Dwarf galaxies: Are there not enough or are there too many? The new discovery of 50 "ghost" galaxies located up to 100 million light-years away from us may solve this puzzle.
For decades, astronomers studying the surroundings of the Milky Way have determined there are too few dwarf satellite galaxies orbiting our own galaxy when compared to predictions made by the standard model of cosmology. This problem of "missing satellite galaxies" became even more complicated recently when studies of the local universe revealed some galaxies may be surrounded by too many galactic satellites. In addition to this, scientists are also still puzzled about how dwarf galaxies are distributed around the larger galaxies they orbit.
To answer these questions, a team of astronomers searched further afield, looking for faint satellite galaxies around three galaxies. The first was NGC 5068, a spiral galaxy located 17 million light-years away, the second was NGC 5084, a lenticular galaxy 80 million light-years away, and the last was NGC 5087, an elliptical galaxy 100 million light-years away. The researchers discovered almost 50 faint dwarf galaxies orbiting these distant galaxies.
"Studying these islands of nearly invisible stars is not just astronomical stamp collecting," team leader Crescenzo Tortora, a researcher at the Italian National Institute for Astrophysics (INAF), said in a statement. "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."
Smashing new dwarf galaxies
The team obtained the deepest images to date of the three galaxies' surroundings using the Very Large Telescope (VLT) as part of its VST (VLT Survey Telescope) SMASH survey. Rather than use an algorithm to identify possible dwarf galaxies, Tortora, also the VST SMASH survey principal investigator, and his colleagues conducted a meticulous visual inspection of VLT images to ensure no potential dwarf galaxies were missed.
This involved masking light from foreground stars and other astronomical bodies to allow the faint ghostly galaxies to shine through. This led to the observation of 47 dwarf galaxies, of which only four had previously been discovered.
The team then identified the nature of the candidate dwarf galaxies by comparing their colors and structures to observations of other dwarf galaxies. The ghost galaxies took various shapes — there were dwarf spheroidals, dwarf ellipticals and dwarf irregulars. Some demonstrated internal structures that suggested they're dwarf spirals. And all of these candidate dwarf galaxies were found to have a mass between one million and one billion times the mass of the sun.
"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," team member and INAF researcher Rossella Ragusa said.
"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," Ragusa added.
Confirming the newly observed objects are definitely dwarf galaxies will require further observations. Thankfully, however, the region studied in the VST-SMASH survey falls within the field of view of the Euclid space telescope. Euclid's first images have already led to the discovery of a wealth of dwarf galaxies, with more likely to come in future data releases.
"We look forward to studying these same fields with Euclid's eyes, so we can confirm or update our findings," Tortora said.
The team's research was published on Friday (Sept. 11) in the journal Astronomy & Astrophysics.
