Radio Continuum Observations of the Candidate Supermassive Black Hole in the Dwarf Elliptical VCC 128

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 VCC 128 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. estimated its mass as M-center dot similar to 6 x 10(6)-5 x 10(7) M-circle dot. 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 is the best option here. We have tried to detect the X-band radio emission coming from the putative black hole in VCC 128 when it accretes gas from the surrounding ISM. While we made a positive 4 sigma detection of a point source 4.63 '' southwest of the binary nucleus, no statistically significant evidence for emission associated with the nuclei themselves was detected. This implies either that VCC 128 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 difficult to detect at radio wavelengths.