The highest angular resolution image in Astronomy reveals the insides of a galactic nucleus

The space mission RadioAstron (Russian Space Agency) has observed, along with fifteen other radio telescopes distributed around the globe, the environment of the black hole at the core of the active galaxy BL Lacertae


Since 1974, observations with very long baseline interferometry (VLBI) have combined the signals from a cosmic object received at different radio telescopes spread around the globe to synthetize an antenna with the equivalent size of the largest separation between them. This has provided unprecedented sharpness of the images, with over 1000 times better resolution than the Hubble Space Telescope can achieve in visible light. Now, an international collaboration has broken all records by combining fifteen radio telescopes on Earth and the radio dish of the RadioAstron mission (Russian Space Agency), in orbit around Earth. The work, lead by the Instituto de Astrofísica de Andalucía (IAA-CSIC), provides new insights into the nature of active galaxies, where an extremely massive black hole swallows surrounding matter while simultaneously shooting out a pair of jets of high-energy particles and magnetic fields at nearly light speed.


MPIfR/A. Lobanov.

Observations of microwave light are essential for exploring these jets, since high-energy electrons moving in magnetic fields are very proficient at producing microwaves. But most active galaxies with bright jets are billions of light years away from Earth, so their jets are tiny on the sky. High resolution is essential for viewing the jets in action to reveal phenomena like shock waves and turbulence that control how much light is produced at any given time. “Combining for the first time ground-based radio telescopes with the space radio telescope of the RadioAstron mission, operating at its maximum resolution, has allowed our team to imitate an antenna with a size of eight times the Earth’s diameter, corresponding to about twenty microarcseconds”, said José L. Gómez, the team leader at the Instituto de Astrofísica de Andalucía (IAA-CSIC).

Seen from Earth, twenty microarcseconds corresponds to the size of a two euro coin on the Moon; this high resolution probes with unprecedented detail the central regions of BL Lacertae, an active galactic nucleus located nine hundred million light-years from Earth, powered by a supermassive black hole two hundred million times more massive than our Sun.

Extreme sources

Active galactic nuclei (AGN) are the most energetic objects in the Universe, harboring a giant black hole at the center. Accretion of material toward the black hole leads to the formation of an accretion disk that tightly orbits the black hole, plus a pair of jets of particles shooting out of the nucleus in opposite directions at speeds nearly equal to that of light. “It is thought that jets originate from material drawn toward the black hole, but how the jets are collimated and accelerated is still largely unknown,” said Gómez. “We know, however, that the magnetic field should play an important role”.

Current models suggest that, due to the rotation of the black hole and accretion disk, the magnetic field lines are “twisted” into a spiral structure. Such a coiled field confines the jet to a narrow beam and accelerates its motion. This model is confirmed by the BL Lacertae observations, which reveal the existence of a large-scale spiral magnetic field in one of the jets.


Artist concept of an active galactic nuclei. Credit: Wolfgang Steffen, UNAM.

The exceptional resolution obtained with RadioAstron also reveals an unusually intensity of light at the upstream end of BL Lacertae’s jet not observed before in other AGN. This is making astronomers wonder whether their established ideas on how the jets produce microwave light is correct.

“Our current understanding of how the emission is generated in AGN establishes a clear limit on the intensity of microwaves that their cores can produce over long time spans. The extreme intensity observed in BL Lacertae exceeds that limit, requiring either velocities in the jet even closer to the speed of light than thought before or a revision of our theoretical models”, concludes Jose L. Gómez (IAA-CSIC).

  • J. L. Gómez et al. "Probing the innermost regions of AGN jets and their magnetic fields with Radioastron. I. Imaging BL Lacertae at 21 microarcsecond resolution". The Astrophysical Journal, 817, 96 (2016). DOI: 10.3847/0004-637X/817/2/96

Instituto de Astrofísica de Andalucía (IAA-CSIC)
Unidad de Divulgación y Comunicación
Silbia López de Lacalle - sll[arroba] - 958230532