Dissertation
Dual active galactic nuclei from rainbows to radio: testing the radio-selection method with optical spectroscopy in the stripe 82 field
University of Iowa
Doctor of Philosophy (PhD), University of Iowa
Spring 2022
DOI: 10.17077/etd.006392
Abstract
Simulations of galactic mergers predict that tidally-induced gas inflows can trigger synchronized accretion onto the supermassive black holes at the center of each galaxy. These should be observable as dual active galactic nuclei (dAGN) at kpc-scale separations. We test this prediction using a carefully selected sample of 35 radio galaxy pairs from the Stripe 82 field. Using optical spectroscopy from the Keck LRIS instrument, we confirm that 21 of these pairs have consistent redshifts, and thus are kinematic pairs in mergers; the remaining 14 are found to be isolated galaxies. We further classify the optical spectral signatures of the kinematic pairs via emission line ratios, equivalent widths, and excess of radio power above star-formation predicted outputs. We find 6 galaxies are classified as low ionization LINER- type AGN, and 7 are AGN/starburst composites. Most of the LINERs are found to be retired galaxies, while the composite types are more likely to have some AGN contribution to their ionizing spectra. All our kinematic pairs show evidence of radio power more than 1 dex above the level expected just from star-formation, suggestive of a radio AGN contribution. To rule out the possibility of radio emission from one galaxy overlapping with its companion, we analyze high resolution (0.3′′) imaging from the Very Large Array for 17 of the kinematic pairs. We find that 6 pairs host two separate radio cores thus confirming their status as dAGNs. The remaining 11 are single AGNs in each pair, with most exhibiting prominent jets/lobes overlapping their companion. Our final census will allow us to compute a corrected dAGN duty cycle, and test whether it is higher than predictions of stochastic fueling. While dAGNs are themselves rare, the merger process likely plays some part in their evolution.
Details
- Title: Subtitle
- Dual active galactic nuclei from rainbows to radio: testing the radio-selection method with optical spectroscopy in the stripe 82 field
- Creators
- Arran Gross
- Contributors
- Hai Fu (Advisor)Casey DeRoo (Committee Member)Keri Hoadley (Committee Member)Andrea Prestwich (Committee Member)Adam Myers (Committee Member)
- Resource Type
- Dissertation
- Degree Awarded
- Doctor of Philosophy (PhD), University of Iowa
- Degree in
- Physics
- Date degree season
- Spring 2022
- DOI
- 10.17077/etd.006392
- Publisher
- University of Iowa
- Number of pages
- xiv, 208 pages
- Copyright
- Copyright 2022 Arran Gross
- Comment
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- Language
- English
- Description illustrations
- illustrations (chiefly color), graphs, tables
- Description bibliographic
- Includes bibliographic references (pages 121-135).
- Public Abstract (ETD)
- Throughout the nearby Universe, there are stunning examples of galaxies in the process of colliding and merging together. Computer simulations predict that these violent events disrupt gas in the galactic disk, funneling it to the central nucleus where it can fuel black hole accretion in both galaxies involved. We search for evidence of these dual active galactic nuclei (dAGN) in closely separated galaxy pairs. Our sample consists of 35 radio galaxy pairs selected as having radio and optical detections. We analyze optical spectra and confirm that 21 of these pairs contain two galaxies at the same redshift, and thus are at the same cosmic distance and genuinely in a pair. The remaining 14 contain galaxies with wildly di↵erent redshifts and are thus isolated galaxies. We further investigate the source of the ionized emission lines in the spectra of the true pairs via emission line ratios, equivalent widths, and excess of radio power above star-formation predicted outputs. None of the galaxy pairs shows unambiguous evidence of an optical dAGN, and many have at least some starburst contributions. All of our true pairs do show evidence of radio power well above the level expected just from star-formation, suggestive of a radio AGN contribution. To rule out the possibility of radio emission from one galaxy overlapping with its companion, we analyze high-resolution radio images for 17 of the true pairs. We find that 6 pairs host two separate radio cores thus confirming their status as dAGNs. The remaining 11 contain a single AGN in each pair, with most exhibiting prominent radio jets overlapping their companion. We give optical preselection cuts to further boost the success rate of finding dAGNs in future all-sky surveys.
- Academic Unit
- Physics and Astronomy
- Record Identifier
- 9984271254202771
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