It is a new discovery by serendipity which has just been made by astrophysicists, in this case that of a trail of stars behind a supermassive black hole ejected from a galaxy. It is the movement of this compact star through the intergalactic medium which caused this formation of stars and it might be the emerged point of an iceberg flushed out by the observations of Hubble.
There have never been so many active astronomers and so many instruments on Earth or in space to make astronomical discoveries but, remarkably, more and more of these discoveries are made not by making new observations but also by analyzing those already made and which are archived.
Recently, Pieter van Dokkum of Yale University in New Haven, Connecticut in the United States, well known for his work on the enigma of ultra-diffuse galaxies, had undertaken with colleagues to delve into the memory strata Hubble search for globular clusters surrounding a dwarf galaxy. They then discovered an astonishing phenomenon which led to a publication in The Astrophysical Journal Letters and which can be consulted freely at arXiv.
« We stumbled upon it by chance.explains Van Dokkum in a press release from NASA. I had just browsed a Hubble image and then noticed that we had a small linear structure. I immediately thought oh, a cosmic ray hitting the CCD camera detector and causing a linear imaging artifact. When we cleared the cosmic rays, we realized she was still there. It was unlike anything we had seen before. »
A triple collision of galaxies?
To find out more, Pieter van Dokkum and his team decided to take a closer look, this time taking spectroscopic measurements with instruments from the WM Keck Observatory in Hawaii. Researchers had to face the facts, they had just discovered a trail of young stars behind a supermassive black hole ejected from a nearby galaxy so quickly it might travel the distance from Earth to the Moon in just fourteen minutes .
The measurements made now allow to say that the black hole contains regarding 20 million solar masses (that of the Milky Way just over 4 million) and the trail of stars behind it extends over regarding 200,000 light years, almost twice the diameter of our Galaxy.
Astrophysicists suggest the following scenario to explain the phenomenon. Perhaps less than 50 million years ago, a galactic collision occurred leading two supermassive black holes to sediment in the heart of the galaxy newly formed by fusion of the two previous ones (recall that all large galaxies seem to have a supermassive black hole at their core and that triple, albeit rare, collisions of these galaxies are observed).
Before the black holes merged losing energy in the form of gravitational waves, a third merger took place leading the first binary black hole to enter into violent gravitational interaction with the newcomer third supermassive black hole. The gravitational forces would then have ejected at a notable fraction of the speed of light the last black hole and perhaps also the binary black hole because a second linear structure behind the galaxy associated with the first can be guessed and it would extend into the opposite direction to the first.
This would be consistent with the conservation of momentum and especially the fact that there does not appear to be any evidence in the X-ray domains of supermassive black holes at the center of the associated galaxy. Observations in the future with Chandra, but also with the James-Webb, should tell us more.
Intergalactic gas compressed by shock waves
The trail of young stars observed would be due to the fact that by sinking into the intergalactic medium like a boat leaving a wake, the supermassive black hole would have produced shock waves, equivalent to bow waves. By compressing the intergalactic gas, they would have initiated the gravitational collapse progenitor of new stars.
In fact, nomadic supermassive black holes have already been known for some time and as Futura explained in a previous article, perhaps 10% of all giant black holes are wandering between galaxies.
Still, the phenomenon of creating stars according to a linear trail is indeed new…
A presentation of the discovery made with Hubble. To obtain a fairly accurate French translation, click on the white rectangle at the bottom right. The English subtitles should then appear. Then click on the nut to the right of the rectangle, then on “Subtitles” and finally on “Translate automatically”. Choose “French”. © NASA Goddard
