The large-scale ionization cones in the Galaxy

Abstract: There is compelling evidence for a highly energetic Seyfert explosion (10^{56-57} erg) that occurred in the Galactic Centre a few million years ago. The clearest indications are the x-ray/gamma-ray "10 kpc bubbles" identified by the Rosat and Fermi satellites. In an earlier paper, we suggested another manifestation of this nuclear activity, i.e. elevated H-alpha emission along a section of the Magellanic Stream due to a burst (or flare) of ionizing radiation from Sgr A*. We now provide further evidence for a powerful flare event: UV absorption line ratios (in particular CIV/CII, SiIV/SiII) observed by the Hubble Space Telescope reveal that some Stream clouds towards both galactic poles are highly ionized by a source capable of producing ionization energies up to at least 50 eV. We show how these are clouds caught in a beam of bipolar, radiative "ionization cones" from a Seyfert nucleus associated with Sgr A*. In our model, the biconic axis is tilted by about 15 deg from the South Galactic Pole with an opening angle of roughly 60 deg. For the Stream at such large Galactic distances (D > 75 kpc), nuclear activity is a plausible explanation for all of the observed signatures: elevated H-alpha emission and H ionization fraction (X_e > 0.5), enhanced CIV/CII and SiIV/SiII ratios, and high CIV and SiIV column densities. Wind-driven "shock cones" are ruled out because the Fermi bubbles lose their momentum and energy to the Galactic corona long before reaching the Stream. The nuclear flare event must have had a radiative UV luminosity close to the Eddington limit (f_E ~ 0.1-1). Our time-dependent Seyfert flare models adequately explain the observations and indicate the Seyfert flare event took place T_o = 3.5 +/- 1 Myr ago. The timing estimates are consistent with the mechanical timescales needed to explain the x-ray/gamma-ray bubbles in leptonic jet/wind models (2-8 Myr).

• The paper attributes the formation of large-scale ionization cones to a Seyfert flare at Sgr A*, rejecting wind-driven shock models.
• The study uses time-dependent numerical models and Hubble data to date the flare event to approximately 3.5 ±1 Myr ago and confirms ionization energies up to 50 eV.
• The findings advance our understanding of AGN feedback by linking nuclear activity at Sgr A* to extensive ionization in the Galactic halo and Magellanic Stream.

Overview of "The Large-Scale Ionization Cones in the Galaxy"

The paper explores the ionization phenomenon observed in the Galactic halo, providing a comprehensive analysis of the ionization cones resulting from the radiative activity of the supermassive black hole at the center of the Milky Way, associated with Sagittarius A* (Sgr A*). The research leverages data from the Hubble Space Telescope, which reveals significant ionization levels within certain clouds of the Magellanic Stream, a result that implicates the presence of a powerful ionizing source capable of generating ionization energies up to at least 50 eV.

Main Findings and Conclusions

1. Ionization Sources and Models: The paper rejects wind-driven shock cones resulting from the Fermi bubbles as viable mechanisms for the observed ionization effects, citing insufficient momentum and energy transfer to the Galactic corona. Instead, a Seyfert flare model is proposed, wherein ionization cones emanating from AGN-like activity at Sgr A* serve as the primary mechanism. This model includes a biconically tilted axis by approximately 15 degrees from the South Galactic Pole, with a proposed opening angle of about 60 degrees.
2. Numerical Modeling: Utilizing time-dependent Seyfert flare models, the researchers quantify the approximate timing of the event generating these ionization cones at around 3.5 million years ago (Myr), with a confidence interval of ±1 Myr. The model considers the evolutionary history of Sgr A* and posits an Eddington-limited or sub-Eddington event.
3. Ionization and Radiation Field Analysis: The researchers provide a thorough examination of the ionization and radiation field within the Galactic context, explaining how these ionization cones likely extend to distances of around 75 kpc or more, as evidenced by the projected surface brightness and line ratios observed in \Ha emission and various UV metal ion transitions such as \CIV/\CII and \SiIV/\SiII.
4. Astrophysical Implications: The findings suggest that the energy output at the time of flare activity was nearly approaching or at the Eddington limit, thereby contributing critical insight into the episodic radiative history of the Galaxy's central black hole. Implications extend to understanding the interplay between nuclear activity and the ionization of gaseous streams within the Galactic halo, advancing theoretical frameworks concerning AGN feedback processes.
5. Recommendations for Future Research: Future observational campaigns and modeling efforts are encouraged to further ascertain the distance and structural characteristics of the Magellanic Stream, refine the chronological models of flare-induced ionization events, and explore the broader consequences for ionization in other extragalactic systems.

Implications and Speculative Prospects

The research provocatively positions these Galactic-scale ionization cones as manifestations of past AGN phenomena, analogous in some respects to those observed in other galaxies harboring active galactic nuclei (AGN). This potentially alters the understanding of the Milky Way's evolutionary timeline and suggests that similar processes may be ongoing but subdued in other galaxies.

Furthermore, these findings raise the suggestion that nuclear activity cycles may recur on timescales akin to $\sim$10 Myr, and such episodic events could significantly influence the chemical evolution and ionization state of Galactic halo structures.

The potential for these insights to refine cosmological models and inform the understanding of AGN feedback across different scales and galactic environments is significant. As large telescopic arrays and advanced sensors improve observational capabilities, not only in the optical and UV spectra but also across other wavelengths, the theoretical models discussed in this paper will be subject to rigorous testing and development.