AGN-Driven Biconical Outflows

• AGN-driven biconical outflows are twin, high-velocity flows emanating from active galactic nuclei, defined by conical geometries and key physical parameters such as opening angles and spatial extent.
• High-resolution spectroscopy and IFU mapping reveal detailed kinematics with characteristic acceleration, turnover radii, and deceleration profiles that are essential for understanding their dynamics.
• Emission-line diagnostics yield mass outflow rates and kinetic energetics that demonstrate the significant feedback impact of these outflows on star formation, black hole growth, and host galaxy evolution.

Active galactic nuclei (AGN) frequently drive high-velocity, ionized gas outflows exhibiting biconical geometry on scales of tens to thousands of parsecs. These biconical outflows are a fundamental mechanism by which AGN can inject mass, momentum, and energy into their host galaxies, thereby regulating star formation, SMBH growth, and circumnuclear gas dynamics. Their detailed velocity structure, geometry, physical conditions, and impact have been extensively characterized across a range of Seyfert and radio galaxies using high-spatial-resolution spectroscopy, IFU mapping, and analytic/numerical modeling.

1. Geometry and Physical Structure of AGN-Driven Biconical Outflows

AGN-driven biconical outflows are typically modeled as pairs of oppositely directed, hollow or filled cones, each defined by inner and outer half-opening angles ($\theta_{\rm in}$, $\theta_{\rm out}$), maximum radial extent (per cone), and a symmetry axis inclined at angle $i$ to the observer's line of sight. The cones may be "filled," with emission throughout the volume, or "hollow," with emission/walls between $\theta_{\rm in}$ and $\theta_{\rm out}$. The axis is frequently aligned with, but can be arbitrarily oriented with respect to, the host-galaxy disk and radio jet.

• Opening angles: Typical internal half-opening angles range from $\sim$15°–55°, with outer angles up to $\sim$90° in some IFU samples; e.g., NGC 5728: $\beta=50.2^\circ\pm2^\circ$ and inclination $i=47.6^\circ\pm3^\circ$ (Durré et al., 2018); NGC 3227: $\theta_{\rm in}=47^{+6}_{-2}~{\rm deg}$, $\theta_{\rm out}=68^{+1}_{-1}~{\rm deg}$, $i=40^{+5}_{-4}~{\rm deg}$ (Falcone et al., 30 May 2024).
• Spatial extent: Outflows are traced on scales from sub-100~pc to $\sim$5~kpc; in HE1353–1917, ionization cones reach $R_{\rm ENLR}\sim25~{\rm kpc}$, but the fast outflow is restricted to $\sim$1~kpc (Husemann et al., 2019).
• Cone–host orientation: The bicone can be co-planar, perpendicular, or randomly oriented with respect to the host disk or radio jet. Random orientations dominate in SDSS-selected AGN samples (Nevin et al., 2017).

Projection effects cause the apparent opening angle ($\theta_{\rm app}$) measured in line images to underestimate the true angle, particularly if the cone axis is inclined with respect to the sky plane. The relation is

$$\tan\theta_{\rm app} = \tan\theta_{\rm true}\,\cos i\,,$$

so corrections must be made to infer intrinsic bicone geometries (Fischer et al., 2010).

2. Kinematics: Velocity Laws and Turnover Radii

The kinematic structure of biconical outflows is recovered from resolved emission-line mapping (e.g., [O III], [Fe II], Hα), showing a characteristic acceleration and, often, deceleration profile.

• Acceleration phase: The material accelerates radially from the nucleus, typically following $v(r) \sim v_{\rm turn} \sqrt{r/r_{\rm turn}}$ up to a turnover radius $r_{\rm turn}$.
• Turnover and deceleration: Beyond $r_{\rm turn}$, deceleration sets in as clouds interact with the ISM, with velocities dropping linearly or following empirical broken-law prescriptions, e.g.,

$$v_{\rm LOS}(r) = \begin{cases} v_{\rm turn} \sqrt{r/r_{\rm turn}}, & 0 < r < r_{\rm turn} \ v_{\rm turn} - a(r - r_{\rm turn}), & r_{\rm turn} < r < r_{\rm max} \end{cases}$$

where $a$ is a deceleration parameter (Durré et al., 2018).

Empirical examples:

• NGC 5728: $|v_{\rm LOS}|\simeq 250$~km/s at $r_{\rm turn}\simeq 250$~pc, decelerating to 130~km/s by 500~pc, with true deprojected $V_{\rm out}\simeq 390$~km/s.
• NGC 3516: $v_{\rm max} \approx 1000\pm150$~km/s at $r_{\rm turn}=210\pm30$~pc (Tutterow et al., 8 Sep 2025).
• NGC 3227: $v(r)\sim 500$~km/s at $r_t=26\pm6$~pc, declining beyond (Falcone et al., 30 May 2024).

Turnover radii and velocity profiles are well reproduced by 1D dynamical models balancing radiative driving (parameterized by force multiplier $\mathcal{M}$ and bolometric luminosity $L_{\rm bol}$) against gravitational deceleration from the SMBH and bulge: $$v(r) = \sqrt{\int_{r_1}^r \left[4885\,L_{44}\,\mathcal{M}/r'^2 - 8.6\times 10^{-3}\,M_{\rm enc}(r')/r'^2\right]\,dr'}$$ where $L_{44} = L_{\rm bol}/10^{44}\,{\rm erg\,s}^{-1}$ (Tutterow et al., 8 Sep 2025, Falcone et al., 30 May 2024).

3. Mass Outflow Rates and Kinetic Energetics

The physical impact of biconical outflows is quantified through the mass outflow rate ($\dot{M}_{\rm out}$) and kinetic luminosity ($\dot{E}_k$), typically inferred from emission-line luminosities (e.g., Br$\gamma$, H$\alpha$, [O III]), electron density estimates, and geometric modeling.

$\dot{M}_{\rm out} = n_e\,m_p\,V_{\rm out}\,A\,f$

where $n_e$ is the electron density, $V_{\rm out}$ the deprojected outflow velocity, $A$ the cross-sectional area or cone wall area at radius $r$, and $f$ the volume filling factor ($\lesssim0.1$–$10^{-3}$ in different works).

• Typical values: In NGC 5728, $$\dot{M}_{\rm out}\approx 38\,M_\odot\,{\rm yr}^{-1}$$ (from Br$\gamma$ flux analysis with $n_e\sim10^3\,$cm$^{-3}$, $f\lesssim0.1$) (Durré et al., 2018); in Cygnus A, $100$–$280\,M_\odot\,{\rm yr}^{-1}$ at $v_{\rm out}\sim600$–$700$~km/s ($f\sim10^{-3}$) (Riffel, 2021); in moderate-luminosity IFU samples, values fall in the range $\sim0.03$–$3\,M_\odot\,{\rm yr}^{-1}$ (Kim et al., 2023).
• Kinetic power: $$\dot{E}_k = \frac{1}{2}\,\dot{M}_{\rm out}\,(V_{\rm out}^2 + \sigma^2)$$, where $\sigma$ is the velocity dispersion. Cygnus A demonstrates $\dot{E}_k \sim 6$–$16 \times 10^{43}\,$erg/s ($0.3$–$3.3\%$ of $L_{\rm bol}$), while typical Seyfert values are in the $10^{41}$–$10^{42}$ erg/s range, or $0.01$–$1\%$ of $L_{\rm bol}$ (Durré et al., 2018, Riffel, 2021).
• Mass loading: The ratio $\dot{M}_{\rm out} / \dot{M}_{\rm acc}$ is typically $\sim10^3$, indicating extreme mass loading by the ambient ISM (Durré et al., 2018, Müller-Sánchez et al., 2011).

4. Multiphase Structure and Excitation Mechanisms

Biconical outflows exhibit strong stratification in ionization and excitation, with distinct morphologies in coronal lines, low-ionization species, and molecular gas:

• Coronal gas ([Si X], [Si VI]): Traces high-ionization regions within bicone walls, reaching to $>1.5$~kpc in Cygnus A.
• Low-ionization ([Fe II], H$\alpha$): More centrally peaked or extended, often marking the interface of outflow and circumnuclear disk.
• Molecular (CO, H$_2$): Suppressed within bicone, often peaks perpendicular to outflow axis.
• Photoionization and shock diagnostics: [Fe II]/Pa$\alpha$ and H$_2$ line ratios distinguish AGN versus shock excitation; maps often show AGN-like ratios within cones and high-line-ratio shocks at periphery (as in Cygnus A and HE1353–1917) (Riffel, 2021, Husemann et al., 2019).

Spatially resolved BPT diagrams typically show that the broad component (outflow-dominated) is AGN-photoionized over the bicone extent, while the narrow (rotation-dominated) component and outer regions transition to composite or star-forming excitation (Karouzos et al., 2016).

5. Feedback, Host Galaxy Impact, and Global Trends

The ability of AGN-driven biconical outflows to regulate or quench star formation and SMBH accretion is closely tied to the kinetic power, mass ejection efficiency, outflow geometry, and spatial extent.

• Energy coupling and efficiency: Most measured systems exhibit $\dot{E}_{\rm kin}/L_{\rm bol} \sim 0.01\%$–$1\%$ (NGC 5728: $1\%$; Cygnus A: $0.3$–$3.3\%$), exceeding the $\sim0.5\%$ "critical" threshold for negative feedback in the most energetic cases, but often below it in moderate-luminosity AGN (Durré et al., 2018, Riffel, 2021, Karouzos et al., 2016).
• Spatial reach: Outflows are concentrated within the central $1$–$2$~kpc. The majority of the total outflow mass and energy is contained within this region, and kiloparsec-scale star-forming rings can co-exist with active outflows (Karouzos et al., 2016, Kim et al., 2023).
• Delayed quenching: In many moderate-luminosity systems, the star formation rate (SFR) outside the central kpc remains high ($\dot{M}_{\rm out} \ll {\rm SFR}$), indicating that outflows are insufficient to instantaneously quench star formation at galactic scales (Kim et al., 2023).

Table: Outflow Energetics in Selected AGN

Galaxy	$\dot{M}_{\rm out}$ ($M_\odot$ yr$^{-1}$)	$\dot{E}_{\rm kin}$ (erg/s)	$\dot{E}_{\rm kin}/L_{\rm bol}$
NGC 5728	38	$1.5\times10^{42}$	0.01
Cygnus A	100–280	$(6-16)\times10^{43}$	0.3–3.3%
HE1353–1917	1.9 (ion), 170 (mol)	$2.1\times10^{41}$ (ion), $5.2\times10^{42}$ (mol)	2%

6. Theoretical and Numerical Modeling

Biconical outflow models are constructed analytically and numerically to interpret IFU data, constrain intrinsic kinematics, and connect observed emission to physical outflow characteristics.

• 3D Bicone+Dust Models: These reconstruct observed [O III] profiles and velocity/dispersion statistics in large samples (e.g., SDSS; $N\sim4\times10^4$). Dust strongly impacts the appearance and observed velocity centroids, with wider bicones yielding more negative centroid velocities due to less extinction of the approaching side (Bae et al., 2016).
• MCMC and analytic modeling: Markov Chain Monte Carlo techniques optimize for geometric parameters (opening angles, inclination, turnover radii, $v_{\rm max}$) based on spatially resolved kinematic data, supporting a wide range of asymmetric, nested, and symmetric configurations (Nevin et al., 2017).
• Physics of acceleration: Radiative driving models (incorporating a force multiplier $\mathcal{M}$) quantitatively reproduce the observed acceleration–deceleration profiles and reproduce the measured turnover radii and velocities in Seyfert NLR outflows (Tutterow et al., 8 Sep 2025, Falcone et al., 30 May 2024, Meena et al., 2021).
• Hydrodynamical/N-body approaches: Sub-grid, momentum-driven feedback implementations in codes such as GIZMO launch outflows as stochastic bicone "kicks," demonstrating the emergence of the observed geometry even when the launching axis is allowed to evolve with CND or SMBH spin orientation (Sala et al., 2020).

7. Observational Diagnostics and Classification

Characteristic signatures of AGN-driven biconical outflows include:

• Double-peaked emission lines: As seen in SDSS J1347+1217, asymmetric blue/red peaks in [O III] indicate bi-conical outflows viewed at moderate inclination, distinguishing from dual AGN or rotating NLR disks (Cheng et al., 8 Nov 2025).
• Velocity–dispersion diagrams: Outflow-dominated gas occupies upper-left regions (high $\sigma$, $|v|$) distinct from rotation-dominated clouds (Karouzos et al., 2016).
• Multicomponent Gaussian fitting: Both IFU and long-slit spectroscopy employ Gaussian decomposition and Bayesian evidence to isolate disk rotation and multiple outflow components in velocity space (Tutterow et al., 8 Sep 2025, Falcone et al., 30 May 2024).
• Resolved line ratio mapping: AGN-driven biconical structures are confirmed through ionization ([O III]/Hβ, [N II]/Hα), BPT maps, and spatial correlation with host dust lanes or circumnuclear gas features (Karouzos et al., 2016, Fischer et al., 2010).

In summary, AGN-driven biconical outflows represent an energetically significant, morphologically distinctive feedback channel in Seyfert and radio galaxies. Their physical properties and impact depend sensitively on outflow geometry, driving mechanism, launching region, and coupling to host-galaxy gas, as jointly constrained by spectroscopic mapping, analytic/numerical models, and feedback diagnostics.