Scientists have found a way to detect the black holes of the first stars
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- Scientists have found a way to detect the black holes of the first stars
Future Einstein Telescope and Cosmic Explorer gravitational-wave observatories will be able to detect mergers of black holes that arose after the death of the first stars in the universe. The sensitivity of the devices to low frequencies will allow you to determine the distance to these objects and estimate their mass. This was announced on September 9. Phys.org with reference to a study by scientists at the University of Birmingham.
The first stars and the first black holes
The first stars appeared several hundred million years after the Big Bang, during a period that astronomers call the cosmic dawn. They belong to the so-called population III. Unlike modern stars, they consisted almost exclusively of hydrogen and helium, since heavier chemical elements were still practically absent in the early universe.
During such an event, gravitational waves arise — perturbations propagating in space-time. They are able to bring information to the Earth about objects that existed more than 13.5 billion years ago. However, modern observatories do not yet have sufficient sensitivity to confidently detect such distant signals.
The most distant mergers recorded by the LIGO, Virgo, and KAGRA networks are at about a redshift of z = 1.1. This corresponds to a distance of about 8 billion parsecs. To explore the remnants of the first stars, scientists need to observe events with a redshift of z = 15 and above.
Why ancient black holes look heavier
The authors modeled the capabilities of the future network from Einstein Telescope and Cosmic Explorer. Scientists have tested how accurately it can determine the distances, masses, rotation and position in the sky for black holes that existed in the first 180-270 million years after the Big Bang.
One of the main difficulties is related to the expansion of the universe. As the gravitational wave moves towards the Earth, the wavelength increases and its frequency decreases. At the same time, the mass of the source in the detector data seems to be more real.
Thus, a system of two black holes, each with a mass of about 30 solar masses, may look like a binary system with a total mass of about 1.1 thousand solar masses for an Earth instrument. Without accurately determining the distance, such an object can be mistaken for a much more massive, but relatively close system.
To solve this problem, the researchers applied Bayesian analysis, a method that allows estimating the likely characteristics of a source, taking into account measurement uncertainty. The model included a thousand binary systems, the parameters of which corresponded to modern concepts of the remnants of population III stars.
Why do detectors need a frequency of 5 Hz?
The scientists paid special attention to the lower limit of the sensitivity of the devices. They compared two scenarios: a more conservative one, in which the detectors are capable of detecting signals from 10 Hz, and an optimistic one, from 5 Hz.
The difference of only 5 Hz turned out to be fundamental. At the lower limit of 10 Hz, the device sees mainly the last moments of fusion and the subsequent attenuation of the oscillations of the formed black hole. A significant part of the convergence of objects remains outside the available range.
The sensitivity of 5 Hz makes it possible to register more final turns of black holes around the common center of mass. The longer the signal remains available for analysis, the more accurately the properties of its source can be restored.
With the same strict selection criteria in the model catalog, the 5 Hz configuration revealed 395 systems, and the 10 Hz variant revealed 175. Thus, access to the 5-10 Hz range increased the number of sufficiently strong signals by about 2.3 times.
The most distant simulated source was located at a redshift of z = 19.8. In the 5 Hz configuration, the researchers were able to determine with a 90% probability that it is located further than z = 18.5. For the 10 Hz variant, the corresponding lower bound reached approximately z = 17.5.
What can you learn about ancient black holes?
According to calculations, the future network will be able to determine the mass of individual black holes with an average error of about 11-13%. This will make it possible to compare the observed objects with models of the evolution of the first stars. Such measurements are also important for solving the question of the origin of supermassive black holes. Astronomers are already finding them in the very early universe, but it is still unclear how these objects managed to gain millions and billions of solar masses in a relatively short time.
One hypothesis suggests that the starting point of their growth was the relatively light black holes left after the death of population III stars. Alternative scenarios allow for the formation of heavier nuclei in dense star clusters or as a result of the direct collapse of large gas clouds. Data on the mass of the oldest black holes and the frequency of their mergers will help verify these versions.
In about 60% of the simulated cases, the network was able to limit the area of the possible position of the source to no more than 60 square degrees. This is still a significant part of the sky, but it can be compared with data from other instruments, including the Square Kilometer Array radio telescope and the James Webb Space Observatory.
At the same time, the rotation of black holes turned out to be much more difficult to determine. Even an increase in the low—frequency sensitivity did not reliably restore the precession parameters - changes in the orientation of the axis of rotation of the binary system. This is a serious limitation, since it is the rotation that could help distinguish the remnants of the first stars from black holes of other origin.
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