Tag Archives: active galactic nuclei

This is an artist's impression of planets orbiting a supermassive black hole. (Kagoshima University)

Planets could orbit Supermassive Black Holes

This is an artist's impression of planets orbiting a supermassive black hole. (Kagoshima University)
This is an artist’s impression of planets orbiting a supermassive black hole. (Kagoshima University)

The idea of stars orbiting the supermassive black holes that researchers believe lurk at the centre of most galaxies has been long established as a matter of fact in science. In ‘active galactic nuclei’ or AGNs, these black holes are surrounded by haloes of gas and dust in a violent churning environment. Such clouds of gas and dust have the potential to birth not only stars but planets as well. Yet, the question of whether planets can also orbit these spacetime events has yet to be established. 

Enter Keiichi Wada, a professor at Kagoshima University, and Eiichiro Kokubo, a professor at the National Astronomical Observatory of Japan. These scientists from the distinct fields of active galactic nuclei research and planet formation research respectively have calculated that as a result of gas disc growth, an entirely new class of planets may form around supermassive black holes. 

“With the right conditions, planets could be formed even in harsh environments, such as around a black hole,” Wada points out. 

In their research published in the Astrophysical Journal, the duo of theoreticians propose that protoplanetary discs that surround young stars may not be the only potential site for planet formation. The researchers instead focused calculations and mathematical models on the denser dust discs found around supermassive black holes in AGNs, thus arriving at a surprising conclusion. 

“Our calculations show that tens of thousands of planets with 10 times the mass of the Earth could be formed [at a distance of] around 10 light-years from a black hole,” says Eiichiro Kokubo. 

“Around black holes, there might exist planetary systems of astonishing scale.”

One of the hindrances to the formation of planets in such discs of dust has previously been the amount of energy generated in AGNs, Researchers had believed that this energy output would prevent the coagulation of ‘fluffy ice dust’ that can help the growth of dust grains that can lead to planet formation in protoplanetary discs.

But, what Wada and Kokubo discovered was that the huge density of dust discs around supermassive black holes in AGNs —potentially containing as much as a hundred thousand times the mass of the Sun worth of dust, which is a billion times more massive than a typical protoplanetary disc — helps protect the outer layers from bombardment from high-energy radiation such as gamma rays. 

 A schematic picture of the Active Galactic Nucleus (AGN) and the circumnuclear disc. (Wada, Kokubo, 2019)
A schematic picture of the Active Galactic Nucleus (AGN) and the circumnuclear disc. (Wada, Kokubo, 2019)

This helps form a low-temperature region similar to that found in protoplanetary discs, and thus, in turn, increases the likelihood of fluffy deposits building.

The process would lead to the formation of planets within a period of several hundred million years, according to the pair, and also result in much denser and more populated collections of planets. 

Unfortunately, the limits of current methods of identifying exoplanets would make identifying planets around a supermassive black hole challenging to say the least. 

“ Doppler spectroscopy, transit photometry, gravitational micro-lensing, or direct imaging are hopeless,” warn the duo in their paper. They go on to suggest that a method called photometry with an x-ray interferometer located in space could be a possible solution — if a way of distinguishing the effect caused by such planets from the natural variability of the AGN can be developed. 

For now, researchers will have to look to mathematical models alone to theorise about the potential for planets in orbit around black holes. 


Original research: https://arxiv.org/pdf/1909.06748.pdf

NGC1569 is a star-forming galaxy. Galaxies such as this could see their star formation rates affected by strong winds emanating from a central black hole. (HST/NASA/ESA)

Black Holes could stunt the growth of dwarf galaxies

NGC1569 is a star-forming galaxy. Galaxies such as this could see their star formation rates affected by strong winds emanating from a central black hole. (HST/NASA/ESA)
NGC1569 is a star-forming galaxy. Galaxies such as this could see their star formation rates affected by strong winds emanating from a central black hole. (HST/NASA/ESA)

Black holes at the centre of small dwarf galaxies could slow or even halt the formation of stars via the powerful winds they produce, researchers from University of California, Riverside, have discovered. This suppression of star-formation could have a marked influence on the evolution of such galaxies.

The result seems to confirm the long-held suspicion that supermassive black holes at the centre of galaxies can influence that galaxy’s evolution — including how they grow and the way that they age. But, the research also delivers a surprise; the winds that the astronomers measured coming from the black hole were more powerful than the team reckoned for. This means that models of star formation in dwarf galaxies may require a rethink.

“We expected we would need observations with much higher resolution and sensitivity, and we had planned on obtaining these as a follow-up to our initial observations,” said Gabriela Canalizo, a professor of physics and astronomy at UC Riverside who led the research team. “But we could see the signs strongly and clearly in the initial observations.

“The winds were stronger than we had anticipated.”

Gabriela Canalizo

Thus meaning that black holes don’t just influence the development of larger galaxies, but also play a role in the evolution of smaller dwarf galaxies — galaxies containing anywhere from a few thousand to a few billion stars.

Canalizo continues: “Our findings now indicate that their effect can be just as dramatic, if not more dramatic, in dwarf galaxies in the universe.”

The study — the results of which are discussed in the Astrophysical Journal — used data collected in the Sloan Digital Sky Survey (SDSS), a project which maps 35% of the sky above Earth. In doing so, the survey has been able to identify 50 dwarf galaxies — 29 of which demonstrated clear characteristics of possessing black holes at their centres. A further six of these showed evidence of high-velocity outflows of ionised gas — the powerful winds in question.

The next step for the researchers was to use the Keck telescopes — based in Hawaii — to both detect and measure the properties of these winds, marking the first time this has been achieved.

Discussing what her team found, Canalizo adds: “We found some evidence that these winds may be changing the rate at which the galaxies are able to form stars.”

Studying dwarf galaxies could be the key to understanding how galaxies in general evolve

The study of these smaller galaxies could help scientists answer lingering about galactic evolution in general.

“Larger galaxies often form when dwarf galaxies merge together,” explains Christina Manzano-King, a doctoral student in Canalizo’s lab and the first author of the paper. As a consequence of this, she continues, dwarf galaxies are particularly useful in understanding how galaxies evolve.

Dwarf galaxies hosting active galactic nuclei that have spatially extended outflows. (SDSS)
Dwarf galaxies hosting active galactic nuclei that have spatially extended outflows. (SDSS)

“Dwarf galaxies are small because after they formed, they somehow avoided merging with other galaxies,” she adds. “Thus, they serve as fossils by revealing what the environment of the early universe was like.

“Dwarf galaxies are the smallest galaxies in which we are directly seeing winds — gas flows up to 1,000 kilometres per second — for the first time.”

Christina Manzano-King

Explaining what causes these powerful winds, Manzano-KIng points to material being fed into the black hole. This material — usually gas and dust — forms an accretion disc around the black hole. In this disc — which gradually feeds the black hole — conditions are so violent that friction and tremendous tidal forces heats the material. This releases radiative energy which shoves gas out of the galaxy’s centre and into intergalactic space.

This negatively affects the amount of gas available for star formation.

Manzano-King continues: “What’s interesting is that these winds are being pushed out by active black holes in the six dwarf galaxies rather than by stellar processes such as supernovae.

“Typically, winds driven by stellar processes are common in dwarf galaxies and constitute the dominant process for regulating the amount of gas available in dwarf galaxies for forming stars.”

Astronomers believe that winds emanating from black holes can compress gas and thus aid the gravitational collapse of gas clouds, kick-starting star-formation. But, if the wind is too strong and thus expels gas from the galaxy’s centre, rather than aiding the star formation process, gas becomes unavailable and hinders the process.

This is exactly what appears to be happening in the six galaxies that the team’s research highlighted. In these cases, the wind has had a clear detrimental impact on star formation rates.

Rethinking the relationship between black holes and star formation rates

This research may result in a rethinking of models of star formation and the evolution of galaxies. Current models do not take into account the impact of black holes in dwarf galaxies.

From left to right: Laura Sales, Christina Manzano-King, and Gabriela Canalizo. The team’s research could force a rethinking of star formation rates in dwarf galaxies ( Stan Lim, UC Riverside)
From left to right: Laura Sales, Christina Manzano-King, and Gabriela Canalizo. The team’s research could force a rethinking of star formation rates in dwarf galaxies ( Stan Lim, UC Riverside)

“Our findings show that galaxy formation models must include black holes as important, if not dominant, regulators of star formation in dwarf galaxies,” points out Laura V. Sales, assistant professor of physics and astronomy at UC Riverside.

As for the future of this research, the team next plans to investigate characteristics of gas outflows such as mass and momentum.

“This would better inform theorists who rely on such data to build models,” concludes Manzano-King. “These models, in turn, teach observational astronomers just how the winds affect dwarf galaxies.

“We also plan to do a systematic search in a larger sample of the Sloan Digital Sky Survey to identify dwarf galaxies with outflows originating in active black holes.”


Original research: ‘AGN-Driven Outflows in Dwarf Galaxies’ Christina M. Manzano-King, Gabriela Canalizo, and Laura V. Sales.

Science ABC – galactic edition: Seyfert galaxies

This is the first post in a series about types of galaxies; today, we’ll be finding out more about Seyfert galaxies.

NGC 1097 is a Seyfert galaxy. Lurking at the very centre of the galaxy, a supermassive black hole 100 million times the mass of our Sun is gradually sucking in the matter around it. Via Wikipedia.

NGC 1097 is a Seyfert galaxy. Lurking at the very centre of the galaxy, a supermassive black hole 100 million times the mass of our Sun is gradually sucking in the matter around it. Via Wikipedia.

This galactic type has been described for the first time by Carl K. Seyfert in 1943, who noted that their central regions have peculiar spectra with notable emission lines. Basically, what makes them special is the brightness nuclei.

Many galaxies, including spiral ones like the Milky Way have emission nebulae in their spiral arms, due to their stars. However, Seyfert galaxies have very bright nuclei which show large quantities of gas which is not associated with O or B stars. Their nuclei are called Active Galactic Nuclei (AGN’s); they make out the biggest portion of AGN galaxies, but not the only one.

The galaxies have supermassive black holes with masses between 107 and 108 solar masses. It’s still not clear if the emission lines originate in the accretion disk around the black hole or from clouds of gas illuminated by the central engine in an ionization cone.

However, each part of the accretion disk has a different velocity relative to our line of sight, and the faster the gas is rotating around the black hole, the broader the line will be. Similarly, an illuminated disc wind also has a position-dependent velocity. In 1965, Donald E. Osterbrook came up with a very creative theory, that the active nuclei of Seyfert galaxies might be thought of as miniature quasars (quasars had just been discovered in 1963). Since then, this idea hasn’t gained a lot of popularity, but hasn’t been refuted either.

Initially, they were classified as type 1 and type 1 Seyfert galaxies, but since then, it was shown that Type 1 and Type 2 galaxies are in essence the same, and they only differ due to the angle at which they are observed. Osterbrook (1981) has introduced the following subclasses of Seyfert 1 galaxies, based on spectroscopic details: 1.5, 1.8 and 1.9.

After decades of research on not only Seyfert galaxies, but also quasars, supermassive black holes, and many more, astrophysicists believe that all forms of AGNs are caused by the same sort of physical object, a central supermassive object accumulating more and more mass aruond it. The visual differences are caused by the different viewing angles and different rates of matter supply falling into the objects.

NGC 1542

NGC 1542 via High Energy Astrophysics Group