In the past few decades, astronomers have been able to look farther into the Universe (and also back in time), almost to the very beginnings of the Universe. In so doing, they’ve learned a great deal about some of the earliest galaxies in the Universe and their subsequent evolution. However, there are still some things that are still off-limits, like when galaxies with supermassive black holes (SMBHs) and massive jets first appeared.
According to recent studies from the International School for Advanced Studies (SISSA) and a team of astronomers from Japan and Taiwan provide new insight on how supermassive black holes began forming just 800 million years after the Big Bang, and relativistic jets less than 2 billion years after. These results are part of a growing case that shows how massive objects in our Universe formed sooner than we thought.
Astronomers have known about SMBHs for over half a century. In time, they came to realize that most massive galaxies (including the Milky Way) have them at their cores. The role they play in the evolution of galaxies has also been the subject of study, with modern astronomers concluding that they are directly related to the rate of star formation in galaxies.
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Similarly, astronomers have found that SMBHs have tight accretion disks around them where gas and dust are accelerated to close to the speed of light. This causes the center of some galaxies to become so bright – what are known as active galactic nuclei (AGNs) – that they outshine the stars in their disks. In some cases, these accretion disks also lead to jets of hot material that can be seen from billions of light-years away.
According to conventional models, galaxies didn’t have enough time to develop central black holes when the Universe was less than a billion years old (ca. 13 billion years ago). However, recent observations have shown that black holes were already forming at the center of galaxies at the time. Addressing this, a team of scientists from SISSA proposed a new model that offers a possible explanation.
For their study, which was led by Lumen Boco – a Ph.D. student from the Institute for Fundamental Physics of the Universe (IFPU) – the team started with the well-known fact that SMBHs grow in the central regions of early galaxies. These objects, the progenitors of elliptical galaxies today, had a very high concentration of gas and an extremely intense rate of new star formation.
The first generations of stars in these galaxies was short-lived and quickly evolved into black holes that were relatively small, but significant in number. The dense gas that surrounded them led to significant dynamic friction and caused them to migrate quickly to the center of the galaxy. This is where they merged to create the seeds of supermassive black holes – which slowly grew over time.
As the research team explained in recent SISS press release:
“According to classical theories, a supermassive black hole grows at the centre of a galaxy capturing the surrounding matter, principally gas, “growing it” on itself and finally devouring it at a rhythm which is proportional to its mass. For this reason, during the initial phases of its development, when the mass of the black hole is small, the growth is very slow. To the extent that, according to the calculations, to reach the mass observed, billions of times that of the Sun, a very long time would be required, even greater than the age of the young Universe.”
However, the original mathematical model they developed showed that the formation process for central black holes could be very rapid in its initial phases. This not only offers an explanation for the existence of SMBH seeds in the early Universe but also reconciles the timing of their growth with the known age of the Universe.
In short, their study showed that the process of migration and mergers of early black holes can lead to the creation of an SMBH seed of 10,000 to 100,000 solar masses in just 50-100 million years. As the team explained:
“[T]he growth of the central black hole according to the aforementioned direct accretion of gas, envisaged by the standard theory, will become very fast, because the quantity of gas it will succeed in attracting and absorbing will become immense, and predominant on the process we propose. Nevertheless, precisely the fact of starting from such a big seed as envisaged by our mechanism speeds up the global growth of the supermassive black hole and allows its formation, also in the Young Universe. In short, in light of this theory, we can state that 800 million years after the Big Bang the supermassive black holes could already populate the Cosmos.”
In addition to proposing a working model for observed SMBH seeds, the team also suggested a method for testing it. On the one hand, there are the gravitational waves that these mergers would cause, which could be identifiable using gravitational wave detectors like Advanced LIGO/Virgo and characterized by the future Einstein Telescope.
In addition, the subsequent development phases of SMBHs is something that could be investigated by missions like the ESA’s Laser Interferometer Space Antenna (LISA), which is expected to launch by around 2034. In a similar vein, another team of astronomers recently used the Atacama Large Millimeter/submillimeter Array (ALMA) to address another mystery about galaxies, which is why some have jets and others don’t.
These fast-moving streams of ionized matter, which travel at relativistic speeds (a fraction of the speed of light), have been observed emanating from the center of some galaxies. These jets have been linked to a galaxy’s rate of star formation because of the way they expel matter that would otherwise collapse to form new stars. In other words, these jets play a role in the evolution of galaxies, much like SMBHs.
For this reason, astronomers have sought to learn more about how black hole jets and gaseous clouds have interacted over time. Unfortunately, it has been difficult to observe these kinds of interactions during the early Universe. Using the Atacama Large Millimeter/submillimeter Array (ALMA), a team of astronomers managed to obtain the first resolved image of disturbed gaseous clouds coming from a very distant quasar.
The study that describes their findings, led by Prof. Kaiki Taro Inoue of Kindai University, recently appeared in the Astrophysical Journal Letters. As Inoue and his colleagues explained, the ALMA data revealed young bipolar jets emanating from MG J0414+0534, a quasar located roughly 11 billion light-years from Earth. These findings show that galaxies with SMBHs and jets existed when the Big Bang was less than 3 billion years old.
In addition to ALMA, the team relied on a technique known as gravitational lensing, where the gravity of an intervening galaxy magnifies light coming from a distant object. Thanks to this “cosmic telescope” and ALMA’s high resolution, the team was able to observe the disturbed gaseous clouds around MG J0414+0534 and determine that they were caused by young jets emanating from an SMBH at the center of the galaxy.
As Kouichiro Nakanishi, a project associate professor at the National Astronomical Observatory of Japan/SOKENDAI, explained in an ALMA press release:
“Combining this cosmic telescope and ALMA’s high-resolution observations, we obtained exceptionally sharp vision, that is 9,000 times better than human eyesight. With this extremely high resolution, we were able to obtain the distribution and motion of gaseous clouds around jets ejected from a supermassive black hole.”
These observations also showed that the gas was impacted where it followed the direction of the jets, causing particles to move violently and become accelerated to speeds of up to 600 km/s (370 mps). What’s more, these impacted gaseous clouds and the jets themselves were much smaller than the size of a typical galaxy at this age.
From this, the team concluded that they were witnessing a very early phase of jet evolution in the MG J0414+0534 galaxy. If true, these observations allowed the team to witness a key evolutionary process in galaxies during the early Universe. As Inoue summarized:
“MG J0414+0534 is an excellent example because of the youth of the jets. We found telltale evidence of significant interaction between jets and gaseous clouds even in the very early evolutionary phase of jets. I think that our discovery will pave the way for a better understanding of the evolutionary process of galaxies in the early Universe.”
Together, these studies demonstrate that two of the most powerful astronomical phenomena in the Universe emerged earlier than expected. This discovery also provides astronomers with the opportunity to explore how these phenomena evolved over time, and the role they played in the evolution of the Universe.