AGN and the Characteristics of their Host Galaxies

The vast majority of optically identified active galactic nuclei (AGNs) in the local universe reside in host galaxies with prominent bulges, supporting the hypothesis that black hole formation and growth is fundamentally connected to the buildup of galaxy bulges. However, recent mid-infrared spectroscopic studies with Spitzer of a sample of bulgeless galaxies reveal strong evidence for AGNs in these galaxies. We present follow-up X-ray observations recently obtained with XMM-Newton of two such sources, the late-type optically normal galaxies NGC 3367 and NGC 4536. Detailed spectral analysis reveals that for both galaxies, the 2-10 keV emission is consistent with low-luminosity AGNs. These observations therefore add to the growing evidence that the fraction of late-type galaxies hosting AGNs is significantly underestimated using optical observations alone. A comparison of the mid-infrared [NeV] luminosity and the X-ray luminosities suggests the presence of a highly absorbed X-ray source in both galaxies. Next, a mid-infrared spectroscopic study was conducted on a sample of 30 pairs of interacting galaxies with archival Spitzer high-resolution spectroscopic observations. Previous studies of galaxy interactions have focused primarily on luminous galaxies, or were conducted in the optical or the UV where emission from the AGN may be hidden. Based on the detection of the [NeV] 14.3 microm emission line we find an AGN detection rate of 18%, including 4 optically unidentified AGN. Our study therefore reveals that optical studies miss AGNs even in non-U/LIRG interactions. Furthermore, we find that these AGNs are found in both disturbed and undisturbed hosts with a wide range of pair separations and Hubble types. Combining our study with previously published studies of U/LIRGs, we find that the AGN detection rate is highest in ULIRGs. In addition, we find tentative evidence that the incidence of AGNs is bimodal, with merger stage, with the highest incidence found in the earliest and latest merger stages. Finally, we find evidence based on mid-infrared spectroscopy for three possible dual AGNs, suggesting that a significant population of dual AGNs is hidden in the optical. The study of a larger sample of interacting galaxies is important to understand the relation between the incidence of AGN and the interaction stage of merging galaxies. Using infrared observations from the Wide-field Infrared Survey Explorer (WISE) we find that galaxy pairs have a higher AGN fraction than isolated galaxies and the AGN fraction increases with decreasing pair separation. While WISE only detects AGNs that dominate the emission of their host galaxies, it is able to detect obscured AGN. WISE finds a similar trend with AGN fraction and pair separation as optical studies, however WISE selects a different population of AGN since only a third of the optically selected AGN are categorized as AGN by WISE.