Whatever dark matter is, cosmologists are busy trying to understand the role it plays in the structure of the Universe. Our standard cosmological model, also called Lambda Cold Dark Matter (LCDM), makes a number of predictions about how galaxies form and evolve, largely focused on dark matter haloes. DM haloes are fundamental building blocks for the cosmological structure. Scientists often describe them as the scaffolding on which the Universe is built.
One of LCDM’s predictions concerns satellite galaxies. Theory says that every galaxy forms and grows within a dark matter halo, including dwarf and satellite galaxies. LCDM theory predicts more small dark matter haloes than there are observed satellite galaxies around the Milky Way. New research presented at the Royal Astronomical Society’s National Astronomy Meeting might have the answer.
The presentation is “The contribution of “orphan” galaxies to the ultrafaint population of MW satellites,” and the lead researcher is Dr. Isabel Santos-Santos, from the Institute for Computational Cosmology in Department of Physics at Durham University, UK.
“The last decade has seen a rise in the number of known Milky Way (MW) satellites, primarily thanks to the discovery of ultrafaint systems at close distances,” Santos-Santos writes. “These findings suggest a higher abundance of satellites within ~ 30kpc than predicted by cosmological simulations of MW-like halos in the CDM framework.”
Astronomers have found about 60 satellite galaxies around the Milky Way. The Large and Small Magellanic Clouds are the most well-known satellite galaxies, and there are others like the Sagittarius Dwarf Spheroidal Galaxy and the Sculptor Dwarf. Santos-Santos says there should be dozens more of them.
Some of the known satellite galaxies of the Milky Way, including the well-known Large and Small Magellanic Clouds. There could be many more of them according to simulations, and if scientists can find them, it supports the Lambda Cold Dark Matter model. Image Credit: ESA/Gaia/DPAC. CC BY-SA 3.0 IGO
“We know the Milky Way has some 60 confirmed companion satellite galaxies, but we think there should be dozens more of these faint galaxies orbiting around the Milky Way at close distances,” she said in a press release.
The problem is that these small galaxies can be extremely difficult to detect. Scientists think that these galaxies might have had their dark matter stripped away through interactions with the much more massive Milky Way. Without their dark matter, which acts as a gravitational anchor, gas, dust and even stars are more easily stripped away. That means there’s little active star formation, and only a dimmer population of older stars. This is why satellite galaxies can be so challenging to detect.
“If our predictions are right, it adds more weight to the Lambda Cold Dark Matter theory of the formation and evolution of structure in the universe. Observational astronomers are using our predictions as a benchmark with which to compare the new data they are obtaining,” Santos-Santos said. “One day soon we may be able to see these ‘missing’ galaxies, which would be hugely exciting and could tell us more about how the universe came to be as we see it today.”
The work is based on the Aquarius simulation produced by the Virgo Consortium. Aquarius simulates the evolution of the MW’s dark matter halo in the highest resolution ever. It was created to investigate the fine-scale structure around the MW.
The researchers used Aquarius and other analytic galaxy formation models to watch as dwarf galaxies formed and evolved and to “estimate the true abundance and radial distribution of MW satellites” that LCDM predicts. They determined that small dark matter haloes that could host satellite galaxies have been orbiting the Milky Way for billions of years, but since they’ve been stripped, they’re dim and hard to see. These are sometimes called ‘orphaned’ galaxies. The simulation showed that there could be up to 100 more MW satellites.
“Strikingly, orphans make up half of all satellites in our highest-resolution run, primarily occupying the central regions of the MW halo,” the researchers write.
This illustration shows galaxies forming as part of the large-scale structure of the Universe. Image Credit: Ralf Kaehler/SLAC National Accelerator Laboratory
The other piece of the puzzle concerns the approximately 30 satellite galaxies discovered recently, all small and dim. If these are stripped or orphaned galaxies, then their discovery is additional evidence in support of LCDM. They could be a subset of the dim satellite population the simulation predicts. However, they could also be globular clusters (GC).
Professor Carlos Frenk of the Institute for Computational Cosmology in the Department of Physics at Durham University is one of the co-researchers. Frenk said, “If the population of very faint satellites that we are predicting is discovered with new data, it would be a remarkable success of the LCDM theory of galaxy formation.”
“It would also provide a clear illustration of the power of physics and mathematics,” Frenk added. “Using the laws of physics, solved using a large supercomputer, and mathematical modelling we can make precise predictions that astronomers, equipped with new, powerful telescopes, can test. It doesn’t get much better than this.”
The Vera Rubin Observatory and its 10-year Legacy Survey of Space and Time might uncover the presence of these dim, orphaned satellites. “We predict that dozens of satellites should be observable within ~30 kpc of the MW, awaiting discovery through deep-imaging surveys like LSST,” the researchers explain.
Scientists have been puzzling over the connections between the Milky Way, dark matter haloes, and satellite galaxies for a long time. Some research suggests that not only does the MW have more satellites that we haven’t detected yet, but that those satellites may have had their own satellites that they dragged towards the MW with them. If that turns out to be true, then the MW may have another 150 dim satellites waiting to be found by observatories like the Rubin.
Scientists think that there are different sizes of DM haloes, some with only a few Earth masses, while some are enormously massive. They also think that they could’ve formed hierarchically, with smaller haloes merging with larger haloes, slowly building up the cosmic web that largely defines the modern Universe. If that’s true, then the MW’s orphaned galaxies might be strong evidence supporting their hierarchical nature.
Now that the Vera Rubin Observatory has achieved its long-awaited first light, we could get confirmation soon.
Maybe that will help us figure out what dark matter actually is one day .