The Intermediate Mass Black Hole Candidate in the Center of NGC 404: New Evidence from Radio Continuum Observations

TL;DR

This study provides new radio evidence supporting the presence of an intermediate mass black hole in NGC 404, using high-resolution EVLA observations to detect and analyze radio emission at 5 GHz.

Contribution

First detection of radio emission at frequencies above 1.4 GHz from NGC 404's nucleus, supporting the intermediate mass black hole hypothesis with high-resolution data.

Findings

Detected radio emission at 5 GHz in NGC 404 nucleus

Radio source position aligns with galaxy center and X-ray source

Most likely an accreting intermediate mass black hole

Abstract

We present the results of deep, high-resolution, 5 GHz Expanded Very Large Array (EVLA) observations of the nearby, dwarf lenticular galaxy and intermediate mass black hole candidate (M ~4.5 x 10^5 M_sun), NGC 404. For the first time, radio emission at frequencies above 1.4 GHz has been detected in this galaxy. We found a modestly resolved source in the NGC 404 nucleus with a total radio luminosity of 7.6 +/- 0.7 x 10^17 W/Hz at 5 GHz and a spectral index from 5 to 7.45 GHz of alpha = -0.88 +/- 0.30. NGC 404 is only the third central intermediate mass black hole candidate detected in the radio regime with subarcsecond resolution. The position of the radio source is consistent with the optical center of the galaxy and the location of a known, hard X-ray point source (Lx ~1.2 x 10^37 erg/s). The faint radio and X-ray emission could conceivably be produced by an X-ray binary, star formation, a supernova remnant or a low-luminosity AGN powered by an intermediate mass black hole. In light of our new EVLA observations, we find that the most likely scenario is an accreting intermediate mass black hole, with other explanations incompatible with the observed X-ray and/or radio luminosities or statistically unlikely.