What is a galaxy without stars called?
This month, radio astronomers announced that they had discovered the
darkest galaxy ever seen
, a cloud of hydrogen gas similar to our Milky Way in many ways, such as its mass and rotation, but with no stars that anyone can discern.
"We may be looking at the discovery of a primordial galaxy, a galaxy so diffuse that it has not been able to form stars easily," Karen O'Neil of the Green Bank Observatory in West Virginia said at a news conference in the meeting of the American Astronomical Society held in New Orleans on January 8.
That same week, a group of Spanish astronomers led by Mireia Montes, a researcher at the Institute of Astrophysics of the Canary Islands, revealed the discovery of another almost starless galaxy that they called Nube, "cloud" in Spanish.
A composite image of the area around the Cloud galaxy, with a black and white version in the background and color in the foreground to highlight Cloud in the center.
"With our current knowledge, we do not understand how a galaxy with
such extreme characteristics
can exist ," Montes said in a statement released by the institute.
Montes is the first author of the new work, published in the journal
Astronomy & Astrophysics.
And so, we can add "dark galaxies" to "dark matter", "dark energy" and the other dark terms already present in the cosmic lexicon.
Dark galaxies are entities whose stars are so few and faint that their light cannot be distinguished other than as a thin, transparent haze that does not appear to contain any stars.
(At first, dark galaxies were called "low surface brightness galaxies" or "ultradiffuse galaxies," but time and jargon move on.)
As astronomers continue to delve deeper into the skies with more powerful and intelligent eyes, dark galaxies have begun to appear more frequently, calling into question long-held views about the formation and evolution of galaxies.
Sloan Digital Sky Survey image of the area around Cloud, with previously known galaxies in the field of view labeled.
The inset shows how Cloud appears in images with different surface brightness limits.
Image Montes et al., Astronomy & Astrophysics
The accidental galaxy
These faint ghosts are hard to find and even harder to study, requiring hours or days of observing to focus their visible starlight.
One way to do this is to scan the skies with radio telescopes tuned to the frequency of the interstellar hydrogen gas that invades galaxies.
O'Neil participated in a study of this type, with various telescopes, on about 350 galaxies with low surface luminosity.
"
I mistyped
the coordinates of the galaxy I wanted to observe, which caused the telescope to point to a different part of the sky than expected," he said recently in an email.
The telescope landed on something it had never seen before.
"It is a galaxy formed only by gas, without visible stars," he explains.
"There could be stars. We just can't see them."
The galaxy, known as J0613+52, is located about 270 million light years away.
It's swimming amid 2 billion solar masses of early hydrogen produced in the Big Bang, but the galaxy isn't forming any stars, probably because the gas is
too diffuse
to clump together in clouds that become stars.
Furthermore, there are no nearby galaxies that exert a gravitational influence capable of triggering this agglomeration.
"J0613+52 appears to be underdisturbed and underdeveloped," says O'Neil.
"This could be our first discovery of a nearby galaxy made up of primordial gas."
A tiny cloud
The other new dark galaxy, Cloud, is much smaller than J0613+52 but equally intriguing, Montes said.
The name was suggested by the 5-year-old daughter of his colleague Ignacio Trujillo, because the stars in the galaxy are so sparsely spread over such a large volume that the collective was almost undetectable.
Trujillo was looking at images of a strip of sky that had been recorded by the
Sloan Digital Sky Survey,
a long-term study of the universe, when he saw a small, mysterious blur of light.
After training the
Gran Telescopio Canarias
, the largest optical telescope in the world, on the faint spot of light, he concluded that it was a small galaxy.
It was so transparent that I could see other, more distant galaxies behind it.
The team estimates that Cloud is a dwarf galaxy only a tenth brighter than others of its type, but 10 times larger than other galaxies with a comparable number of stars.
"To show what this means to anyone who knows a little about astronomy, this galaxy is a third the size of the Milky Way but has a mass similar to that of the Small Magellanic Cloud," he said, referring to a galaxy that is a satellite of the Milky Way and is visible in the southern hemisphere.
invisible scaffolding
Throughout the last century, starting with work carried out in the 1930s by Fritz Zwicky, a Swiss astronomer of Bulgarian origin at the
California Institute of Technology
, astronomers have gradually come to the conclusion that most of the Universe is made up of things that we cannot see.
Galaxies are enveloped in clouds of so-called dark matter, which exceeds the ordinary atomic matter of which we and stars are made by 6-1.
No one knows what
dark matter
is , but its gravity is what attracts ordinary matter into clouds that light up into stars and galaxies.
For 40 years, the prevailing opinion is that dark matter is made up of hypothetical and exotic particles called weakly interacting massive particles.
They are invisible and slow compared to the speed of light, and can only be detected through their gravitational effect on other objects.
In computer simulations, this "cold dark matter" reproduces the large-scale structures seen in the universe, such as galaxies and galaxy clusters.
But in computer simulations of smaller-scale structures, this dark matter model breaks down;
In dwarf galaxies, for example, such dark matter should give rise to larger central concentrations of stars than are actually observed.
Many astrophysicists attribute this discrepancy to their inability to model complex and disordered phenomena, such as shock waves and magnetic fields - so-called
gastrophysics
- that prevail when atoms come close to each other.
Furthermore, particle physicists have not been able to produce or detect any of these hypothetical particles in the laboratory.
Some scientists have proposed the existence of other versions of dark matter.
Among them, "diffuse" or "warm" dark matter, composed of hypothetical ultralight particles called axions, and even an entire "dark sector" of invisible particles and forces that would create galaxies with a different footprint.
The study of dark galaxies offers a unique opportunity to explore the origin and evolution of galaxies, and to test ideas about the dark matter that permeates them.
Because of Nube's optical haze, Montes said, astronomers can see the shape of the dark matter halo underlying the dwarf galaxy.
This shape - similar to a banana, a cigar or a surfboard - seems incompatible with the expected behavior of cold dark matter and more in line with diffuse dark matter.
Such a conclusion would revolutionize physics and cosmology, but it is far from settled.
Jeremiah P. Ostriker, a Columbia University expert on the evolution and structures of galaxies, agrees.
According to him, in low-mass galaxies, the cold dark matter model predicts that there should be more stars in the center of the galaxy and progressively fewer towards the edges.
But other types of dark matter could clump together differently, leading to a more uniform distribution of starlight.
"These new low surface brightness systems are a better fit for diffuse dark matter than for cold dark matter," he says in an email.
Montes and his co-authors are not so sure.
"Although diffuse dark matter could alleviate some of the small-scale stresses that appear in the cold dark matter scenario, more work is needed to evaluate this model," they wrote in their paper.
O'Neil and his team hope to detect visible starlight from the invisible galaxy J0613+52.
Doing so would help them figure out what types of stars inhabit that galaxy.
"If we don't detect it, it will also be exciting, since it will mean that we have detected something whose stellar content is much more diffuse than anything seen to date," he says.
What if they don't find anything?
At what point is something too dark to be called a galaxy?
"It's a very interesting question," he says.
Traditionally, the term galaxy referred to a collection of stars and gas;
Then dark matter was added.
"It now appears that the need for stars may not be necessary in the definition," he said.
"This new type of object really opens up and forces us to revisit the concept of what a galaxy is."
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