Astronomers using ESO’s Very Large
Telescope have observed reservoirs of cool gas around some of the earliest
galaxies in the Universe. These gas halos are the perfect food for supermassive
black holes at the centre of these galaxies, which are now seen as they were
over 12.5 billion years ago. This food storage might explain how these cosmic
monsters grew so fast during a period in the Universe’s history known as the
Cosmic Dawn.
“We are now
able to demonstrate, for the first time, that primordial galaxies do have
enough food in their environments to sustain both the growth of supermassive
black holes and vigorous star formation,” says Emanuele Paolo Farina, of the
Max Planck Institute for Astronomy in Heidelberg, Germany, who led the research
published today in The Astrophysical Journal.
“This adds a fundamental piece to the puzzle that astronomers are building to
picture how cosmic structures formed more than 12 billion years ago.”
Astronomers have wondered how
supermassive black holes were able to grow so large so early on in the history
of the Universe. “The presence of these early monsters, with masses several
billion times the mass of our Sun, is a big mystery,” says Farina, who is also
affiliated with the Max Planck Institute for Astrophysics in Garching bei
München. It means that the first black holes, which might have formed from the
collapse of the first stars, must have grown very fast. But, until now,
astronomers had not spotted ‘black hole food’ — gas and dust — in large enough
quantities to explain this rapid growth.
To complicate matters further,
previous observations with ALMA, the Atacama Large Millimeter/submillimeter
Array, revealed a lot of dust and gas in these early galaxies that fuelled
rapid star formation. These ALMA observations suggested that there could be
little left over to feed a black hole.
To solve this mystery, Farina and
his colleagues used the MUSE instrument on ESO’s Very Large Telescope in the
Chilean Atacama Desert to study quasars — extremely bright objects powered by
supermassive black holes which lie at the centre of massive galaxies. The study
surveyed 31 quasars that are seen as they were more than 12.5 billion years
ago, at a time when the Universe was still an infant, only about 870 million
years old. This is one of the largest samples of quasars from this early on in
the history of the Universe to be surveyed.
The astronomers found that 12
quasars were surrounded by enormous gas reservoirs: halos of cool, dense
hydrogen gas extending 100,000 light years from the central black holes and
with billions of times the mass of the Sun. The team, from Germany, the US,
Italy and Chile, also found that these gas halos were tightly bound to the
galaxies, providing the perfect food source to sustain both the growth of
supermassive black holes and vigorous star formation.
The research was possible thanks to
the superb sensitivity of MUSE, the Multi Unit Spectroscopic Explorer, on ESO’s
VLT, which Farina says was “a game changer” in the study of quasars. “In a
matter of a few hours per target, we were able to delve into the surroundings
of the most massive and voracious black holes present in the young Universe,”
he adds. While quasars are bright, the gas reservoirs around them are much
harder to observe. But MUSE could detect the faint glow of the hydrogen gas in
the halos, allowing astronomers to finally reveal the food stashes that power
supermassive black holes in the early Universe.
In the future, ESO’s Extremely Large
Telescope will help scientists reveal even more details about galaxies and
supermassive black holes in the first couple of billion years after the Big
Bang. “With the power of the ELT, we will be able to delve even deeper into the
early Universe to find many more such gas nebulae,” Farina concludes.
Journal article: https://iopscience.iop.org/article/10.3847/1538-4357/ab5847
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