Gravitational Torques from a Lopsided Young Stellar Component Sustain High Black Hole Accretion in NGC 4593

Abstract: Supermassive black holes (SMBHs) grow primarily through gas accretion, observed as active galactic nuclei (AGNs). While mergers can drive luminous AGN episodes, secular processes may fuel a substantial portion of cosmic BH growth. Whether these mechanisms can sustain high BH accretion rates remains uncertain. This study aims to identify the secular mechanism driving high BH accretion rates, by targeting a galaxy with moderately massive SMBH, high central gas densities, accretion rates of a few percent of the Eddington limit, and gas kinematics resolved close to black hole-dominated scales. A blind search led to the identification of NGC 4593, which is representative of the AGN population driving BH mass density growth since $z=1$. A prominent single-arm ("$m=1$") molecular gas spiral with ${\rm log}\,M_{\rm mol}/{\rm M}\odot=8.1\pm0.3$ extends from 1.3$\,$kpc down to the SMBH's sphere of influence ($1.7^{+0.5}{-0.2}\,{\rm pc}$). Star formation in the spiral is inefficient (${\rm SFR} = 4.9 \times 10^{-2} \,{\rm M}\odot {\rm /yr}$, $\langle t{\rm dep}\rangle=3.9 \pm 0.6\,\rm{Gyr}$), whereas inflow rate exceeds the SFR by two orders of magnitude and is sufficient to sustain the current BH accretion rate for $\geq$35$\,$Myr, enabling $\sim$10% SMBH growth. A young, lopsided stellar component (${\rm log}\,M_\star/{\rm M}_\odot=7.5-9.3$) exerts torques on the molecular gas, likely driving the gas inflow. This young stellar component may serve as both a cause and a product of sustained gas funnelling towards the SMBH. These findings provide evidence for the sustained secular $m=1$ feeding mode at high BH accretion rates, linking kpc-scale gas dynamics to the black hole's sphere of influence. This mechanism, consistent with simulation predictions, may represent a key contributor to SMBH growth in luminous AGNs since cosmic noon.