Evidence of a Runaway Supermassive Black Hole Confirmed by Supersonic Shock Observations
Astronomers confirm the existence of a runaway supermassive black hole, previously hinted at by Hubble, now observed by Webb, moving at supersonic speeds and creating a shock wave in the intergalactic medium.
The existence of runaway supermassive black holes, theorized to arise from galaxy mergers, has long been anticipated by astronomers. In March 2023, the Hubble Space Telescope provided a crucial hint of such an object, revealing a long trail of shocked gas. Now, the team behind that initial observation has utilized the James Webb Space Telescope (JWST) to conduct further investigations, confirming that this object is indeed moving at supersonic speeds through the intergalactic medium, creating a shock wave as it disrupts the surrounding gas. Their findings are published in The Astrophysical Journal Letters.
The candidate runaway black hole was serendipitously discovered in a Hubble image, with its host galaxy located at a redshift of 0.964. The image displayed a peculiar linear structure, suggesting a connection to a neighboring galaxy. Follow-up spectroscopy indicated that this structure might correspond to a supermassive black hole traveling at supersonic speeds, producing a shock in the circumgalactic medium. However, the spectroscopic data also left room for alternative interpretations, such as the possibility of a spiral galaxy viewed edge-on without a central bulge.
To investigate this enigmatic structure, which spans 62 kiloparsecs, Pieter Van Dokkum from Yale University and his colleagues employed the NIRSpec spectrograph on the JWST. This instrument allows each pixel in the image to contain a spectrum, enabling astrophysicists to compare the spectral data across different physical locations.
The Doppler effect plays a crucial role in this analysis; light from objects moving toward us is shifted to shorter wavelengths (blue shift), while light from receding objects is shifted to longer wavelengths (red shift). The researchers observed that at the edge of the structure, the gas velocity varied by over 600 km/s across just 1 kiloparsec, a steep gradient indicative of a supersonic shock wave. Such a dramatic change in gas velocity over a small astronomical scale cannot be explained by any other effect. Additionally, the morphology of the gas at the end of the trail and the intensity ratios of emission lines [O III]/Hα, [N II]/Hα, [S II]/Hα, and [S III]/[S II] support the presence of rapid radiative shocks and quick cooling, with shock speeds aligning with predictions based on the black hole's velocity and shock geometry.
The team meticulously modeled the expected kinematics of a shock wave, predicting how velocities should vary across different spaxels. The shock does not move perfectly parallel to the plane of the sky; instead, it is inclined toward us by about 30 degrees. Consequently, one side of the shock (the "near limb") moves toward us while the other side (the "far limb") recedes. This angle results in a significant amount of blue-shifted emissions from the near limb and some red-shifted emissions from the far limb.
The authors simulated the shock and aimed to match the observed kinematics to their model. They calculated the brightness of two emission lines in each spaxel to determine their positions along the shock and trail, allowing them to predict velocities at various distances. A sharp transition from negative velocities (blue-shifted) to positive velocities (red-shifted) indicates whether the observation is from the near or far limb. The peak of the shock demonstrates how well their model fits the data.
As the black hole traverses the circumgalactic medium, it leaves behind a narrow trail of hot gas. Over time, additional gas from the environment mixes with this trail, forming a stable structure. Eventually, the gas cools and becomes dense enough to form stars within the trail.
By measuring the light emitted from the trail, Van Dokkum and his team estimate that it contains the equivalent of 200 million solar masses in stars. This figure is close to the total amount of gas that could mix with the trail over its lifetime, suggesting that conditions within the trail may favor the formation of more massive stars than typically expected. Higher-mass stars emit significantly more light per unit mass, indicating that a smaller total mass of stars could produce equivalent brightness if the stellar population consists of many high-mass stars.
The researchers performed a simple mass estimation of the supermassive black hole using energy conservation principles. As the black hole moves through the medium, it heats the gas, transferring some of its kinetic energy. By measuring the energy transferred to the medium in the shock and trail, they establish a lower limit for the black hole's mass at a minimum of 10 million solar masses. This mass aligns with expectations based on the stellar mass of the host galaxy's bulge.
The black hole's escape velocity is accurately measured at 954 (±120) km/s, with an inclination angle of 29 degrees. There are two primary mechanisms through which a supermassive black hole can be ejected from its host galaxy: the recoil induced by gravitational waves during a binary merger or interactions in a three-body system. When two similarly massive black holes with opposing spins orbit each other, they emit more gravitational waves in one direction than the other. This asymmetry creates a recoil force in the opposite direction, allowing the resulting black hole to be ejected from the galaxy at high speeds. While this configuration is relatively rare, nearly all galaxy mergers are expected to eventually lead to black hole mergers, suggesting that a population of black holes could be ejected from their galaxies via this gravitational "kick." In a three-body system, the instability of three orbiting black holes can result in the ejection of the lowest-mass black hole from the galaxy.
Given that the estimated mass of 10 million solar masses closely matches the stellar mass of the bulge of the host galaxy, Van Dokkum and his collaborators conclude that it is unlikely for the runaway black hole to be the result of a lower-mass ejection from a triple system. Instead, they suggest it is more likely the outcome of a recoil event following a merger.
Several questions remain about this intriguing object. Definitively proving the presence of a black hole within this structure poses significant challenges. If one exists, it is likely surrounded by a much larger envelope of hot gas, making direct observation impossible. However, for Van Dokkum and his team, the observations align well with expectations for a runaway supermassive black hole, representing the most concrete evidence to date of such an object. Future data collection on this remarkable object will undoubtedly yield further insights.
