[Paper Review] Molecular gas in NUclei of GAlaxies (NUGA) VII. NGC4569, a large scale bar funnelling gas into the nuclear region

F. Boone, A. J. Baker+9 more|DIGITAL.CSIC (Spanish National Research Council (CSIC))|May 24, 2007

Galaxies: Formation, Evolution, Phenomena50 references4 citations

TL;DR

This study investigates molecular gas dynamics in NGC 4569 using high-resolution CO observations to determine how a large-scale stellar bar drives gas inflow toward the nucleus. Using orbital modeling in a weakly barred potential, the authors identify the inner Lindblad resonance (ILR) and show gravitational torques efficiently funnel gas to within 300 pc, though an additional mechanism is required to fuel the central starburst below 100 pc.

ABSTRACT

This work is part of the NUGA survey of CO emission in nearby active galaxies. We present observations of NGC4569, a member of the Virgo Cluster. We analyse the molecular gas distribution and kinematics in the central region and we investigate a possible link to the strong starburst present at the nucleus. 70% of the 1.1x10^9 Msolar of molecular gas detected in the inner 20" is found to be concentrated within the inner 800 pc and is distributed along the large scale stellar bar seen in near-infrared observations. A hole in the CO distribution coincides with the nucleus where most of the Halpha emission and blue light are emitted. The kinematics are modelled in three different ways, ranging from the purely geometrical to the most physical. This approach allows us to constrain progressively the physical properties of the galaxy and eventually to emerge with a reasonable fit to an analytical model of orbits in a barred potential. Fitting an axisymmetric model shows that the non-circular motions must be comparable in amplitude to the circular motions (120 km/s). Fitting a model based on elliptical orbits allows us to identify with confidence the single inner Lindblad resonance (ILR) of the large scale bar. Finally, a model based on analytical solutions for the gas particle orbits in a weakly barred potential constrained by the ILR radius reproduces the observations well. The mass inflow rate is then estimated and discussed based on the best fit model solution. The gravitational torques implied by this model are able to efficiently funnel the gas inside the ILR down to 300 pc, although another mechanism must take over to fuel the nuclear starburst inside 100 pc.

Motivation & Objective

• To investigate the role of a large-scale stellar bar in funneling molecular gas toward the nucleus of NGC 4569.
• To determine whether non-circular motions and resonant structures in the gas kinematics can explain the observed concentration of molecular gas.
• To model the gravitational torques in a barred potential and estimate the mass inflow rate into the nuclear region.
• To assess whether the observed gas dynamics can account for the strong nuclear starburst activity in this LINER-type galaxy.
• To identify the physical mechanisms responsible for fueling the central starburst, distinguishing between bar-driven inflow and other processes.

Proposed method

• High-resolution (0.5") CO(1-0) and CO(2-1) line observations were obtained using the IRAM Plateau de Bure Interferometer and 30 m telescope to map molecular gas distribution and kinematics.
• Three modeling approaches were applied: purely geometrical, elliptical orbit modeling, and analytical solutions for gas particle orbits in a weakly barred potential.
• The inner Lindblad resonance (ILR) was identified by fitting orbital models to the observed kinematic structure, using the condition Ω - κ/2 = Ω_p.
• Gravitational torque calculations were derived from the perturbed potential Φ_b(r,φ) = εΦ₀ cos(2φ), with Φ₀ from a logarithmic potential model.
• Mass inflow rate was estimated using the radial velocity derived from angular momentum change: v_rad = T̄ / (∂M/∂r), where T̄ is the net torque over an orbit.
• The model was constrained by the ILR radius and pattern speed, with the bar strength kept below 5% to remain in the weak perturbation regime.

Experimental results

Research questions

• RQ1 What is the spatial distribution and kinematic structure of molecular gas in the central 20″ of NGC 4569?
• RQ2 To what extent does the large-scale stellar bar drive gas inflow toward the nucleus via gravitational torques?
• RQ3 Where is the inner Lindblad resonance (ILR) located, and how does it influence gas dynamics and inflow efficiency?
• RQ4 Can the observed gas concentration and kinematics be explained by orbital models in a barred potential?
• RQ5 What is the estimated mass inflow rate into the nuclear region, and what mechanism fuels the starburst below 100 pc?

Key findings

• 70% of the total molecular gas mass (~7.7 × 10⁸ M☉) is concentrated within the inner 800 pc, with a significant concentration along the large-scale stellar bar.
• A hole in the CO distribution coincides with the nucleus, where most Hα emission and blue light are observed, indicating a lack of molecular gas at the very center.
• The inner Lindblad resonance (ILR) is confidently identified at a radius of approximately 300 pc, based on elliptical orbit modeling.
• The analytical model of gas orbits in a weakly barred potential, constrained by the ILR radius, provides the best fit to the observed kinematics.
• Gravitational torques from the bar efficiently funnel gas down to within 300 pc, with a mass inflow rate consistent with the model, though not sufficient to fully explain fueling below 100 pc.
• The pattern speed of the bar is estimated to be ~0.11 v_p / r_p, and the corotation radius is ~6.26 times the ILR radius, consistent with a logarithmic potential model.