While wanting to observe a dwarf galaxy, researchers came across a strange phenomenon: a line of stars in the process of formation, extending over nearly 200,000 light years. The culprit has finally been identified: it is a wandering supermassive black hole, which was ejected from its galaxy!
At the center of most large galaxies is a supermassive black hole. The Milky Way is no exception, with its recently imaged Sagittarius A*. If most of the time, these giant stars remain at the heart of their galaxy, it happens that on very rare occasions they are ejected from it, and flee into the circumgalactic environment, a structure that surrounds the galaxy. This type of event requires a large amount of energy, and likewise releases a lot of it. Until today, no phenomenon of this type had been observed: wandering stellar black holes, yes, but never supermassive ones.
That’s why researchers were shocked when they discovered a strange straight line in Hubble Space Telescope readings as they searched for images of the nearby dwarf galaxy RCP 28. They ended up identifying the unexpected culprit: a supermassive black hole on the run, initiating the formation of stars in its path! They describe their results in a study accepted into Astrophysical Journal Letters and pre-published on ArXiv.
A strange burst of light indicates the presence of a supermassive black hole
It all started with observations via the Hubble telescope of the dwarf galaxy RCP 28 in September 2022. The researchers accidentally unearthed a “ thin, linear feature that spans 62 kpc (1 kiloparsec = 3261.56 light years, so 62 kpc is regarding 200,000 light years) of the nucleus of a compact galaxy “, they describe. Without a known analogue, it actually corresponds to very recently formed and aligned stars! The team first assumed a galactic origin. Or rather, a supermassive black hole at the center of its galaxy.
Such phenomena occur regularly, and come from the accretion disk of the giant black hole. The matter attracted by the star swirls around it at high speed, to the point where it ionizes and passes into the form of plasma. Extreme temperature and pressure conditions prevail within the disk, and sometimes magnetohydrodynamic interactions cause the high-speed emission of ionized matter. And these jets are capable of causing the birth of stars!
Indeed, during their passage, they compress the matter they cross, sometimes triggering the gravitational collapse of a molecular cloud and thus initiating the process of stellar accretion. But, various elements contradict the hypothesis of a black hole jet. According to the theory, the strongest interactions should occur closest to the home galaxy, and then decay. And their observations do not agree: the researchers noted the presence of very bright points far from the galactic center!
Moreover, the morphology of the feature does not match that of a supermassive black hole jet: the line should be more and more dispersed as one moves away from the origin. And it is not. The only possible conclusion according to the study: the feature is the wake of a runaway supermassive black hole”. Or rather on the run, ejected from its host galaxy.
A merger of three galaxies in the past
How it got there remains to be seen. According to the researchers, “The first step is always a galaxy merger, which leads to the formation of a binary supermassive black hole at the center of the fusion remnant.” The two monsters can thus merge, without one being ejected. But if a third galaxy is added, that’s where it gets complicated. If the third black hole arrives before the other two have completed their merger, then “a three-body interaction may impart great velocity to one of the supermassive black holes leading to its escape from the galactic core”, describes the study.
They estimated a time since the ejection of 39 million years, and calculated the current speed of the isolated black hole at nearly 1,600 km/s! The study also focused on the supermassive binary remaining following the interaction, for which they estimate a speed of 900 km/s. Indeed, two hyper-compact star systems have been detected on either side of the line of nascent stars, corresponding to dense clusters of stars around supermassive black holes. Confirming the scenario described below.
