This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Dwarf galaxies—with a mass equal to about a billion suns, but often considerably less—are the most abundant in the universe. However, because of their low luminosity, they are very difficult to detect.
They have mostly been discovered in the vicinity of the Milky Way, in what is called the Local Group, but also, and in large quantities, at greater distances, where current instrumentation cannot distinguish individual stars. It is precisely around dwarf galaxies that significant discrepancies have emerged over recent decades between observations and the predictions of the standard cosmological model, challenging our understanding of galaxy formation and evolution processes in the cosmos. Initially, there seemed to be too few dwarf galaxies near ours and other galaxies in the local universe compared with numerical simulation predictions—the "missing satellite galaxies" problem.
More recently, however, the issue appears to have changed scope, with some studies even indicating an overabundance of satellite galaxies in observational data compared with simulations. The spatial distribution of these dwarfs around their host galaxies—they are generally arranged along a sort of plane perpendicular to the central galaxy's disk, as in the case of the Milky Way—is also still a topic of debate. To untangle the knot and determine whether these issues concern only the Local Group or represent a recurring behavior, it is essential to extend the search for dwarf galaxies around other galaxies or galaxy groups beyond our immediate cosmic neighborhood.
This is what an international team of researchers led by the Italian National Institute for Astrophysics (INAF) did, cataloging nearly 50 dwarf galaxies and bringing some of these extremely faint "ghost galaxies" out of the darkness around three host galaxies: NGC 5068, NGC 5084 and NGC 5087, a spiral, a lenticular and an elliptical galaxy located 17, 80 and 100 million light-years away, respectively. The results, obtained through observations from the VST (VLT Survey Telescope)—an Italian telescope operated by INAF at the ESO Paranal Observatory in Chile—are published in the journal Astronomy & Astrophysics. "Studying these islands of nearly invisible stars is not just astronomical stamp collecting," explains Crescenzo Tortora, INAF researcher, principal investigator of the VST-SMASH survey used to obtain the data, and lead author of the new paper.
"It is the only way we have to solve some of the major puzzles of the cosmological model and finally understand how mysterious dark matter shapes and molds the entire universe." The work presents the deepest optical images yet obtained for the fields surrounding the three galaxies, covering an area of sky about 12 times that of the full moon. Through a visual inspection of the images in both individual filters and color, the team identified 47 candidate dwarf galaxies. Only four were previously known, significantly increasing the number of dwarf galaxies discovered in this portion of the sky.
To find the galaxies, the team did not use automated algorithms, which would have easily missed such faint objects. Instead, three authors working independently conducted a meticulous visual inspection of the images. It was also necessary to "cover" and mask the light from foreground stars and other artifacts, a necessary step to accurately measure the faint light of these galaxies.
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