On the disappearance of broad-line region in low-luminosity active galactic nuclei: the role of the outflows from advection dominated accretion flows

The broad-line region (BLR) disappears in many low-luminosity AGNs, the reason of which is still controversial. The BLRs in AGNs are believed to be associated with the outflows from the accretion disks. Most of the low-luminosity AGNs (LLAGNs) contain advection dominated accretion flows (ADAFs), which are very hot and have a positive Bernoulli parameter. ADAFs are therefore associated with strong outflows. We estimate the cooling of the outflows from the ADAFs, and find that the gases in such hot outflows always cannot be cooled efficiently by bremsstrahlung radiation. The ADAF may co-exist with the standard disk, i.e., the inner ADAF connects to the outer thin accretion disk at radius R_tr, in the sources accreting at slightly lower than the critical rate. For the ADAFs with >0.001 L_edd, a secondary small inner cold disk is suggested to co-exist with the ADAF due to the condensation process. We estimate the Compton cooling of the outflow, of which the soft seed photons either come from the outer cold disk or the secondary inner cold disk. It is found that the gas in the outflow far from the ADAF may be efficiently cooled to form BLR clouds due to the soft seed photons emitted from the cold disks, provided the transition radius of the ADAF to the outer cold disk is small or/and the secondary small cold disk has a luminosity >0.003L_edd. The BLR clouds can still be formed in the outflows from the outer cold thin disks, if the transition radius is not very large. For the sources with <0.001L_edd, the inner small cold disk is evaporated completely in the ADAF and outer thin accretion disk may be suppressed by the ADAF, which leads to the disappearance of the BLR. The physical implications of this scenario on the double-peaked broad-line emitters are also discussed.

💡 Research Summary

The paper addresses the long‑standing puzzle of why many low‑luminosity active galactic nuclei (LLAGN) lack a broad‑line region (BLR). The authors start from the widely accepted view that BLR clouds are supplied by outflows launched from the accretion flow. In LLAGN the inner accretion flow is typically an advection‑dominated accretion flow (ADAF), which is extremely hot (ion temperatures 10⁹–10¹¹ K) and possesses a positive Bernoulli parameter, guaranteeing the presence of strong, high‑velocity outflows.

First, the authors calculate the radiative cooling of these hot outflows via thermal bremsstrahlung. Using realistic densities (10⁴–10⁶ cm⁻³) and temperatures (∼10⁸ K) they find that the bremsstrahlung cooling time far exceeds the dynamical time required for the gas to travel to the canonical BLR radius (∼10⁴–10⁵ R_S). Consequently, bremsstrahlung alone cannot lower the gas temperature to the ∼10⁴ K regime needed for line emission, and the outflow remains too hot to condense into clouds.

The study then explores two additional cooling channels that can operate if soft photon sources are present: (1) Compton cooling by photons from an outer standard thin disk, and (2) Compton cooling by photons from a secondary, small, cold disk that may form inside the ADAF through condensation. The Compton cooling rate scales with the soft‑photon energy density, which in turn depends on the luminosity of the photon source and the distance from it. If the transition radius (R_tr) between the outer thin disk and the inner ADAF is small (≲10³–10⁴ R_S) or if the inner cold disk shines with L ≳ 0.003 L_Edd, the photon field is intense enough that the outflow can be efficiently cooled to ∼10⁴ K at radii where BLR clouds are normally observed. In this regime, the cooled gas can fragment, forming the BLR clouds that produce the characteristic broad emission lines.

Conversely, when the overall accretion power falls below ∼0.001 L_Edd, the inner cold disk evaporates completely and the outer thin disk may be suppressed by the ADAF’s pressure. The photon field then becomes too weak for Compton cooling to be effective, and the hot outflow remains uncooled, leading to the disappearance of the BLR.

The authors also discuss the implications for double‑peaked broad‑line emitters. Such objects are thought to arise when the line‑emitting region is confined to the transition zone between a thin disk and an ADAF. In the present framework, a moderate transition radius yields a photon field that partially cools the outflow, allowing a limited population of BLR clouds that produce the double‑peaked profiles. If the transition radius is very small, the BLR becomes extensive and the line profiles resemble those of classical Seyfert 1 galaxies; if it is very large, the BLR vanishes, consistent with the observed properties of many LLAGN.

Overall, the paper proposes a unified picture in which the presence or absence of a BLR in low‑luminosity AGN is governed not merely by the total luminosity but by the geometry of the accretion flow (the size of the transition radius) and the existence of a secondary cold disk inside the ADAF. The key physical mechanism is Compton cooling of the ADAF‑driven outflow by soft photons from cold disks. This model makes testable predictions: (i) LLAGN with detectable BLR should show evidence for either a compact outer thin disk (small R_tr) or a faint inner cold disk with L ≳ 0.003 L_Edd; (ii) high‑resolution X‑ray/UV spectroscopy should reveal signatures of cooled, dense outflow material at distances of ∼10⁴ R_S in sources that possess a BLR. Future numerical simulations that couple ADAF dynamics, condensation physics, and radiative transfer will be essential to validate the scenario.