[Paper Review] Molecular Gas in NUclei of GAlaxies (NUGA): IV. Gravitational Torques and AGN Feeding

S. García‐Burillo, F. Combes+3 more|ArXiv.org|Jul 4, 2005

Galaxies: Formation, Evolution, Phenomena64 references106 citations

TL;DR

This paper investigates gravitational torques from stellar potentials as a mechanism to drive molecular gas inflow toward AGN in four low-luminosity galaxies (NGC 4321, 4826, 4579, 6951). It finds that torques are predominantly positive within 200 pc, hindering inflow, and proposes that transient torques or viscosity in high-density nuclear rings may overcome this barrier, enabling recurrent AGN fueling episodes over a galaxy's lifetime.

ABSTRACT

We discuss the efficiency of stellar gravity torques as a mechanism to account for the feeding of the central engines of four low luminosity AGN: NGC4321, NGC4826, NGC4579 and NGC6951. These galaxies have been observed as part of the NUGA CO project, aimed at the study of AGN fueling mechanisms. Our calculations allow us to derive the characteristic time-scales for gas flows and discuss whether torques from the stellar potentials are efficient enough to drain the gas angular momentum in the inner 1 kpc of these galaxies. Results indicate paradoxically that feeding should be thwarted close to the AGN: in the four cases analyzed, gravity torques are mostly positive inside r~200pc, resulting in no inflow on these scales. As a possible solution for the paradox, we speculate that the agent responsible for driving inflow to still smaller radii is transient and thus presently absent in the stellar potential. Alternatively, the gravity torque barrier associated with the ILR of the bars in these galaxies could be overcome by other mechanisms that become competitive in due time against gravity torques. We find that viscosity can counteract moderate--to--low gravity torques on the gas if it acts on a nuclear ring of high gas surface density contrast and a few 100pc size. We propose an evolutionary scenario in which gravity torques and viscosity act in concert to produce recurrent episodes of activity during the typical lifetime of any galaxy. In this scenario the recurrence of activity in galaxies is indirectly related to that of the bar instabilities.

Motivation & Objective

• To assess the efficiency of stellar gravity torques in driving molecular gas inflow to the central engines of low-luminosity AGN.
• To determine whether gravity torques alone can account for the observed fueling of AGN in the inner 1 kpc of galaxies.
• To evaluate the role of viscosity and gas self-gravity as alternative or complementary mechanisms to overcome the torque barrier near the AGN.
• To propose a scenario in which recurrent AGN activity is linked to bar evolution, even when active phases are not synchronized with peak bar strength.

Proposed method

• Derived stellar potentials from high-resolution near-infrared (NIR) images of the four galaxies.
• Estimated averaged effective torques on molecular gas using high-resolution CO(1–0) and CO(2–1) maps with angular resolution ~0.5”–2”.
• Calculated characteristic inflow timescales based on the net torque and gas surface density profiles.
• Evaluated the relative efficiency of viscosity versus gravity torques by comparing viscous timescales to dynamical timescales in nuclear rings.
• Assessed gas mass fractions as a function of radius to estimate the influence of gas self-gravity on the gravitational potential.
• Used case-by-case analysis to compare viscous and gravitational torque effects in each target galaxy.

Experimental results

Research questions

• RQ1 Are stellar gravity torques sufficient to drive significant gas inflow in the inner 1 kpc of low-luminosity AGN hosts?
• RQ2 Why is gas inflow hindered at radii <200 pc despite the presence of large-scale bars?
• RQ3 Can viscosity in high-surface-density nuclear rings overcome the positive torque barrier and enable AGN fueling?
• RQ4 How do gas self-gravity and non-axisymmetric structures influence the gravitational potential and inflow efficiency?
• RQ5 What is the connection between bar evolution cycles and the recurrence of AGN activity in low-luminosity galaxies?

Key findings

• In all four galaxies, gravity torques are predominantly positive within ~200 pc, indicating a net outward torque that hinders gas inflow.
• The torque barrier at ~200 pc suggests that gravity alone cannot explain nuclear fueling in these systems.
• Viscosity can counteract moderate-to-low gravity torques if acting on a nuclear ring with high gas surface density contrast and size ~few 100 pc.
• Gas self-gravity is significant in NGC 4826 and NGC 4321, with gas mass fractions of 10–15% at ~100 pc, potentially influencing non-axisymmetric instabilities.
• In NGC 6951 and NGC 4579, gas mass fractions remain low (1–3%) from ~100 to 1000 pc, indicating minimal self-gravity influence.
• A scenario is proposed in which gravity torques and viscosity act in concert over a bar’s lifetime, enabling recurrent AGN episodes even when not synchronized with peak bar strength.