Steep-spectrum radio emission from the low-mass active galactic nucleus GH 10

GH10 is a broad-lined active galactic nucleus (AGN) energized by a black hole of mass 800,000 M_⊙. It was the only object detected by Greene et al. in their Very Large Array (VLA) survey of 19 low-mass AGNs discovered by Greene & Ho. New VLA imaging at 1.4, 4.9, and 8.5 GHz reveals that GH 10's emission has an extent of less than 320 pc, has an optically thin synchrotron spectrum with a spectral index α = -0.76 ± 0.05 (S _ν ∝/ ν^+α), is less than 11% linearly polarized, and is steady - although poorly sampled - on timescales of weeks and years. Circumnuclear star formation cannot dominate the radio emission, because the high inferred star formation rate, 18 M_⊙ yr^-1, is inconsistent with the rate of less than 2 M_⊙ yr^-1 derived from narrow Hα and [O II] λ3727 emission. Instead, the radio emission must be mainly energized by the low-mass black hole. GH 10's radio properties match those of the steep-spectrum cores of Palomar Seyfert galaxies, suggesting that, like those galaxies, the emission is outflow-driven. Because GH 10 is radiating close to its Eddington limit, it may be a local analog of the starting conditions, or seeds, for supermassive black holes. Future imaging of GH 10 at higher linear resolution thus offers an opportunity to study the relative roles of radiative versus kinetic feedback during black hole growth.