Chandra X-ray and Hubble Space Telescope Imaging of Optically Selected Kiloparsec-Scale Binary Active Galactic Nuclei II: Host Galaxy Morphology and AGN Activity

TL;DR

This study uses Hubble and Chandra imaging to analyze the host galaxy morphology and activity of kiloparsec-scale binary AGNs, revealing diverse merger types and similar accretion properties compared to single AGNs, with implications for X-ray detection biases.

Contribution

It provides detailed morphological analysis of binary AGN host galaxies and compares their properties to single AGNs, highlighting potential biases in X-ray selected samples.

Findings

Binary AGNs occur in diverse merger morphologies.

Binary AGNs have similar luminosities and Eddington ratios as single AGNs.

Clumpy star formation may bias X-ray detection of binary AGNs.

Abstract

Binary active galactic nuclei (AGNs) provide clues to how gas-rich mergers trigger and fuel AGNs and how supermassive black hole (SMBH) pairs evolve in a gas-rich environment. While significant effort has been invested in their identification, the detailed properties of binary AGNs and their host galaxies are still poorly constrained. In a companion paper, we examined the nature of ionizing sources in the double nuclei of four kpc-scale binary AGNs with redshifts between 0.1~0.2. Here, we present their host galaxy morphology based on F336W (U-band) and F105W (Y-band) images taken by the Wide Field Camera 3 (WFC3) onboard the Hubble Space Telescope. Our targets have double-peaked narrow emission lines and were confirmed to host binary AGNs with follow up observations. We find that kpc-scale binary AGNs occur in galaxy mergers with diverse morphological types. There are three major mergers with intermediate morphologies and a minor merger with a dominant disk component. We estimate the masses of the SMBHs from their host bulge stellar masses and obtain Eddington ratios for each AGN. Compared with a representative control sample drawn at the same redshift and stellar mass, the AGN luminosities and Eddington ratios of our binary AGNs are similar to those of single AGNs. The U-Y color maps indicate that clumpy star forming regions could significantly affect the X-ray detection of binary AGNs, e.g., the hardness ratio. Considering the weak X-ray emission in AGNs triggered in merger systems, we suggest that samples of X-ray selected AGNs may be biased against gas-rich mergers.