Astrophysics

Jellyfish Galaxies Are Feeding Supermassive Black Holes

Streaming cosmic tentacles of gas, dust, and stars extending for tens of thousands of light-years are helping to explain how black holes acquire matter.

There are all sorts of strange “creatures” in outer space. Astronomers study objects called blitzars and magnetars, while citizen scientists with the Galaxy Zoo project have found objects that they’ve affectionately dubbed “green peas” and “voorwerps.” Some of these objects are well understood while others are more mysterious.

One such object that has long been observed but not well understood are so-called “jellyfish galaxies,” named for their remarkably long tentacles of gas, dust, and stars that can extend for tens of thousands of light-years beyond the bodies of their galactic discs.

A new study of these galaxies suggests that their streaming tentacles are feeding supermassive black holes, and the process by which this cosmic material is fed to the black holes is a previously unknown means of black holes acquiring matter.

Astronomers say the discovery is an important clue in the long-standing puzzle to understand how galaxies form and evolve.

In their paper, published this week in Nature, the team said that while it is now well established that most, if not all, galaxies host a supermassive black hole at their center, the process for how galaxies accrete — or acquire — matter is not well understood.

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When supermassive black holes at the center of galaxies accrete matter (usually gas), they give rise to a highly energetic phenomena named Active Galactic Nuclei (AGN). The AGN is a condensed region at the center of a galaxy that can be more luminous than the light from the rest of the galaxy and is visible over most of the electromagnetic spectrum (X-Rays, ultraviolet, etc.).

They also have other characteristics that indicate the excess, and sometimes brief, luminosity is not produced by stars.

But one of the central questions about black holes is that only a small percentage are AGNs — that is, just a few are accreting matter. So how do they acquire the material to make them active?

“We want to understand why only a small fraction of supermassive black holes are active,” said Benedetta Vulcani, an astronomer from the University of Melbourne, in a statement. She was part of a team of scientists using the MUSE (Multi-Unit Spectroscopic Explorer) instrument on the Very Large Telescope to study AGNs. “Supermassive black holes are present in almost all galaxies,” she noted, “so why are only a few accreting matter and shining brightly?”

The team wrote in their paper that “a number of physical processes have been proposed to account for the funneling of gas towards the galaxy centers to feed the AGN.  There are also several physical processes that can remove (strip) gas from a galaxy, and one of them is ram pressure stripping.”

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Ram pressure stripping takes place when gas from within the galaxy is “blown off.” Galaxy clusters are filled with hot, X-ray emitting gas known as the intra-cluster medium. As individual galaxies move within a galaxy cluster, they experience this intra-cluster gas as a “wind” — like the wind you feel when you ride a bike or motorcycle. Ram pressure stripping occurs if this wind is strong enough to overcome the gravitational pull of the galaxy, and the gas contained within it is “peeled off.”

The jellyfish galaxies are one of the spectacular and beautiful examples of galaxies undergoing gas stripping by ram pressure, and so an observational program known as GASP (GAs Stripping Phenomena in galaxies with MUSE) has been specifically targeting jellyfish galaxies to study where, how and why gas can be removed from galaxies. (Their logo even includes a cute likeness of a jellyfish).

The team focused on seven jellyfish galaxies, and amazingly, six out of the seven jellyfish galaxies were found to host an active supermassive black hole at the center, feeding on the surrounding gas. They said this fraction is surprisingly high, as among galaxies in general the fraction is less than one in ten. Their research shows that the ram pressure funnels the gas towards the galaxy center, feeding the black hole.

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“This strong link between ram pressure stripping and active black holes was not predicted and has never been reported before,” said team leader Bianca Poggianti from the INAF-Astronomical Observatory of Padova in Italy. “It seems that the central black hole is being fed because some of the gas, rather than being removed, reaches the galaxy center.”

Ram pressure stripping has been studied previously, and other research found this process to have profound effects on the evolution of galaxies, but in a different way. One study showed that the gas being stripped out included the cool, denser gas that is the source of continued star formation and so the galaxies appear to be dying from a lack of new stars being formed. Another study of galaxies moving through galaxy clusters found that the ram pressure did not strip the galaxies of gas but instead compressed the gas, thereby accelerating star formation.

The research team said they will continue to study ram pressure stripping to better understand the process and its consequences. The GASP team has an ongoing study, obtaining deep, detailed data for 114 jellyfish galaxies in various environments (out of about 400 known). They hope that they can provide more pieces in the puzzle of the poorly understood connections between supermassive black holes and their host galaxies.

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