The Silent Majority - Jets and Radio Cores from Low-Luminosity Black Holes

Abstract

They are weak, they are small, and they are often overlooked, but they are numerous and an ubiquitous sign of accreting black holes: compact radio cores and jets in low-power AGN. Here I summarize our work concerning these radio cores and jets in recent years, specifically focusing on the large population of low-luminosity AGN. Special attention is also given to Sgr A*, the supermassive black hole candidate at the Galactic Center, whose radio properties are reviewed in more detail. This source exhibits a submm-bump, possibly from an ultra-compact region around the black hole which should allow imaging of the event horizon of the black hole in the not too distant future. A jet model is proposed which explains the basic feature of Sgr A*: its slightly inverted radio spectrum, the submm-bump, the lack of extended emission, and the X-ray emission. This model also works for famous sources like M81*, NGC4258, or GRS1915+105 based on the argument that radio cores are jets whose emission can be scaled with the accretion power over many orders of magnitude. This scaling is corroborated by the detection of many Sgr A*-like radio cores in nearby Low-Luminosity AGN (LLAGN), some of which show jet structures on the VLBI (Very Long Baseline Interferometry) scale. These cores confirm an AGN origin of about half of the known low-luminosity AGN classified as LINERs and dwarf-Seyferts. It is argued that in fact most of the compact radio emission at centimeter waves in LLAGN is produced by a compact radio jet and not an Advection Dominated Accretion Flow (ADAF). In general one can say that compact radio cores are a genuine feature of AGN, allowing one to precisely pinpoint black holes in many galaxies - not only in luminous quasars.