How Compact are the Cores of AGN? Sub-Parsec Scale Imaging with VLBI at Millimeter Wavelengths

TL;DR

This paper presents high-resolution mm-VLBI imaging of AGN cores, revealing structures down to a few Schwarzschild radii, and discusses future prospects for even more detailed imaging of black hole environments.

Contribution

It introduces a new 86 GHz VLBI survey of compact radio sources and demonstrates the potential of mm-VLBI for imaging black hole regions at unprecedented resolutions.

Findings

VLBI at 86 GHz achieves micro-arcsecond resolution.

The core of M87 is smaller than 15 Schwarzschild radii.

Future observations at 1 mm could resolve regions a few Schwarzschild radii across.

Abstract

We study the most central regions of AGN jets with an angular resolution of tens of micro-arcseconds using VLBI at millimeter wavelengths (mm-VLBI). We present and discuss a new 86 GHz VLBI survey of compact radio sources. We show new high dynamic range images of two nearby radio galaxies (3C 120 and M87). In M87 the size of the compact VLBI core (the jet base) is < 15 Schwarzschild radii. Future mm-VLBI observations at 1 mm and shorter wavelengths should lead to images of galactic and extragalactic radio sources with a spatial resolution down to a few Schwarzschild radii of the central super massive black holes. To achieve this, the participation of large and sensitive millimeter and sub-millimeter telescopes in VLBI is essential. Owing to their high sensitivity existing and planned phased interferometers like the IRAM Plateau de Bure interferometer, CARMA, the SMA and ALMA could play an important role.