Steep-Spectrum Radio Emission from the Low-Mass Active Galactic Nucleus GH 10

TL;DR

GH 10, a low-mass AGN, exhibits steep-spectrum, steady radio emission driven by a low-mass black hole, resembling Seyfert galaxy cores, providing insights into early black hole growth and feedback mechanisms.

Contribution

This study presents detailed radio observations of GH 10, revealing its emission characteristics and ruling out star formation as the primary source, highlighting black hole-driven outflows in low-mass AGNs.

Findings

Radio emission is optically-thin synchrotron with spectral index -0.76.

Emission is less than 320 pc in extent and less than 11% polarized.

Star formation rate estimates are inconsistent with radio emission origin.

Abstract

GH 10 is a broad-lined active galactic nucleus (AGN) energized by a black hole of mass 800,000 Solar masses. 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 -0.76+/-0.05, is less than 11 percent 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 Solar masses per year, is inconsistent with the rate of less than 2 Solar masses per year derived from narrow Halpha and [OII] 3727 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 Seyferts, 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 resolution thus offers an opportunity to study the relative roles of radiative versus kinetic feedback during black-hole growth.