Emerging picture: More than 10,000 times fainter than the Milky Way, the Leo II dwarf spheroidal galaxy is 500 light years across and is approximately 600,000 light years away from the Sun.
Credit: Josh Simon using data made available by the Sloan Digital Sky Survey.
Bullock likens it to counting cars on a road on a dark night. Only nearby car lights would be visible, but many more cars could be out there but too far away to be seen, especially if their lights were dim.
However, if you knew the approximate spacing between cars on the road, you could infer how many cars there were in total based on the few car lights that you could see, he said. "This is basically what we did — except that we were counting dwarf galaxies."
Formation of galaxies
Tollerud said that the results have implications for our understanding of the formation of galaxies. The small size of these dwarf galaxies makes it easier to construct detailed scenarios about their formation in a way that's impossible for big galaxies like the Milky Way, he said. "We can then generalise this to understand other types of galaxies."
The experts also believe that the existence of 400 satellite galaxies may set a limit on how the fast the elusive particles that are dark matter can move, and hence how 'warm' dark matter can be.
"If [dark matter particles] are moving too quickly, they can't clump up into subhalos [which contain dwarf galaxies]. So the existence of about 400 satellites means that the dark matter has to be moving slow enough to clump into at least 400 subhalos," said Tollerud.
Nice and novel
Commenting on the research, astrophysicist Geraint Lewis, from the University of Sydney in Australia, said it was "novel" and a "nice prediction". But he said the presence of these extra galaxies hinged on whether the CDM theory was correct and that he was only half convinced the missing galaxies would be found in future, deeper sky surveys.
"If we look up [to the sky] and see them that would be great and everyone would be happy. If we don't see [the missing galaxies], people will still be running around trying to explain why they should still be there," he said.
Larger sky surveys, such as those possible with the Australian National University's SkyMapper telescope, which became operational in July this year, should reveal if the researchers are correct in their predications within five to ten years, added Lewis.
Revising star numbers
Does this research have implications for current estimates of the number of stars and galaxies out there?