Different telescopes reveal different signatures of active black holes. LOFAR (left) detects radio jets; WISE (centre) detects hot dust; SDSS/MaNGA (right) provides spatially resolved optical spectra of stars and gas. Credits: ASTRON, NASA/JPL-Caltech, SDSS, MPA
A schematic view of the proposed AGN population sequence created with the help of AI. Left: a radiative AGN with abundant gas and a prominent dusty torus, typical of star-forming hosts. Centre: a mixed AGN with compact radio emission alongside radiative signatures. Right: a radio-dominated AGN with extended jets, typical in quiescent hosts. This is a map of connected AGN-galaxy states, not a timeline for any individual galaxy. Credit: MPA/G. Jin
Host-galaxy properties change systematically from the nucleus outward for different AGN types. Each column shows one AGN class, with radial profiles of star formation (top), stellar age (middle), and gas ionisation (bottom); grey lines show reference profiles for normal galaxies. Infrared and optical AGN hosts show enhanced central star formation and younger stellar populations, while radio AGN hosts resemble quiescent galaxies at all radii. Credit: SDSS, MPA/G. Jin
Active supermassive black holes do not all look the same. Some are obscured by hot dust, some are surrounded by fast-moving ionised gas, and some can eject giant radio jets extending over millions of light-years. By mapping about two thousand such objects from their galactic centres outward, researchers at the Max Planck Institute for Astrophysics found that these variations are related to the properties of their host galaxies. Infrared and optical AGNs are found in star-forming galaxies with young centres and ionised winds, whereas radio AGNs are mostly found in older, quieter systems. A small group showing both kinds of activity sits in between.
Seeing Active Black Holes in a Different Light
From the Nucleus to the Outskirts
A Bridge Between AGN Modes
A small group of AGNs exhibits both optical/infrared and radio signatures. Several independent measurements show that these 'mixed' AGNs lie between the radiative and radio-dominated populations, providing an observational link rather than a clear boundary between the two. Their radial star-formation profiles lie between those of optical/infrared-only and radio-only AGNs. Their stacked spectra reveal a combination of strong emission lines and a significant 4000-Ångström break, suggesting ongoing black-hole accretion within an ageing stellar population. While their radio emission is present, it remains relatively compact compared with the extended jets of radio-only AGNs.
Seeing Active Black Holes in a Different Light
An active galactic nucleus (AGN) can appear in several forms, such as a hot, dusty torus that glows in infrared light; broad emission lines from gas moving close to the black hole; narrow lines from gas farther out; or a radio jet blasting away from the centre. Some of this diversity is simply due to our viewing angle. For example, an AGN seen edge-on through its dusty torus looks different from one seen face-on. However, orientation alone cannot explain everything. A long-standing question is whether these different 'faces' of AGN activity are also connected to what is happening in the surrounding galaxy, and if so, how.
From the Nucleus to the Outskirts
To find out, the team combined data on around ten thousand nearby galaxies, including nearly two thousand identified AGNs, taken from three surveys, each of which contributes a different piece of the picture. WISE, an infrared satellite, reveals hot dust. MaNGA is an integral-field spectroscopic survey that supplies optical AGN signatures and spectra at multiple positions across each galaxy. This allows the team to map properties rather than just measuring them in aggregate. LoTSS, a state-of-the-art radio survey using the LOFAR telescope, traces synchrotron emission from jets. Together, these datasets enable the researchers to trace the evolution of each galaxy from its inner regions to its outskirts.
The first differences appear in the central regions. Galaxies hosting infrared AGN exhibit the clearest boost in central star formation, forming stars at a faster rate near their nuclei than similar galaxies without AGN activity. These infrared and optical AGN hosts also have younger stellar populations at their centres, as determined by a spectral indicator of stellar age. In contrast, radio AGN hosts have old central populations and reduced central star formation, much like normal quiescent galaxies. The simultaneous presence of black-hole growth and central star formation in the same populations is consistent with both processes being fed by the same gas supply, though the data do not show one triggering the other. The gas emission and kinematics also carry the AGN's signature. Infrared, broad-line and narrow-line AGNs all exhibit stronger gas ionisation towards the centre, and this excess compared to normal galaxies extends over several kiloparsecs before fading. Fast-moving, ionised winds are strongest in infrared and broad-line AGNs, and their average signal extends to around 2 kiloparsecs from the nucleus. Radio AGNs show no comparable outflow on average. These winds clearly disturb the ionised gas around the black hole. However, the data do not show that star formation is immediately shut down by the current AGN. The link between black-hole and stellar growth is strongest near the centre and gradually weakens towards the outskirts.
The first differences appear in the central regions. Galaxies hosting infrared AGN exhibit the clearest boost in central star formation, forming stars at a faster rate near their nuclei than similar galaxies without AGN activity. These infrared and optical AGN hosts also have younger stellar populations at their centres, as determined by a spectral indicator of stellar age. In contrast, radio AGN hosts have old central populations and reduced central star formation, much like normal quiescent galaxies. The simultaneous presence of black-hole growth and central star formation in the same populations is consistent with both processes being fed by the same gas supply, though the data do not show one triggering the other. The gas emission and kinematics also carry the AGN's signature. Infrared, broad-line and narrow-line AGNs all exhibit stronger gas ionisation towards the centre, and this excess compared to normal galaxies extends over several kiloparsecs before fading. Fast-moving, ionised winds are strongest in infrared and broad-line AGNs, and their average signal extends to around 2 kiloparsecs from the nucleus. Radio AGNs show no comparable outflow on average. These winds clearly disturb the ionised gas around the black hole. However, the data do not show that star formation is immediately shut down by the current AGN. The link between black-hole and stellar growth is strongest near the centre and gradually weakens towards the outskirts.
A Bridge Between AGN Modes
A small group of AGNs exhibits both optical/infrared and radio signatures. Several independent measurements show that these 'mixed' AGNs lie between the radiative and radio-dominated populations, providing an observational link rather than a clear boundary between the two. Their radial star-formation profiles lie between those of optical/infrared-only and radio-only AGNs. Their stacked spectra reveal a combination of strong emission lines and a significant 4000-Ångström break, suggesting ongoing black-hole accretion within an ageing stellar population. While their radio emission is present, it remains relatively compact compared with the extended jets of radio-only AGNs.
What's missing?
Several observations could clarify this picture. Molecular gas from CO observations would reveal the fuel reservoir directly. X-ray observations could reveal the hot atmospheres thought to exist around radio-mode AGNs. Higher-resolution radio images would clarify whether the compact jets in mixed AGNs are younger. Comparing these with spatially resolved simulations would reveal whether the observed states arise from one evolutionary route or several. Ultimately, these observations will reveal whether the empirical map uncovered here reflects a common evolutionary route, repeated cycles or multiple paths through black hole and galaxy growth.
Authors:
Jin Gaoxiang
PhD student
Tel: 2298
Email: gxjin@mpa-garching.mpg.de
Guinevere Kauffmann
Director
Tel: 2013
Email: gamk@mpa-garching.mpg.de
Original publication
Jin et al.
'A spatially resolved evolutionary sequence of multi-wavelength AGN host galaxies
Monthly Notices of the Royal Astronomical Society, Volume 546, Issue 4
DOI
