Two new papers use HST ultraviolet spectroscopy and absorption-line variability to estimate where AGN-driven outflow components originate in the Seyfert galaxy NGC 5548. Both studies focus on UV C IV absorption troughs measured across multiple epochs, including HST/COS data from the 2014 AGN STORM campaign and additional HST observations from 2013. The key observable is how the absorption features vary in response to changes in the ionizing continuum emitted by the active nucleus. Each paper links this variability to the gas recombination timescale, which then constrains the outflow’s radial distance from the central source. One study introduces probability-based diagnostic events (labeled G1 and G2) derived from variability behavior, combined with mock continuum light curves modeled with a damped random walk (DRW) prescription. It reports distances for specific outflow components (components 1 and 6), finding about 0.77 pc for component 1 and about 1.72 pc for component 6 (with large uncertainties on the latter). The other paper analyzes detection-rate curves of variability and uses multi-step structures in blended troughs to separate different velocity components, concluding that four of six identified components lie a few parsecs from the nucleus while two are at tens of parsecs. Both studies report consistency with prior literature results for well-studied components.
HST spectroscopy studies constrain UV outflow component distances in NGC 5548
Two new papers use HST ultraviolet spectroscopy and absorption-line variability to estimate where AGN-driven outflow components originate in the Seyfert galaxy NGC 5548. Both studies focus on UV C IV...
- Both studies use multi-epoch HST/COS spectroscopy of UV absorption lines (notably C IV) to track variability in NGC 5548’s AGN outflow.
- They infer radial distance by relating absorption-line variability to the absorbers’ recombination timescale (t_r).
- One study uses diagnostic events (G1/G2) and DRW-based mock light curves to map event probabilities to t_r and distance.
- The other study uses variability detection-rate curves, including multi-step profiles in blended troughs, to estimate t_r for individual velocity components.
- Both papers find outflow components located at parsec scales, with the broader picture including components at a few pc and at tens of pc.
arXiv:2607.21038v1 Announce Type: new Abstract: AGN-driven outflows serve as a key channel through which the energetic central engine influences host galaxy evolution. Among the physical properties of outflows, their radial distance from the galactic nucleus is particularly important for assessing AGN feedback. In this study, we investigate the UV outflow components in NGC 5548 by analyzing the variability of C IV absorption troughs in optical spectra obtained through multiple HST observations during 2013 and 2014. We construct a set of variability-based diagnostic events, labeled G1 and G2, which are sensitive to the recombination timescale ($t_r$) of ionized gas. By combining these with mock light curves generated using a damped random walk (DRW) model, we numerically establish a mapping between the G1 event probability and $t_r$. This approach allows us to constrain the radial distances of outflow components 1 and 6, whose absorption variability is primarily driven by changes in the incident ionizing continuum, to be $0.77^{+0.10}_{-0.10}$ and $1.72^{+1.74}_{-1.72}$ pc, respectively. These results are consistent with those obtained using a different method in our previous study, as well as with values reported in the literature.
2 hours agoarXiv:2607.21029v1 Announce Type: new Abstract: AGN-driven outflows are routinely invoked as a key agent of supermassive black holes to regulate the evolution of galaxies. The radial distance from the central engine is a crucial parameter for evaluating the impact of these outflows on the host galaxy. In this work, we estimate the radial distances of ultraviolet (UV) outflow components in NGC 5548 using the most up-to-date absorption-line variability method, combined with multi-epoch HST/COS spectroscopy from the 2014 AGN STORM campaign and archival data observed in 2013. The recombination timescale (tr) of the absorbers are measured by analyzing the detection rate curves of absorption-line variability. In particular, the detection rate curves of the absorption troughs showing blended multiple velocity components are featured by distinct ``multi-step' profiles, allowing for measuring tr for individual components. Among the 6 identified outflow components, four are found to be a few pc from the center and two are 30-40 pc away. Our results agree well with the more reliable results in the literature on components 1 and 4, and show overall consistency with previous works, demonstrating the power of our new methodology especially when it is aided by densely sampled HST spectra.
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