2025A&A...700A.285W

Context. 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 black hole growth. Whether these mechanisms can sustain high black hole accretion rates remains uncertain.
Aims. The aim of this study is to identify the secular mechanism driving high SMBH accretion rates, by targeting a galaxy with a 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.
Methods. 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. Combining HST imaging, VLT/MUSE spectroscopy, and ALMA imaging, we resolve molecular and ionised gas kinematics close to the sphere of influence of the SMBH.
Results. A prominent single-arm (“m = 1”) molecular gas spiral with log M_mol/M_⊙ = 8.1 ± 0.3 extends from 1.3 kpc down to the SMBH's sphere of influence ($ 1.7^{+0.5}_{-0.2}\,\mathrm{pc} $). Star formation in the spiral is inefficient (SFR = 4.9 × 10^–2 M_⊙/yr, ⟨t_dep⟩=3.9 ± 0.6 Gyr), whereas the gas inflow rate exceeds the SFR by two orders of magnitude and is sufficient to sustain the current SMBH accretion rate for ≥35 Myr, enabling ∼10% SMBH growth. A young, lopsided stellar component (log M_⋆/M_⊙ = 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.
Conclusions. These findings provide evidence for the sustained secular m = 1 feeding mode at high SMBH 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.