Energy outbursts observed from supermassive black holes

The center of the Perseus galaxy cluster (Image courtesy NASA/CXC/IoA/A.Fabian et al.)
The center of the Perseus galaxy cluster (Image courtesy NASA/CXC/IoA/A.Fabian et al.)

An article published in the journal “Monthly Notices of the Royal Astronomical Society” reports the results of the study of molecular gas outflows in twelve massive galaxies at the center of galaxy clusters. A team of researchers analyzed data collected with the ALMA radio telescope to conduct this study finding that the active galactic nuclei (AGNs) of the observed galaxies cause periodic energy outbursts in the form of radiation or as jets of particles that move at speeds close to the speed of light. The energy releases cause gas outflows which can be pushed even beyond the galaxy borders. This mechanism limits the approach of the gas to the central supermassive black hole, decreasing the outbursts in what’s called a feedback. The researchers are especially studying the consequences on star formation and the possibility that a galaxy might become quenching.

For years we have known that it’s normal for galaxies to have a supermassive black hole at their center and that if it’s surrounded by a great amount of gas and dust it’s an active galactic nucleus. In this case, the black hole can heat those materials to the point that they emit significant quantities of electromagnetic radiation. One of the topics of study in recent years is the relationship between supermassive black holes, in particular in cases of active galactic nuclei, and star formation within the galaxies that host them to try to understand which conditions favor the birth of new stars and which ones inhibit it.

This new research targeted twelve massive galaxies with active galactic nuclei at the center of galaxy clusters observed with the ALMA (Atacama Large Millimeter/submillimeter Array) radio telescope, inaugurated in March 2013. When large amounts of gas and dust surround the central black hole, in addition to forming a very bright disk in what is generally an object of the type called a quasar, there are times when a lot of those materials get swallowed causing an outburst of energy. That pushes the gas outward, in some cases even out of the galaxy, limiting the amount available for star formation. This mechanism called AGN feedback is self-limiting because it prevents other materials from being swallowed until the outburst activity is over. At that point, the galaxy’s gravity can attract the gas released in the outflow, starting the cycle again.

The ALMA radio telescope has the sensitivity needed to investigate the AGN feedback, in particular by tracing structures composed of carbon monoxide gas. Most of the galaxies studied contain structures in the form of filaments that apparently form when the hot bubbles of the gas that is moving outward cools. Those outflows eventually stop and return to the galaxy.

According to the researchers there could be a trend that connects the mass of the molecular gas directly around the active galactic nucleus and the power of the jets it emits. They’re the first to express caution regarding that possible trend due to the uncertainty margins in the measurements and the small sample of galaxies examined.

Observations with a higher spatial resolution with ALMA are needed to examine the structures around the galactic nuclei and check if that trend really exists. These are complex studies, the reason why there were already several researches on the subject but without certain answers. Observations keep on being conducted and will continue also with other instruments that will enter service in the coming years to understand when a supermassive black hole favors star formation and when instead it inhibits it, sometimes to the point of making the galaxy quenching, the state prior to its death.

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