Astronomers have used, for the first time, the combination of LOFAR and WSRT-Apertif, the phased array upgrade of the Westerbork Synthesis Radio Telescope, to measure the life cycle of supermassive black holes emitting radio waves. This study, part of the LOFAR deep fields surveys, opens the possibility of timing this cycle for many objects in the sky and explore the impact it has on galaxy evolution.

Published by the editorial team, 12 January 2021

Supermassive black holes are an important component of galaxies. When in their active phase, they eject huge amounts of energy, which eventually can expel gas and matter from galaxies and impact the entire formation of new stars.

These ejections represent only a phase in the lifecycle of a supermassive black hole. They are believed to last from tens of millions to a few hundreds of millions of years, only a short moment in the life of a galaxy. After this, the supermassive black hole enters a quiet phase.  However, astronomers think that this cycle can actually repeat multiple times in which the black hole starts a new phase of ejections.  But timing this cycle is hard because the timescales involved are far too long to be directly probed: other ways to easily measure them in a large number of objects are needed.

Radio wave ejections

Some of the energy – also called ‘flux’ – is ejected by the supermassive black hole in the form of radio waves. Both radio waves at low and high frequencies are emitted and can be detected by sensitive radio telescopes like LOFAR (low frequency radio waves) and WSRT-Apertif (high frequency radio waves). “High frequency radio waves quickly lose their energy – and, as consequence, their flux – while those in the lower frequency do so much more slowly,” Prof. Dr. Raffaella Morganti, first author of the paper called The best of both worlds: Combining LOFAR and Apertif to derive resolved radio spectral index images, says.

Observing these supermassive black holes with both LOFAR and WSRT-Apertif, scientists have been able to say which supermassive black holes are, at present, ‘switched off’ and how long ago it happened. They also have identified a case where the ejection phase of the supermassive black hole has ‘recently’ restarted.

Dying supermassive black holes

In a previous study, LOFAR was used to find possible supermassive black holes in the dying or restarting phase, by taking advantage of their properties at low frequencies. In this study these same sources were surveyed also using WSRT-Apertif, and thus measuring radio waves at higher frequencies. The relative strength of the emission at these two frequencies is used to derive, to first order, how old a radio source is and whether it is already in a dying phase (see Figure 1).

Supermassive black holes
Figure 1: LOFAR and WSRT-Apertif detection of supermassive black hole radio waves. The difference in flux at which LOFAR and WSRT-Apertif detect a supermassive black hole determines if it is in its ejection phase (a) or not (b). The lower the flux of b, the longer it has been since the supermassive black hole was in its ejection phase. (Copyright: Studio Eigen Merk/ASTRON)

Morganti: “Because of our earlier studies using LOFAR, we knew the expected relative difference in flux between the lower and higher frequencies if the supermassive black holes are in the active, ejection phase. Comparing them with the, now available, Apertif data, we were able to tell, for each of them, whether the on-going activity was confirmed or whether the ejected phase had stopped.

“Interestingly, the relative number of radio galaxies found in the ‘out’ phase is also telling for how long a supermassive black hole has been ‘switched off’.  These objects are rare, therefore large surveys are necessary to collect enough data about them so that we have a large enough data size for statistical analysis.”

Great combination

With this proof of concept study Morganti and colleagues have demonstrated that a combined survey of LOFAR and WSRT-Apertif can indeed detect the phase in which a supermassive black hole currently is. Morganti: “LOFAR is unique in sensitivity and spatial resolution at the low frequencies. And while there are other radio telescopes that can observe the higher frequencies, Apertif is now covering in-depth large areas of the northern sky, instead of focusing on a single source.” That is key, because Morganti and colleagues plan to chart all detectable supermassive black holes with radio emission, in order to learn more about the birth and life cycles of galaxies.

A next step will be to create an automated way to detect these sources over much larger areas, using the large surveys that LOFAR and Apertif are doing. This is too big a job to do manually for a small group and approaches like Radio Galaxy Zoo and machine learning will be the way forward.

Combined LOFAR/WSRT-Apertif observation of supermassive black holes
Figure 2: Part of the radio sky observed by this project where many galaxies with supermassive black holes emitting radio waves can be seen. The colours give an indication of the phase in the active life of the supermassive black hole. The red colours represent emission from black holes, in the later phase, at the end of their active life. Greener colours represent black holes in their “youth”. (Copyright: ASTRON)



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