The Milky Way has more than 30 known satellite galaxies. The Large and Small Magellanic Clouds are the largest and most well-known; other lesser-known ones, like the Sagittarius Dwarf Galaxy, are also on the list. Astronomers think there are many more small satellites that are difficult to detect but essential in understanding the Milky Way. The Vera Rubin Observatory should help astronomers find many more of them.
New research shows that the VRO and its Legacy Survey of Space and Time (LSST) could detect dozens more dwarf galaxies, possibly more than 100. Their work is based on a simulated sky survey for the LSST from the LSST Dark Energy Science Collaboration (DESC) Data Challenge 2 (DC2). In this new work, researchers injected stellar population data into the simulated survey and tested the LSST’s ability to retrieve it.
The research is “Predictions for the Detectability of Milky Way Satellite Galaxies and Outer-Halo Star Clusters with the Vera C. Rubin Observatory.” It’s been submitted to the Open Journal of Astrophysics, and the lead author is Kabelo Tsiane from the Department of Physics at the University of Michigan.
In the standard cosmological model with cold dark matter (CDM), galaxies form as dark matter haloes merge. The lowest-mass dark matter haloes that contain stars are ultra-faint dwarf galaxies. These are the smallest and dimmest galaxies in existence, and understanding them is a way to better understand how galaxies form. However, as their name implies, they’re difficult to detect.
In the past, astronomers were reasonably sure they could tell dark-matter-dominated dwarf galaxies and baryon-dominated halo star clusters apart. But, as observational capabilities improve, astronomers have found ultra-faint systems with unclear origins. Finding more of them and classifying them will enhance our understanding of galaxy formation and growth.
“The discovery and measurement of these systems will continue to be an invaluable probe into the fundamental nature of our universe.” – from Predictions for the Detectability of Milky Way Satellite Galaxies and Outer-Halo Star Clusters with the Vera C. Rubin Observatory
“Each newly discovered system increases our understanding of the Milky Way satellite population, as well as providing opportunities for unique, fortuitous discoveries among the most extreme stellar systems,” the authors write. In this work, they collectively refer to both satellite galaxies and halo star clusters as Milky Way satellites.
The population of satellites is not well understood.
“More than 65 confirmed and candidate dark-matter-dominated satellite galaxies have been detected around the Milky Way to date,” the authors write. They also point out that the known population of satellites agrees with CDM model predictions. “However, observational and theoretical arguments suggest that the current census of Milky Way satellites is incomplete, and recent models predict that the total Milky Way satellite galaxy populations consists of ~100–300 systems,” they explain.
This figure shows the Milky Way dwarf galaxy population. The size of the dot indicates the galaxy’s relative size, and the colour bar represents its distance from the Sun. Image Credit: Pace 2024.
There are also about one dozen ultra-faint compact stellar systems in our galaxy’s halo with unclear origins. Are they star clusters that initially formed in dwarf galaxies that were later disrupted? Or are they “… an extension of the dwarf galaxy population into the small, hyper-faint regime,” as the authors write.
The LSST is poised to find out when it begins its 10-year survey sometime in 2025. To do that, the Vera Rubin Observatory must be able to differentiate between Milky Way satellite galaxies and distant galaxies.
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Satellites like the Large Magellanic Cloud are easy to spot and have been known since antiquity. However, there are increasingly small and dim objects that are also satellites. The LSST is poised to find many more of them. Image Credit: By ESO/VMC Survey – CC BY 4.0, https://commons.wikimedia.org/w/index.php?curid=82120974
This work largely depends on star/galaxy separation, a vital image analysis technique in astronomy based on the fact that stars are an average of five light-years apart in our galaxy. In contrast, galaxies are typically separated by millions of light years. However, this study focuses on satellite galaxies, which are generally hundreds of thousands of light-years away.
In this research, astronomers need to be able to identify individual stars in satellite galaxies. When light from distant background galaxies pollutes the light from individual stars, it creates false positives. When missed stars are classified as galaxies, the results are negatively affected.
If scientists can generate perfect star/galaxy separation, that would result in optimal detection of Milky Way satellite galaxies. Of course, that’s not how things work. Current star/galaxy separation techniques are far from perfect, and developing more powerful techniques is essential for the upcoming Legacy Survey of Space and Time.
In this work, the researchers injected simulated stars with properties from the DC2 data into the known data from the DC2 catalogue. This neat scientific trick allows them to test how well the LSST will detect the injected stars. Detecting stars accurately means detecting dwarf galaxies accurately.
“We predict the sensitivity of the Vera C. Rubin Observatory Legacy Survey of Space and Time (LSST) to faint, resolved Milky Way satellite galaxies and outer-halo star clusters,” the paper states. “We inject simulated stars into the DC2 catalogue with realistic photometric uncertainties and star/galaxy separation derived from the DC2 data itself.”
The researchers found that the ability to detect stars in satellite galaxies depends on the magnitude of the star in question and its galaxy’s half-light radius. For a moderately compact, bright stellar system that could be either a dwarf galaxy or a star cluster, their method is 50% efficient at successfully identifying dwarf galaxies out to about 250 kiloparsecs (800,000 light-years) from the Sun.
This means that their system can potentially detect anywhere from 89 to 119 satellite galaxies. Since astronomers know of about 30 satellite galaxies now, the LSST could triple that number. “When assuming perfect star/galaxy classification and a model for the galaxy-halo connection fit to current data, we predict that 89 +/- 20 Milky Way satellite galaxies will be detectable with a simple matched-filter algorithm applied to the LSST wide-fast-deep data set,” the researchers explain.
This figure illustrates some of the research results. Models predict there could be about 300 Milky Way satellites, labelled the Fiducial Satellite Population. The other lines show three predictions for the LSST, and the red line indicates currently known satellites. Image Credit: Tsiane et al. OJA 2025.
According to the authors, the LSST will preferentially find satellites at larger distances, lower luminosities, and fainter surface brightness. This amounts to a new population of Milky Way satellites that has gone unobserved so far. “This new population of satellites will help advance our understanding of the threshold of galaxy formation,” they write.
The authors also explain that there’s significant room for improvement when it comes to detecting satellites. Upcoming missions like the Nancy Grace Roman Telescope will help, unless it becomes a victim of massive cuts to NASA’s budget.
“The wealth of information arising from our continued detection of small-scale structure holds valuable insights into galaxy formation, reionization, the formation of heavy elements and dark matter microphysics,” the authors conclude. “The discovery and measurement of these systems will continue to be an invaluable probe into the fundamental nature of our universe.”