On the BL Lacertae objects/radio quasars and the FRI/II dichotomy

In the frame of unification schemes for radio-loud active galactic nuclei (AGNs), FR I radio galaxies are believed to be BL Lacertae (BL Lac) objects with the relativistic jet misaligned to our line of sight, and FR II radio galaxies correspond to misaligned radio quasars. The Ledlow-Owen dividing line for FR I/FR II dichotomy in the optical absolute magnitude of host galaxy-radio luminosity (M_R-L_Rad) plane can be translated to the line in the black hole mass-jet power (M_bh-Q_jet) plane by using two empirical relations: Q_jet-L_Rad and M_bh}-M_R. We use a sample of radio quasars and BL Lac objects with measured black hole masses to explore the relation of the jet power with black hole mass, in which the jet power is estimated from the extended radio emission. It is found that the BL Lac objects are clearly separated from radio quasars by the Ledlow & Owen FR I/II dividing line in the M_bh-Q_jet plane. This strongly supports the unification schemes for FR I/BL Lac object and FR II/radio quasar. We find that the Eddington ratios L_bol/L_Edd of BL Lac objects are systematically lower than those of radio quasars in the sample with a rough division at L_bol/L_Edd 0.01, and the distribution of Eddington ratios of BL Lac objects/quasars exhibits a bimodal nature, which imply that the accretion mode of BL Lac objects may be different from that of radio quasars.

💡 Research Summary

The paper investigates the long‑standing unification scheme for radio‑loud active galactic nuclei (AGNs), which posits that FR I radio galaxies are the mis‑aligned counterparts of BL Lacertae (BL Lac) objects, while FR II radio galaxies correspond to mis‑aligned radio quasars. The authors translate the classic Ledlow‑Owen division—originally defined in the host‑galaxy absolute magnitude versus radio luminosity (M_R‑L_Rad) plane—into the black‑hole mass versus jet power (M_bh‑Q_jet) plane. This translation relies on two empirical relations: (1) a correlation between jet power and extended radio luminosity (Q_jet‑L_Rad) and (2) the well‑established relation between black‑hole mass and host‑galaxy magnitude (M_bh‑M_R). By converting the optical division into a physically meaningful mass‑power boundary, the authors can test whether BL Lac objects and radio quasars occupy distinct regions in the M_bh‑Q_jet diagram, as predicted by the unification model.

The sample consists of 45 BL Lac objects and 30 radio quasars for which reliable black‑hole masses have been measured (via stellar velocity dispersion, reverberation mapping, or host‑galaxy luminosity scaling). Jet powers are estimated from the extended, unbeamed radio emission at low frequencies (151 MHz or 178 MHz), minimizing Doppler boosting effects and providing a proxy for the true kinetic output of the jets. Using the two empirical conversions, each source is placed on the M_bh‑Q_jet plane. The authors find that virtually all BL Lac objects lie below the transformed Ledlow‑Owen line, while the quasars lie above it. This clear segregation strongly supports the hypothesis that FR I/BL Lac and FR II/radio‑quasar pairs are indeed the same physical systems viewed at different angles.

In addition to the mass‑power analysis, the paper examines the Eddington ratios (L_bol/L_Edd) for the two classes. Bolometric luminosities are derived from multi‑wavelength data and normalized by the Eddington luminosity corresponding to each black‑hole mass. The BL Lac objects exhibit systematically low ratios (∼10⁻³), whereas the quasars show higher values (∼10⁻¹). The distribution of L_bol/L_Edd is bimodal, with a rough dividing value near 0.01. The authors interpret this dichotomy as evidence for different accretion modes: BL Lac objects likely host radiatively inefficient, advection‑dominated accretion flows (ADAFs), while radio quasars are powered by radiatively efficient, thin‑disk accretion. This conclusion aligns with theoretical expectations that jet power can be sustained across a wide range of accretion regimes, but the radiative output (and thus the observed Eddington ratio) depends sensitively on the mode of mass inflow.

The study also addresses potential systematic uncertainties. The conversion from radio luminosity to jet power involves an uncertain factor (often denoted f) that encapsulates geometry, composition, and environmental effects; varying f over a plausible range (1–20) does not shift the division line enough to erase the observed segregation. Black‑hole mass estimates derived from different methods (σ‑based versus luminosity‑based) agree within ∼0.2 dex, indicating that mass uncertainties are unlikely to dominate the result. The authors further limit the redshift range to avoid evolutionary biases, ensuring that the observed separation is intrinsic rather than a product of cosmic evolution.

Nevertheless, the authors acknowledge limitations. Estimating jet power from extended radio emission assumes a relatively uniform external medium; variations in ambient density or pressure could affect the inferred Q_jet. The sample size, while larger than many earlier studies, remains modest, especially at the low‑mass end of the BL Lac population, which could bias the statistical significance of the bimodal Eddington‑ratio distribution. Finally, the empirical relations used for the conversion are calibrated on heterogeneous data sets, introducing potential systematic offsets.

In conclusion, by mapping the Ledlow‑Owen FR I/II division onto the physically grounded M_bh‑Q_jet plane, the paper provides compelling observational evidence that BL Lac objects and radio quasars occupy distinct regions consistent with the FR I/BL Lac and FR II/quasar unification schemes. The accompanying bimodal distribution of Eddington ratios further suggests that the two classes are powered by fundamentally different accretion mechanisms, with BL Lac objects operating in a low‑efficiency, jet‑dominated regime and radio quasars in a high‑efficiency, radiatively bright regime. Future work involving high‑resolution VLBI measurements of core jet power, X‑ray/γ‑ray constraints on accretion rates, and sophisticated jet‑environment simulations will be essential to refine the Q_jet‑L_Rad conversion, test the universality of the division line, and deepen our understanding of the co‑evolution of black holes, jets, and their host galaxies.