Broad He i 1.08-µm absorption from the obscurer in the active galaxy NGC 5548

Wildy, Conor; Landt, Hermine; Ward, Martin J; Czerny, Bożena; Kynoch, Daniel

Abstract

The nucleus of the active galaxy NGC 5548 was the target of two intensive spectroscopic monitoring campaigns at X-ray, ultraviolet (UV), and optical frequencies in 2013/2014. These campaigns detected the presence of a massive obscuration event. In 2016/2017, Landt et al. conducted a near-IR spectroscopic monitoring campaign on NGC 5548 and discovered He I 1.08-μm absorption. Here, we decompose this absorption into its components and study its time variability. We attribute the narrow He  I absorption lines to the warm absorber (WA) and, as for the newly appeared low-ionization WA lines in the UV, their presence is most likely due to a reduction in ionization parameter caused by the obscurer. The observed variability of the narrow He I absorption is consistent with what is expected for the WA. Most importantly, we also detect fast, broad He I absorption, which we attribute to the obscurer. This He I broad absorption, which is indicative of a high column density gas, is unsaturated and variable on time-scales of a few months. The observed variability of the obscurer is mainly due to changes in ionization, although density changes also play a role. We test the physical cycle model of Dehghanian et al. which proposes that helium recombination can account for how the obscurer influences the physics of the WA gas. Our results support their model, but also indicate that the reality might be more complex.

Citation

Wildy, C., Landt, H., Ward, M. J., Czerny, B., & Kynoch, D. (2021). Broad He i 1.08-µm absorption from the obscurer in the active galaxy NGC 5548. Monthly Notices of the Royal Astronomical Society: Letters, 500(2), 2063-2075. https://doi.org/10.1093/mnras/staa3301

• Journal Article Type: Article
• Acceptance Date: Oct 20, 2020
• Online Publication Date: Oct 27, 2020
• Publication Date: 2021-01
• Deposit Date: Jan 8, 2021
• Publicly Available Date: Jan 8, 2021
• Journal: Monthly Notices of the Royal Astronomical Society: Letters
• Electronic ISSN: 1745-3933
• Publisher: Oxford University Press
• Peer Reviewed: Peer Reviewed
• Volume: 500
• Issue: 2
• Pages: 2063-2075
• DOI: https://doi.org/10.1093/mnras/staa3301
• Public URL: https://durham-repository.worktribe.com/output/1247845