X-ray nuclear activity in S4G barred galaxies: No link between bar strength and co-occurrent supermassive black hole fueling

TL;DR

This study investigates whether stellar bar strength in galaxies correlates with supermassive black hole activity and finds no significant link, suggesting other factors influence black hole fueling in low-luminosity active galactic nuclei.

Contribution

It provides a detailed analysis of bar strength and nuclear activity in nearby galaxies, demonstrating no correlation between the two, which challenges previous assumptions about bar-driven black hole fueling.

Findings

No significant correlation between bar strength and nuclear activity.

Most galaxies host low-luminosity AGN activity.

Bar strength does not predict black hole fueling efficiency.

Abstract

Stellar bars can lead to gas inflow toward the center of a galaxy and stimulate nuclear star formation. However, there is no compelling evidence on whether they also feed a central supermassive black hole: by measuring the fractions of barred active and inactive galaxies, previous studies have yielded conflicting results. In this paper, we aim to understand the lack of observational evidence for bar-driven active galactic nucleus (AGN) activity by studying a sample of 41 nearby (d < 35 Mpc) barred galaxies from the Spitzer Survey for Stellar Structure in Galaxies. We use Chandra observations to measure nuclear 2--10 keV X-ray luminosities and estimate Eddington ratios, together with Spitzer 3.6um imaging to quantify the strength of the stellar bar in two independent ways: (1) from its structure, as traced by its ellipticity and boxiness, and (2) from its gravitational torque Q_b, taken as the maximum ratio of the tangential force to the mean background radial force. In this way, rather than discretizing the presence of both stellar bars and nuclear activity, we are able to account for the continuum of bar strengths and degrees of AGN activity. We find nuclear X-ray sources in 31 out of 41 galaxies with median X-ray luminosity and Eddington ratio of L_X=4.3x10^{38} erg/s and L_bol/L_Edd=6.9x10^{-6} respectively, consistent with low-luminosity AGN activity. Including upper limits for those galaxies without nuclear detections, we find no significant correlation between any of the bar strength indicators and the degree of nuclear activity, irrespective of galaxy luminosity, stellar mass, Hubble type, or bulge size. Strong bars do not favor brighter or more efficient nuclear activity, implying that at least for the low-luminosity regime, supermassive black hole fueling is not closely connected to large scale features.