[Paper Review] Observational Overview of the Feeding of Active Galactic Nuclei

Thaisa Storchi‐Bergmann|ArXiv.org|Dec 21, 2007

Pulsars and Gravitational Waves Research1 references3 citations

TL;DR

This paper identifies nuclear gaseous spirals and inflows along these structures as the key mechanism transporting gas from kiloparsec to sub-parsec scales to feed supermassive black holes in active galactic nuclei. Using high-resolution integral field spectroscopy, the authors present the first direct observational evidence of mass inflow along nuclear spirals, estimating inflow rates of ~10⁻² to 10⁻¹ M☉/yr, resolving a long-standing question about the final fueling stage of AGN.

ABSTRACT

I present an overview of the observational signatures of feeding of Active Galactic Nuclei, discussing briefly the role of interactions among galaxies on extragalactic scales, and of non-axisymmetric gravitational potentials -- such as bars -- on galactic scales. Then I discuss at larger length the feeding signatures on hundred of parsec scales, for which new results include: (1) recent star formation surrounding the active nucleus on tens of parsec scales; (2) excess of gas and dust in active galaxies relative to non-active ones, in the form of nuclear spirals and disks; (3) new kinematic signatures of gas inflow along nuclear spiral arms, which may be the long sought mechanism to bring gas from kiloparsec scales down to the nucleus to feed the supermassive black hole.

Motivation & Objective

• To identify and characterize the final stage of gas inflow that feeds supermassive black holes in active galactic nuclei.
• To resolve the discrepancy between theoretical predictions of gas transport and observational lack of clear association between large-scale structures (bars, interactions) and nuclear activity.
• To determine whether nuclear spirals and associated inflows are the dominant mechanism for transporting gas from ~1 kpc to the innermost regions of galaxies.
• To quantify the mass inflow rate through nuclear spirals and assess their role in triggering and sustaining AGN activity.
• To test the evolutionary scenario in which interactions and bars initiate gas inflow, followed by a delayed or sequential onset of starburst and AGN activity.

Proposed method

• Utilized high spatial and spectral resolution integral field unit (IFU) spectroscopy from Gemini Multi-Object Spectrographs to map 2D velocity fields and kinematics of ionized and molecular gas in AGN host galaxies.
• Analyzed Hα and other emission line profiles to identify deviations from symmetric rotation, indicating inflow or outflow motions.
• Constructed model velocity fields to subtract from observed data, isolating kinematic structures such as inflowing arms and bi-conical outflows.
• Mapped gas density and velocity dispersion to identify regions of enhanced turbulence and shocks, particularly near the nucleus and along spiral arms.
• Correlated the presence of nuclear spirals and inflows with AGN activity, stellar population age, and circumnuclear star formation.
• Estimated mass inflow rates by combining kinematic data with gas density and velocity gradient measurements.

Experimental results

Research questions

• RQ1 What is the dominant physical mechanism responsible for transporting gas from kiloparsec scales to the immediate vicinity of the supermassive black hole in AGN?
• RQ2 Are nuclear gaseous spirals associated with measurable inflow signatures, and can they account for the angular momentum loss required for accretion?
• RQ3 Why is there no clear observational correlation between large-scale bars or galaxy interactions and the presence of AGN activity?
• RQ4 How do the kinematics of circumnuclear gas relate to the age of the stellar population in AGN host galaxies?
• RQ5 Can the observed inflow rates along nuclear spirals sustain the observed accretion luminosities in AGN?

Key findings

• Nuclear gaseous spirals are strongly associated with AGN activity, suggesting they are the primary channel for transporting fuel to the central black hole.
• The first direct observational evidence of gas inflow along nuclear spiral arms was obtained using IFU spectroscopy, revealing systematic velocity gradients consistent with inward motion.
• Mass inflow rates of ~10⁻² to 10⁻¹ M☉/yr were measured along nuclear spirals, providing a viable mechanism to feed the SMBH after gas reaches the inner kiloparsec.
• The presence of intermediate-age stellar populations (10⁸ years) in AGN hosts supports a delayed triggering scenario, where interactions or bars initiate gas inflow, followed by a starburst that may later fuel the AGN via stellar mass loss.
• Kinematic signatures of shocks and enhanced velocity dispersion near the nucleus are consistent with interaction between radio jets and circumnuclear gas, indicating feedback processes.
• The lack of clear correlation between large-scale bars and AGN activity may be due to a delay between bar-induced inflow and final fueling, or bar destruction during the active phase.