In the heart of huge galaxies like our own, you’ll find a supermassive black hole (SMBH). They maintain their commanding position in the center of the galaxy, where they consume gas, dust, stars, and everything else that comes too close, causing them to become ever more massive over time. However, in extremely unusual cases, an SMBH can be expelled from its orbit and sent hurtling across space.
The new study!
A new publication by Canadian, Australian, and American scientists shows evidence of a rogue Supermassive Black Hole racing through space and interacting with the circumgalactic medium (CGM.) In the process of moving, the giant is generating shock waves and firing off the birth of stars.
The title of the study is “A candidate runaway supermassive black hole identified by shocks and star formation in its wake.” Professor of Astronomy and Physics at Yale University Pieter van Dokkum is the paper’s primary author. Currently, there has been no round of peer review for this research.
Runaway SMBH!
You may now be the only one who has never heard of a runaway Supermassive Black Hole. Most SMBHs live in the galactic core. SMBHs can leave their galaxies under certain conditions, according to experts. This paper describes how a supermassive black hole (SMBH) can be expelled from its galaxy.
When galaxies collide, it always begins. Because of this, a binary SMBH forms at the core of the merger remnant. The binary SMBH can live for up to a billion years before collapsing. At that point, if a third SMBH has made it to the galactic core, a three-body collision could give one of the SMBHs a velocity boost, propelling it out of the galaxy.
However, locating these rogue SMBHs is challenging despite their theoretical underpinnings. In 2021, at a distance of roughly 230 million light-years, astronomers discovered one of the best candidates. The authors witnessed unusual activity and speeds, which they interpreted as signs of recent upheaval. A galaxy merger, a binary black hole system, or a gravitational-wave recoil event? They couldn’t tell.
Spotting a rogue Supermassive Black Hole!
It is possible to spot a rogue SMBH in a couple of ways, as astronomers are well aware. The hole is actively swallowing matter like an active galactic nucleus. And its brightness is very useful as a telltale sign of its presence. The authors state, “For such objects, “the presence of an SMBH is not in doubt, but it can be difficult to determine whether they are ‘naked’ black holes or the nuclei of merging galaxies.”
Another way is the rogue hole’s stellar mass. As it ejects, an SMBH pulls some stars along. Without an AGN, the dimness makes it hard to distinguish the hole and its star partners from afar.
Scientists may be able to discover a rogue SMBH by observing the diffuse gas in the circumgalactic medium (CGM) as it passes through.
The author of the paper wrote: “The interaction of a runaway supermassive black hole with the CGM can lead to the formation of a wake of shocked gas and young stars behind it,”. In their paper, they talk about how they accidentally found a line in Hubble’s Advanced Camera for Surveys pictures that could be one of these wakes.
An SMBH’s passage through the ionized hydrogen of the CGM results in a shock front and a lengthy wake. Shock clouds of gas in the wake can cool and condense into stars, leaving behind a path that looks like a series of knots. They determined the ages and metallicities of three of the knots in the linear feature.
Theory and mathematical method!
Theoretical and numerical methods suggest that the wake’s youngest stars shouldn’t be older than 30 million years old. The three knots have traits consistent with ages in the range by the researcher’s predictions. They have similar metal and dust densities.
If an SMBH was expelled from the host galaxy, the galaxy should display symptoms of disturbance. Supermassive black holes (SMBHs) are so massive that they shape the galaxies they pass through. The team discovered that the galaxy that gave birth to the rogue SMBH had an abnormal shape.
The linear feature could also be explained in another way, though. It could be a jet from a black hole instead of a rogue Supermassive Black Hole. Black hole jets can also shock gas in the CGM and cause stars to form if the conditions are right. The authors say, “There are two well-studied nearby examples of jets triggering star formation,”. One of them is called Minkowski’s Object.
A black hole jet is a likely reason, but it has many issues!
The authors agree that the jet from a black hole is a likely reason, but they tell there are numerous issues with it. Jets from a black hole don’t have visible emission lines, and there is no sign of nuclear activity. There is also a problem with morphology.“A more serious issue is that the morphology of the feature does not match simulations or observations of jet-induced star formation,” the authors say.
Moreover, they write: “The line ratios, colors, and the overall morphology are consistent with an ejected SMBH moving through the CGM at high speed while triggering star formation,”.
Finding the black holes would be proof beyond a doubt that this is true. The researchers write, “The ‘smoking gun’ evidence for this scenario would be the unambiguous identification of the black holes themselves,” Moreover, they explain. “The obvious places to look for them are A and B in Fig. 6.”
Do rogue SMBHs exist?
It’s not surprising that these observations’ lasting characteristic has an uncommon explanation. The JWST can provide more observations to determine if rogue SMBHs are at work.
“Deeper data, for instance, from the JWST NIRSPEC IFU, may show the expected broad, highly red- or blueshifted emission lines of ionized gas that is bound to the black holes themselves. Those data could also spatially resolve flows, shocks, and star formation near A,” the authors say, looking ahead.
And if one Supermassive Black Hole gets out of control and makes one of these features, there will be more.
The authors state: “Looking ahead, the morphology of the feature in the HST images is so striking that it should not be too difficult to find more examples if they exist. Future data from the Nancy Grace Roman telescope can be searched with automated algorithms,”.
A rogue Supermassive Black Hole that has broken away from its galaxy and is wandering through the circumgalactic medium is an interesting thing to see. We’re just starting to learn about SMBHs, how they affect galaxy growth, and how they come together to make gravitational waves.
