According to researchers from Canada, Australia and the United States (CGM), there is evidence that a supermassive black hole is tearing through space and interacting with the circumgalactic medium. The giant also creates shock waves and starts star formation.
Identifying SMBHs on runways is obviously important, but difficult. There are several ways a supermassive black hole (SMBH) can escape from the center of a galaxy. The first step is always a galaxy merger, forming a binary SMBH. If a third SMBH reaches the center of the galaxy before the binary merges, a three-body interaction could give great velocity to one of the SMBHs, leading to its escape from the core.
However, despite their theoretical underpinnings, locating these rogue SMBHs is challenging. In 2021, astronomers discovered one of the leading contenders at a distance of 230 million light-years.
The authors saw unusual movement and speed, indicating a recent glitch. However, they could not determine whether they had witnessed a gravitational wave backlash event, a binary black hole system, or an ongoing galaxy merger.
There are a few methods by which astronomers can spot a runaway SMBH. The simplest method is if the hole can be recognized by its brightness as an active galactic core absorbing the material.
Scientists noted, “For such objects, there is no doubt about the presence of an SMBH, but it can be difficult to determine whether they are ‘naked’ black holes or the cores of merging galaxies.”
The star mass pulling the rogue hole behind it is another way. When an SMBH is ejected, some stars are pulled along by its strong gravity. Still, the lack of the brightness of an AGN makes it challenging to distinguish the hole and its companions at a distance.
The impact of a suspected SMBH on diffused gas in the circumgalactic medium (CGM) as it passes through it is a third technique for astronomers to identify it.
Scientists noted, “The interaction of a runaway supermassive black hole with the CGM could lead to the formation of a wake of shocked gas and young stars behind it. Their paper reports the chance discovery of a linear feature in images from Hubble’s Advanced Camera for Surveys that could be one of these wakes.
An SMBH creates a shock front and significant wake as it passes through ionized hydrogen in the CGM. Clouds of shocked gas can cool and form stars that resemble knots in the path in the aftermath. Three of the linear feature’s nodes were examined, and the scientists determined their age and metallicity.
According to theory and simulation, the youngest stars formed in the wake would be less than 30 million years old. The three knots fall within the age range suggested by models, according to measurements of their attributes by the researchers. In addition, their metallicity and dust content are within the range.
If the receiving galaxy has emitted an SMBH, the galaxy should show signs of disruption. SMBHs are extremely massive, and that much mass cannot move through a galaxy without shaping it. The team examined the galaxy from which the rogue SMBH originated and found that its morphology was disrupted.
Scientists noted, “However, there is another possible explanation for the linear characteristic. It could be a jet with a black hole instead of a rogue SMBH. Under the right conditions, black hole fighter jets can also jerk gas into the CGM, leading to star formation. There are two well-studied examples of nearby jets triggering star formation. One of them is called Minkowski’s Object.”
“The black hole fighter jet explanation is possible, but they say it has too many problems. Visible emission lines are not a property of black hole jets and there is no evidence of nuclear activity. There is also a problem with morphology. A more serious problem is that the feature morphology does not match simulations or observations of jet star formation.”
“They settle on a runaway SMBH as the best explanation for the data and observations.”
“The line proportions, colors and overall morphology are consistent with an ejected SMBH moving at high speed through the CGM and triggering star formation.”
“Locating the black holes would be irrefutable evidence for this conclusion. The ‘smoking gun’ evidence for this scenario would be the unequivocal identification of the black holes themselves.’
“These are candidates for ‘hypercompact stellar systems,’ SMBHs shrouded in stars and gas that escaped with them.”
“Deeper data, for example from the JWST NIRSPEC IFU, can show the expected broad, highly red- or blue-shifted emission lines from ionized gas bound to the black holes. That data can also spatially resolve currents, shocks, and star formation near A. ”
“Looking ahead, the feature morphology in the HST images is so striking that it shouldn’t be too difficult to find more examples if they exist. Future data from the Nancy Grace Roman Telescope can be searched with automated algorithms.”
- Pieter van Dokkum et al., A candidate runaway supermassive black hole identified by shocks and star formation in its wake, arXiv (2023). DOI: 10.48550/arxiv.2302.04888