Active Galactic Nuclei

Active Galactic Nuclei


At the center of most galaxies lies a supermassive black hole that swallows material and in the process produces the brightest radiation in the universe across multiple wavelengths. These black holes  are called active galactic nuclei (AGN) and the galaxies in which they reside are known as “active galaxies”.

Radio AGNs are a particularly important group of AGNs characterised by the emission of plasma jets that is believed to play a key role in galaxy evolution. The structure and life-cycle of radio AGNs, and the way they interact with the surrounding medium provide clues about the evolution of their host galaxies.

Astronomers at ASTRON  use LOFAR  and other radio facilities to study active galaxies on all morphological scales at frequencies covering the whole radio band. The research in radio is also complemented with observations at optical and X-ray frequencies. 

Research staff:  Raffaella Morganti, Tom Oosterloo, Robert Schulz, Joe Callingham, Alexander Kutkin

PhD students: Nika Jurlin, Suma Murthy

To derive the age of the radio emission from AGNs we combining spectral observations at low and high radio frequencies. At the same time, the large field of view of LOFAR allows us to find and study elusive objects like remnants and restarted radio galaxies. Our group also uses the LoTSS survey  to study radio galaxies that are just born. These galaxies are interesting because we can use them to investigate the initial stages of jet evolution when the outflow of gas is strongly affected by the surrounding dense medium. How many young AGNs survive these violent  initial interactions with the environment and continue to grow is still an open question in astronomy. 

To get closer to the answer, we look at observations of

  • 21 cm absorption line of neutral hydrogen (HI) at radio wavelengths (using single dish telescopes or interferometers  such as the VLBI) 
  • molecular gas (using ALMA)
  • warm ionised gas (using the instruments at the ESO-VLT).

When a jet interacts with the surrounding media, it creates an outflow of neutral hydrogen that can be traced and studied with radio observation. In particular, the frequency-dependent opacity effects in AGN jets provide information about their kinematics, geometry, magnetic fields and more.

Our group also uses RadioAstron - a space VLBI mission led by the Levedev Physics Institute in Moscow to probe the innermost regions of AGN jets at very high resolution (pc-scales).