Using WISE to Find Obscured AGN Activity in SDSS Mergers and Interactions

Abstract

In simulations, major encounters between gas-rich galaxies are predicted to drive gas to the centers of interacting and merging systems triggering new star formation (SF) and fueling an active galactic nucleus (AGN). Depending on the rate of SF, large amounts of obscuring dust can make detection of merger-induced activity difficult and may be at the heart of the ongoing merger-AGN connection debate. To provide better constraints on the importance of obscured AGNs, we use data from the Wide-field Infrared Survey Explorer (WISE) for a comprehensive sample of over 1000 major galaxy interactions and ongoing mergers visually selected from the SDSS with 1010 M and 0.01 < z < 0.08. We examine the [3.4]- [4.6] versus [4.6]-[12] color-color plane and find that most interactions and mergers have the same colors as “normal” (non-interacting and non-merging) galaxies, which define a narrow [3.4]-[4.6] locus and span a wide range in [4.6]-[12] colors from spectroscopically quiescent (blue, no warm dust) to galaxies with obscured SF. We find that 2 – 9 percent of mergers (and 1.0 – 2.5 percent of interactions) have unusually red [3.4]-[4.6] colors, which are associated with dust-obscured (Type-2) AGNs. Mergers (interactions) are 5 – 18 (3 – 5) times more likely to host a buried AGN than normal galaxies. This increased likelihood of dusty AGN activity in mergers and interactions supports an AGN-merger connection. We investigate the nature of merging and interacting galaxies with dusty AGN. We find that dusty AGN mergers and interactions favor smaller pair separations, smaller dark matter halo masses, and higher [OIII] luminosities (a proxy for AGN power) than the bulk of mergers and interactions with normal WISE colors. Using SDSS urz colors to distinguish quiescent from star-forming galaxies, we find that more than three-quarters of the WISE AGN subpopulation of mergers and interactions are forming stars. We find that AGNs also classified as ongoing mergers are 2 – 6 times more likely to be obscured than AGNs in nonmerging, non-interacting galaxies. Around half of merging AGNs are obscured, suggesting that shorter wavelengths will be inadequate in selecting AGNs in merging systems. We find no association between merging systems and optically identified AGNs (Seyferts), suggesting that central star formation (and thus dust obscuration) is the key to making an AGN in a merger. Our findings indicate a strong association between ongoing star formation and dust-enshrouded black hole growth in merging galaxies as predicted in the modern merger hypothesis.