A Jellyfish galaxy is an exotic type of doomed and disrupted galaxy normally observed within clusters, which are immense structures that host anywhere from hundreds to thousands of sparkling galactic constituents. Indeed, Jellyfish have been detected inhabiting a number of galaxy clusters, and they are spectacular objects that undergo a dramatic sea-change, as they crash down through the dense core of their host cluster at supersonic speeds, displaying brilliant starbursts along a glittering tail of gas. In July 2019, a team of astronomers announced that they had observed first-hand the sinister role that an excited supermassive black hole was playing in the “murder” of an exotic Jellyfish galaxy. The supermassive heart of darkness, that lurks at the center of the Jellyfish dubbed JO201, is tearing away gas and hurling it out into space. In so doing, the black hole is speeding up the tragic quenching of baby star-birth. This means that the black hole is in the process of “murdering” the doomed exotic galaxy.
Galaxy clusters are the largest known gravitationally bound structures in the Universe. Indeed, galaxy clusters were believed to be the largest known structures in the Universe until as recently as the 1980s, when galactic superclusters were first discovered. One of the most important attributes of galaxy clusters is the intracluster medium (ICM), which is composed of heated gas floating around in the space between galaxies (intergalactic space). The ICM usually has peak temperatures between 2-15 Kelvin, and these temperatures are dependent on the total mass of the cluster. Smaller collections of galaxies are usually referred to as groups, rather than clusters. The galaxy groups and clusters can themselves join together to create superclusters.
It is commonly thought that most, if not all, large galaxies hold a voracious supermassive black hole in their hearts. These gravitational monsters can weigh-in at millions to billions of times solar-mass, and their collections of Jellyfish galaxies are characterized by ram pressure stripping off the gas from the victimized galaxy by the ICM. Ram pressure is exerted on a body traveling through a fluid medium, caused by relative bulk motion of the fluid instead of random thermal motion. It causes a drag force to be exerted on the body.
Voracious Supermassive Black Holes Feed On Jellyfish
In 2017, observations of Jellyfish galaxies with the European Southern Observatory’s (ESO’s) Very Large Telescope (VLT) revealed a previously unknown way that supermassive black holes eat dinner. The mechanism that forms the characteristic tentacles of gas and new-born stars (protostars) that inspired astronomers to give Jellyfish galaxies their nickname, also makes it possible for the gas to reach the central regions of the galaxies. In this central region, where the hungry black hole lurks in sinister secret, the infalling banquet shines brilliantly as it tumbles to its doom.
An Italian-led team of astronomers used the MUSE (Multi-Unit Spectroscopic Explorer) instrument on the VLT at ESO’s Paranal Observatory in Chile to study how gas can be torn from galaxies. The scientists focused on extreme examples of exotic Jellyfish galaxies inhabiting nearby galaxy clusters, whose “tentacles” of material extend for tens of thousands of light-years beyond their galactic discs.
The “tentacles” of Jellyfish are formed in galaxies by ram pressure stripping. Their mutual gravitational attraction causes galaxies to plummet at high speed into clusters, where they then meet up with a searing-hot, dense gas. This gas acts like a ferocious wind that forces tails of gas out of the galaxy’s disc. This powerful wind also triggers starbursts within it.
Seven Jellyfish galaxies were observed for this study, and six of them were found to host a supermassive black hole in their hearts, feeding vorciously on the surrounding gas. This fraction is surprisingly high, because among galaxies in general the fraction is less than one in ten.
“The strong link between ram pressure stripping and active black holes was not predicted and has never been reported before. It seems that the central black hole is being fed because some of the gas, rather than being removed, reaches the galaxy center,” explained team leader Dr. Bianca Poggianti in an August 16, 2017 ESO Press Release. Dr. Poggianti is of the INAF-Astronomical Observatory of Padua in Italy.
A long-standing mystery is why only a small percentage of supermassive black holes lurking in the centers of galaxies are active. Since these gravitational beasts inhabit almost all galaxies, why are only a few accreting matter and shining brightly? The result of this 2017 study is important because it reveals a previously unknown mechanism that feeds a central black hole.
“These MUSE observations suggest a novel mechanism for gas to be funnelled towards the black hole’s neighborhood. This result is important because it provides a new piece in the puzzle of the poorly understood connections between supermassive black holes and their host galaxies,” commented Dr. Yara Jaffe in the August 16, 2017 ESO Press Release. Dr. Jaffee is an ESO fellow who contributed to the paper.
These observations are part of a much more extensive investigation of many more Jellyfish galaxies that is currently in progress.
“This survey, when completed, will reveal how many, and which, gas-rich galaxies entering clusters go through a period of increased activity at their cores. A long-standing puzzle in astronomy has been to understand how galaxies form and change in our expanding and evolving Universe. Jellyfish galaxies are a key to understanding galaxy evolution as they are galaxies caught in the middle of a dramatic transformation,” Dr. Poggianti added.
The Tragic Death Of Jellyfish JO201
The tragic fate of the Jellyfish galaxy JO201 was revealed as part of the study targeting 114 Jellyfish galaxies by the GASP (GAs Stripping Phenomena) collaboration, an international team of astronomers led by Dr. Poggianti.
In order to explore the structure of the Jellyfish galaxies in 3D and calculate the timescales of their transformation, Dr. Callum Bellhouse of the University of Birmingham in the U.K. created interactive models that can also be experienced in virtual reality. Dr. Bellhouse presented the new findings at the Royal Astronomical Society’s (RAS) National Astronomy Meeting held in Lancaster, U.K. on July 3, 2019.
According to the new findings, JO201 was originally a spiral galaxy like our own pin-wheel-shaped, starlit Milky Way. However, the ill-fated JO201 has been plunging through the massive galaxy cluster named Abell 85 at supersonic speeds for approximately a billion years. As this doomed Jellyfish zips along the line of sight, its tentacles appear foreshortened in the model. However the team of astronomers estimate that the tentacles actually trail 94 parsecs behind JO201–which is approximately three times the diameter of our large Galaxy.
“A galaxy sustains itself by constantly forming new stars from gas, so understanding how gas flows into and out of a galaxy helps us learn how it evolves. The example of JO201 shows how the balance tips towards then away from star-formation as it plunges through the galaxy cluster and faces increasingly extreme stripping of its gas,” Dr. Bellhouse explained in a July 3, 2019 RAS Press Release.
JO201’s sea change from a spiral into a Jellyfish galaxy caused a short-lived increase in baby star birth, as a result of the ram-pressure stripping process. Compressed clouds of gas collapsed, and then created a ring of stars in the disk of the galaxy. Within the tentacles, dense blobs of gas condensed like rainclouds on Earth. This triggered the formation of bright new fiery baby stars in the galaxy’s wake.
Alas, over the past few hundred million years, the active and voracious supermassive black hole seems to have torn away the gas, thus leaving a large void around the center of the galaxy’s disc. The team of astronomers propose that the ram-pressure stripping may have forced gas into the central regions of the galaxy, where it caused the black hole to blast out material. This triggered a shock wave that left a tattle-tale cavity behind.
“An important balancing act occurs between processes which either boost or diminish the star formation rate in Jellyfish galaxies. In the case of JO201, the central black hole becomes excited by the ram-pressure stripping and starts to throw out gas. This means that the galaxy is being hollowed out from the inside, as well as torn away from the outside,” Dr. Bellhouse explained in the July 3, 2019 RAS Press Release.
“JO201 is, so far, a unique example of a supermassive black hole and ram-pressure stripping in quenching star formation in a Jellyfish galaxy. Studying these curious objects gives us an insight into the complex processes that galaxies experience,” Dr. Bellhouse added.