Dwarf galaxies – with a mass equal to about a billion suns, but often considerably less – are the most abundant in the universe; however, due to their low luminosity, they are very difficult to detect. They have mostly been discovered in the vicinity of our galaxy, the Milky Way, in what is called the Local Group, but also, and in large quantities, at greater distance, where however current instrumentation is not able to distinguish individual stars. It is precisely around dwarf galaxies that, over recent decades, significant discrepancies have emerged 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 to 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 to simulations. The spatial distribution of these dwarfs around their host galaxy – 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 behaviour, 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, cataloguing 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 at 17, 80 and 100 million light-years from us, respectively. The results, obtained thanks to 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 ever obtained to date for the field surrounding the three galaxies, covering an area of the sky about 12 times that of the full moon. Through a visual inspection of the images both in individual filters and in color, the team identified 47 candidate dwarf galaxies. Only four of them 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; the search was conducted through a meticulous visual inspection of the images by three authors working independently. It was also necessary to "cover" and mask the light from foreground stars and other artifacts, a mandatory step to accurately measure the faint light of these galaxies.

"To give people a taste of the thrill of being a dwarf galaxy hunter, I created a small online game that reproduces a process very similar to the one we used, allowing users to experience firsthand the visual survey we performed and try to spot the dwarf galaxies near NGC 5087", adds Tortora.

The nature of these galaxies was verified based on their color and structural properties, which are consistent with those of other dwarf galaxies, but confirmation can only come in the future with spectroscopic observations that allow their distance and velocity to be estimated: for this reason, they are currently considered "candidates". Their spatial distribution indicates that they are likely satellites of the larger galaxies in this field – namely NGC 5084, NGC 5087, and NGC 5068 (in the case of NGC 5084, there also seems to be a planar distribution similar to that of the Milky Way's satellites) – but only spectroscopic follow-up will be able to prove this hypothesis.

Many of the new discoveries are diffuse galaxies with low surface brightness and lacking internal structures: these are dwarf spheroidals or dwarf ellipticals; others, however, display internal structures and could be classified as dwarf spirals or dwarf irregulars. Each is estimated to have a mass between one million and one billion solar masses.

The result is a consequence of the exceptionally high quality of the images from the VST and its OmegaCam camera, which, by observing for about two hours in each of the three studied bands, allowed the sky to be probed to a depth that a survey such as the one underway at the Rubin Observatory, also in Chile, will only reach in a few years.

"Thanks to the deep VST observations, we were able not only to 'see' some extremely faint galaxies better, but also to characterize their light profiles with a high signal-to-noise ratio: by gathering more light, we better mapped their outer regions", comments Rossella Ragusa, INAF postdoctoral researcher and co-author of the paper. "An example of this is the comparison we conducted with one of the dwarf galaxies in the sample—which had previously been discovered using less deep data—whereby we determined its dimensions and the shape of its light profile (particularly in the galaxy's outermost, fainter regions) with greater precision and accuracy".

The study shows that the light profile of these galaxies is not very steep, unlike that of elliptical galaxies, and is instead very similar to that of disk galaxies. The team managed to measure how the galaxy's color changes as a function of distance from the center: redder at the center and less red toward the periphery, just like in most galaxies in the universe, particularly those in this mass range. The preliminary results are consistent with the scenario in which galaxies like these form through a process of "monolithic collapse," a hypothesis that also remains to be confirmed – along with the role of the environment on the galaxy properties – with spectroscopic data and even deeper imaging in the future.

The VST-SMASH survey was designed so that all the galaxies included in the sample also fall within the field of view of the European Space Agency’s Euclid space telescope. Euclid’s very first images have already led to the discovery of a vast number of dwarf galaxies, and the first data release, due to be published in a few months, will enable the detection and characterization of even more; several co-authors of this paper, including Tortora and Ragusa, are participating in this effort as part of the Euclid collaboration. "We look forward to studying these same fields with Euclid’s eyes, so we can confirm or update our findings", concludes Tortora.

RELATED JOURNAL ARTICLE

"VST-SMASH: VST Survey of Mass Assembly and Structural Hierarchy II. Exploring dwarf galaxies in the vicinity of NGC 5068 and of the two galaxies NGC 5084 and NGC 5087 at the edges of the Virgo Supercluster", C. Tortora, R. Ragusa, L. Hunt, A. Unni, M. Baes, Abdurro’uf, F. Annibali, M. Gatto, N. R. Napolitano, H. Su, A. Venhola and D. Carollo, published in Astronomy&Astrophysics.