Radio continuum observations of the candidate supermassive black hole in the dwarf elliptical VCC128

TL;DR

This study investigates the presence of a supermassive black hole in the dwarf elliptical galaxy VCC128 through radio observations, finding no definitive emission from the black hole but detecting a nearby radio source, raising questions about the black hole's activity or existence.

Contribution

First radio continuum attempt to detect a black hole in VCC128, providing constraints on its activity and challenging previous binary nucleus interpretation.

Findings

No significant radio emission from the nucleus, suggesting a quiescent black hole or absence of one.

Detection of a nearby radio source offset from the nucleus.

Implication that accretion conditions may prevent detectable emission.

Abstract

The presence of black holes (BHs) at the centers of dwarf elliptical galaxies (dEs) has been argued both theoretically and observationally. Using archival HST/WFPC2 data, we found the Virgo cluster dwarf elliptical galaxy VCC128 to harbor a binary nucleus, a feature that is usually interpreted as the observable signature of a stellar disk orbiting a central massive black hole. Debattista et al. 2006 estimated its mass M sim 6 10^6 - 5 10^7 Msun. One of the most robust means of verifying the existence of a BH is radio continuum and/or X-ray emission, however because of the deficiency of gas in dEs, radio continuum emission forms the best option here. We have tried to detect the X-band radio emission coming from the putative black hole in VCC128 when it accretes gas from the surrounding ISM. While we made a positive 4 sigma detection of a point source 4.63'' south-west of the binary nucleus, no statistically significant evidence for emission associated with the nuclei themselves was detected. This implies either that VCC128 has no massive central black hole, which makes the nature of the binary nucleus hard to explain, or, if it has a central black hole, that the physical conditions of the ISM (predominantly its density and temperature) and/or of the surrounding accretion disk do not allow for efficient gas accretion onto the black hole, making the quiescent black hole very hard to detect at radio wavelengths.