It’s the story of a not-so-distant galaxy that turned to the dark side.
Or, to be more exact, which would never have really left it: it could in fact have been extinct since the beginning of its existence and never have formed the slightest star.
However, it is still a little early to say this with certainty.
What we can say about it for now is that it is so dark that existing surveys do not see a single star where it is located, some 270 million light years away ( it may seem far away, but it is actually in our cosmic neighborhood on the scale of the universe).
But how did we go about discovering this object if it is “invisible”?
Both by mistake and by chance, as is often the case in science!
In the 1980s, thanks to advances in imaging, astronomers began to uncover “ghost” galaxies, less luminous than the deepest black of the sky and known as “low surface brightness”.
They only appear at the cost of long exposures and a treatment that makes them stand out from the “noise” of the universe.
Observations in the radio domain, which notably make it possible to trace neutral hydrogen (the most abundant gas in the universe), confirm that these galaxies, although they are very faint, can on the other hand contain immense quantities of gas.
Some are as massive as our Milky Way.
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For twenty years, an international team led by Karen O'Neil, radio astronomer at the Green Bank Observatory, has been searching for "massive" galaxies with low surface brightness with the radio telescopes of Nançay (France), Green Bank (West Virginia). ) and Arecibo (Puerto Rico).
“
The objective was to determine the gaseous and dynamic masses of these ultra-diffuse galaxies
,” says Karen O'Neil in a press release.
To do this, we observed them using several instruments, some of them several times.
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It was during this work that something unexpected recently happened.
For one reason or another, the Green Bank telescope was pointed at the wrong coordinates when it was supposed to observe a galaxy already detected in Nançay.
Astronomers realized this because the data was incompatible with those taken in France.
To their great surprise, they nevertheless showed the presence of a large quantity of gas in a completely dark region of the sky.
They therefore continued their investigations, pointing all around the area in order to carry out a summary map.
They then noticed that they were “looking” at a gigantic disk rotating rapidly on itself (200 km/s).
The mass of hydrogen detected is estimated at 1.69 billion solar masses, making it a fairly large object and the least bright object identified to date in relation to its mass.
Although it contains half as much gas as the Milky Way, it is even poorer in stars: their number is at best equal to less than 1% of the number of those in our galaxy.
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Be careful, however: it has not yet been confirmed that this galaxy, called “J0613 + 52”, is completely dark.
“
We need to have deep images of this galaxy with optical telescopes ,
”
explains Wim van Driel, radio astronomer at the Paris Observatory, who is participating in this work.
Images of the object available in the public domain do not have the necessary sensitivity.
Only deeper images could allow us to estimate how many stars this galaxy contains, or to give a more precise upper limit.
We are in the process of organizing observations with a French optical telescope in this sense.
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It is also planned to carry out much finer mapping of the cloud thanks to the very large network of 27 antennas of 25 meters each of the “Very Large Array” (in New Mexico).
It will then remain to interpret this result and measure its cosmological implications.
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What we can say for now is that having stumbled upon such a dark galaxy, pointing anywhere in a certain way, suggests that this type of objects should not be so rare
, analyzes Samuel Boissier, CNRS researcher, specialist in galaxies with low surface brightness at the Marseille astrophysics laboratory.
Some models quite naturally predict that these very faintly luminous galaxies are very numerous, or even the majority, in the universe.
It's possible that we've actually only studied the tip of the iceberg so far, looking at galaxies that form a lot of stars, like our Milky Way.
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Many other galaxies to discover
We should soon know for certain.
The European Euclid space telescope, which is currently carrying out a systematic survey of the sky of unprecedented depth, will probably allow us to find out more.
The “Large Synoptic Survey Telescope”, which is due to enter service in 2025, should also revolutionize the field, by carrying out an entire survey of the southern sky with a record level of sensitivity and precision (its mirror measures 8.4 meters in diameter ).
The future network of giant radio telescopes SKA (“Square Kilometer Array”), in which France is participating, should enable it to find many other massive and diffuse galaxies.
“
It will be very interesting to see the number of these faintly bright galaxies and the way in which they are structured on the sky
,” estimates Samuel Boissier.
It is not impossible that this could affect the measurement of cosmological parameters based on the observation of large structures and the distribution of galaxies in 3D.
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This would constitute a great victory for the dark side of the universe.