[Paper Review] Molecular Gas in NUclei of GAlaxies (NUGA) I.The counter-rotating LINER NGC4826

S. García‐Burillo, F. Combes+9 more|ArXiv.org|Jun 6, 2003

Astrophysics and Star Formation Studies59 references80 citations

TL;DR

This study presents high-resolution CO(1-0) and CO(2-1) observations of NGC 4826 using the IRAM Plateau de Bure Interferometer, revealing a lopsided, 40 pc nuclear molecular disk with two one-arm spirals driven by m=1 instabilities. The kinematics indicate that inner m=1 modes are consistent with a fast wave (trailing, outside corotation), while outer modes align with a slow wave, suggesting that the current gas distribution does not favor active galactic nucleus (AGN) fueling despite a significant reservoir of molecular gas (3.1 × 10⁸ M☉).

ABSTRACT

We present new high-resolution observations of the nucleus of the counter-rotating LINER NGC4826, made in the J=1-0 and J=2-1 lines of 12CO with the IRAM Plateau de Bure mm-interferometer(PdBI).The CO maps, which achieve 0.8''(16pc) resolution in the 2-1 line, fully resolve an inner molecular gas disk which is truncated at an outer radius of 700pc. The total molecular gas mass is distributed in a lopsided nuclear disk of 40pc radius and two one-arm spirals, which develop at different radii in the disk. The distribution and kinematics of molecular gas in the inner 1kpc of NGC4826 show the prevalence of different types of m=1 perturbations in the gas. Although dominated by rotation, the gas kinematics are perturbed by streaming motions related to the m=1 instabilities. The non-circular motions associated with the inner m=1 perturbations agree qualitatively with the pattern expected for a trailing wave developed outside corotation ('fast' wave). In contrast, the streaming motions in the outer m=1 spiral are better explained by a 'slow' wave. A paradoxical consequence is that the inner m=1 perturbations would not favour AGN feeding. An independent confirmation that the AGN is not being generously fueled at present is found in the low values of the gravitational torques exerted by the stellar potential for R<530pc. The distribution of star formation in the disk of NGC4826 is also strongly asymmetrical. Massive star formation is still vigorous, fed by the significant molecular gas reservoir at R<700pc. There is supporting evidence for a recent large mass inflow episode in NGC4826. These observations have been made in the context of the NUclei of GAlaxies (NUGA) project, aimed at the study of the different mechanisms for gas fueling of AGN.

Motivation & Objective

• To investigate the role of m=1 instabilities in driving gas inflow toward the nucleus of NGC 4826, a counter-rotating LINER galaxy.
• To determine whether the observed molecular gas kinematics and distribution are consistent with secular evolution or external triggers.
• To assess the potential for AGN fueling by analyzing gravitational torques and gas inflow patterns in the nuclear region.
• To link the morphology and kinematics of molecular gas to star formation activity and structural asymmetries in the inner disk.
• To contribute to the NUGA project’s broader goal of identifying mechanisms for gas fueling in active galactic nuclei.

Proposed method

• High-resolution interferometric observations of 12CO J=1-0 and J=2-1 transitions were conducted with the IRAM Plateau de Bure Interferometer, achieving 0.8 arcsec (16 pc) spatial resolution.
• Velocity-channel maps and position-velocity diagrams were used to analyze the kinematics and spatial distribution of molecular gas in the inner 1 kpc.
• The gravitational torque from the stellar potential was computed to evaluate its ability to drive gas inflow, particularly for R < 530 pc.
• The pattern speed of m=1 instabilities was estimated by comparing observed streaming motions with theoretical models of fast and slow waves.
• Molecular gas mass was derived from the CO line luminosity using standard X-factor, yielding a total mass of 3.1 × 10⁸ M☉.
• HST images were used to compare the observed asymmetries in star formation with the morphology of m=1 perturbations in the molecular gas.

Experimental results

Research questions

• RQ1 What drives the m=1 instabilities observed in the molecular gas disk of NGC 4826, and are they consistent with fast or slow wave modes?
• RQ2 Does the current distribution and kinematics of molecular gas in NGC 4826 support efficient fueling of the active galactic nucleus?
• RQ3 How do the gravitational torques from the stellar potential influence gas inflow in the central region of NGC 4826?
• RQ4 To what extent are the observed asymmetries in star formation and gas distribution linked to the m=1 perturbations in the molecular disk?
• RQ5 What role does secular evolution play in the formation and maintenance of the lopsided nuclear disk in NGC 4826?

Key findings

• The molecular gas mass in NGC 4826 is 3.1 × 10⁸ M☉, with 15% (4.65 × 10⁷ M☉) concentrated in a 40 pc radius lopsided nuclear disk.
• The inner molecular disk exhibits two one-arm spiral structures, with streaming motions indicating a fast wave pattern (trailing, outside corotation), consistent with a pattern speed ≥ 1500 km s⁻¹ kpc⁻¹.
• The outer m=1 spiral shows kinematics better explained by a slow wave, suggesting a different dynamical origin or pattern speed.
• Gravitational torques in the central region (R < 530 pc) are low, indicating that the current gas distribution is not conducive to efficient AGN fueling.
• Star formation is strongly asymmetric and spatially correlated with the m=1 perturbations, particularly in the inner disk, indicating ongoing or recent inflow-driven star formation.
• The observed kinematics of ionized gas and stellar velocity dispersion support a recent large-scale gas inflow episode, likely triggered by past accretion or interaction, consistent with a 10⁷–10⁸ yr timescale.