identification in large datasets, leveraging mid-infrared color selection criteria to optimize the discovery of hidden AGNs in a myriad of galactic formations. Additionally, enhancing computational models such as hydrodynamic simulations could provide deeper insights into mergers' roles in galactic evolution.

Overall, this research highlights the overlooked population of dual AGNs in advanced mergers, advocating for wider adoption of mid-infrared color selection techniques to uncover obscured AGNs. As extensive investigations progress in this domain, the astronomical understanding of AGN dynamics and the complex, intertwined history of galaxy interactions will deepen significantly.