Scientists have mapped for the first time a galactic wind, the gas reservoir of a galaxy, and thus observed some of its “lost” matter, according to a study released on Thursday (16).
“Galaxy are rarely passive islands of stars”, but rather dynamic structures, whose formation and evolution are difficult to study, says astrophysicist Nicolas Bouché, interviewed by AFP.
An understanding that becomes even more difficult because, according to the theory, galaxies are constituted mainly by a mysterious dark matter, of unknown nature and, therefore, invisible, and by only about 16% of baryonic matter, that of atoms and molecules of the visible Universe.
As if that weren’t enough, observation of galaxies reveals only 20% of this baryonic matter. The rest—the “lost” matter—would be sent back into space by the galactic wind, a nebula of gas and dust caused by exploding stars within the galaxy.
An international team led by researchers from the Astrophysical Research Center in Lyon (Cral) mapped this nebula of lost matter using the Muse spectrograph, coupled to the Very Large Telescope (VLT) of the European Austral Observatory installed in the Chilean Atacama desert.
“It’s as if we’ve seen an iceberg for the first time”, comments Nicolas Bouché, a researcher at Cral and co-signer of the study published in the prestigious British journal MNRAS, with its main author, Johannes Zabl, from the department of astronomy at the Canadian University of Saint Nary .
Researchers had already observed galaxy nebulae, but much more diffuse. This time, observation of Gal1, a very young galaxy about a billion years old, revealed “a cloud of gas produced by these galactic winds, which escapes from both sides of the galaxy’s disk by two asymmetrical cones”.
Gigantic in size, this persistent cloud spans more than 80,000 light years from the center of Gal1. For comparison, the diameter of our Milky Way is about 100,000 light years.
Despite its size, this gas nebula represents only “about 10 to 20% of the mass of the galaxy” observed, according to Bouché. It acts as a reservoir of matter, from which the galaxy draws to fuel its star formation.
Part of the cloud would fall back into the galactic disk to form these stars, some of which, eventually exploding, would send matter back to the nebula, and so on.
Scientists were able to establish a “map” of this nebula, providing information about its volume and mass, thanks to a happy combination of circumstances and the remarkable Muse instrument.
The wide-field spectrograph provides a three-dimensional image, in which the two dimensions of the optics are added to that of light analysis, which makes it possible to detect the presence of primordial elements.
In this case, a quasar – a particularly bright object in the Universe – served, because of its proximity to the galaxy Gal1, as a “lighthouse”, betraying the presence of magnesium.
“We then detected this same element in the galaxy and, therefore, the presence of the gas associated with it”, says Bouché.
The observation of the two gas cones was possible, moreover, only because the galaxy was presented almost in profile for observation.
Astronomers already knew of this type of nebula in the universe near us, and therefore recent, notes a press release from the CNRS. But its existence was only supposed to exist for young galaxies, still in formation, such as Gal1, which was apprehended when the more distant Universe was seven billion years old, that is, about half its current age.
The researchers will now look at several galaxies to “understand why Gal1 has a cloud and others don’t, and what conditions favor its presence,” according to Bouché.