X-ray and radio observations of central black holes in nearby low-mass early-type galaxies: Preliminary evidence for low Eddington fractions

TL;DR

This study investigates the accretion activity of central black holes in nearby low-mass early-type galaxies using radio and X-ray observations, finding they generally exhibit very low Eddington ratios, which challenges existing detection methods.

Contribution

First direct evidence of extended radio emission from a low-mass galaxy's black hole and a preliminary assessment of their low accretion rates compared to more massive galaxies.

Findings

NGC 5102 shows extended radio lobes but no core radio emission.

NGC 205 shows no detectable radio or X-ray emission.

Low Eddington ratios (~10^-6.57) are common in low-mass galaxy black holes.

Abstract

We present new radio and X-ray observations of two nearby ($< 4$ Mpc) low-mass early-type galaxies with dynamically-confirmed central black holes: NGC 5102 and NGC 205. NGC 5102 shows a weak nuclear X-ray source and has no core radio emission. However, for the first time we demonstrate that it shows luminous extended radio continuum emission in low-resolution, low-frequency ($< 3$ GHz) data, consistent with jet lobes on scales $\gtrsim 100$ pc formed from past accretion and jet activity. By contrast, in new, extremely deep, strictly-simultaneous Very Large Array and Chandra observations, no radio or X-ray emission is detected from the black hole in NGC 205. We consider these measurements and upper limits in the context of the few other low-mass early-type galaxies with dynamically-confirmed black holes, and show that the mean ratio of bolometric to Eddington luminosity in this sample is only $\textrm{log} \, (L_{\rm bol}/L_{\rm Edd}) = -6.57\pm0.50$. These Eddington ratios are lower than typical in a comparison sample of more massive early-type galaxies, though this conclusion is quite tentative due to our small sample of low-mass galaxies and potential biases in the comparison sample. This preliminary result is in mild tension with previous work using less sensitive observations of more distant galaxies, which predict higher X-ray luminosities than we observe for low-mass galaxies. If it is confirmed that central black holes in low-mass galaxies typically have low Eddington ratios, this presents a challenge to measuring the occupation fraction of central black holes with standard optical emission line, X-ray, or radio surveys.