It is a galactic mystery that created an outcast.
Researchers studying a nearby galaxy have discovered it is a ‘lonely giant’.
The University of Michigan team studied the satellite galaxies of Messier 94, or M94, a galaxy similar in size to our Milky Way.
They expected it to be in a similar neighborhood as the Milky Way, which has about 10 smaller, satellite galaxies surrounding it, each with at least a million stars, and up to more than a billion, such as the Magellanic Clouds.
Instead, they detected just two galaxies near M94, with very few stars each.
‘More than just an observational oddity, we show that the current crop of galaxy formation models cannot produce such a satellite system,’ Adam Smercina, a National Science Foundation fellow in the U-M Department of Astronomy, wrote in the journal Astrophysical Letters.
‘Our results indicate that Milky Way-like galaxies most likely host a much wider diversity of satellite populations than is predicted by any current model.’
Smercina says their results have implications for the current understanding of how galaxies form – which is in much larger halos of dark matter.
These halos of dark matter surrounding galaxies have immense gravitational force, and can pull in gas from their immediate vicinity.
Large galaxies like the Milky Way generally form in halos of about the same mass.
But these smaller satellite galaxies, which form in smaller ‘subhalos,’ are not nearly so dependable.
M94 indicates that galaxy formation in intermediate-sized dark halos may be much more uncertain than previously thought.
To observe the number of satellite dwarf galaxies around M94, the researchers took a composite image of the large galaxy.
The image covered about 12 square degrees of the night sky–the full moon, for comparison, appears as about one square degree.
This kind of image includes layers and layers of ‘noise,’ including cosmic rays and scattered light, which make faint dwarf galaxies difficult to detect.
To make sure they weren’t missing satellite galaxies, Smercina and his team engineered artificial galaxies back into the image and recovered them using the same methods as for real satellites.
With this technique, the researchers confirmed that were no more than two galaxies around M94.
‘The real kicker is whether or not the community expected this could be possible,’ Smercina said.
‘That is the real curiosity of this finding–the result is something the simulations don’t predict.
‘When you can discover something we didn’t really think we could find, you can make a contribution to our understanding of how our universe works, that’s really rewarding.’