Understanding the growth and evolution of galaxies is a critical part of cosmology. We know that massive galaxies like our own Milky Way grew over time by cannibalizing and merging with smaller satellite galaxies. That could be what’s happening right now with the Large and Small Magellanic Clouds, as they’re being tidally disrupted by the Milky Way.
But researchers wonder if dwarf galaxies similar to the Magellanic Clouds have their own, much less massive satellites. New research by astronomers at Dartmouth College aims to broaden our understanding of dwarf galaxies and their satellites. The Lambda CDM model says that galaxies grow massive through mergers, and researchers want to test that idea.
The research is “Identifying Dwarfs of MC Analog GalaxiEs (ID-MAGE): The Search for Satellites around Low-mass Hosts,” and it’s published in The Astrophysical Journal. The lead author is Laura Hunter, a research associate in the Physics and Astronomy Department at Dartmouth College.
ID-MAGE is a survey of low-mass galaxies and their satellites between 4 and 10 megaparsecs away (13 million and 32.6 million light-years). It searched for satellite galaxies around 35 dwarf galaxies roughly as massive as the Large and Small Magellanic Clouds. ID-MAGE is based on data from the Dark Energy Spectroscopic Instrument (DESI) Legacy Imaging Surveys. ID-MAGE found 355 candidate satellite galaxies, 264 of which are new detections.
The 35 galaxies were chosen because they’re different sizes and in different proximities to other galaxies. The idea is to discover how these factors influence the formation of satellite galaxies, and by casting a wide net, the results aren’t skewed by mass and proximity. “A robust sample of satellite galaxies around hosts with a wide range of masses is required to test ΛCDM models and our understanding of galaxy formation and evolution,” the researchers explain.
This figure shows the surrounding environments for three representative low-mass galaxies in the survey. The galaxy on the left is in an isolated environment, while the environment gets more crowded in the middle image and on the right. “The central black point is the host galaxy, and the gray region is the 150 kpc radius search area,” the authors explain. “The green crosses are known galaxies that are less massive than our hosts, the pink squares are SMC-mass galaxies, and the orange diamonds are LMC-mass galaxies.” Image Credit: Hunter et al 2025 ApJ 989 58
Observations show that larger galaxies host more dwarf galaxies, which makes intuitive sense. Their greater mass should allow them to attract and maintain their holds on smaller galaxies. But the question is, does this same relationship hold true for lower mass galaxies? Do they hold even smaller galaxies in thrall?
“With this survey, we’ll be able to test whether those predictions hold true with much smaller host galaxies,” lead author Hunter said in a press release. “Astronomy is a field where you can’t run experiments; all you can do is observe and make as many measurements as you can, and then put that data into a simulation and see whether it reproduces your observations. If it doesn’t, that tells us that there’s something wrong with our assumptions or our model of the universe.”
Determining which of the satellite candidates were actual satellite galaxies was a two-step process. First, the team used an algorithm to remove noise from the images, especially light from other stars or galaxies. Following that, they visually inspected images to eliminate things like image defects.
This figure illustrates how the team’s detection algorithm works. The first image on the left shows the candidate dwarf satellite galaxy. The second panel is the image after masking objects from the Guide Star Catalog. The third image is after further processing, and the final image shows the object detected above the background noise. After the algorithm processed the images, they are visually inspected. Image Credit: Hunter et al 2025 ApJ 989 58
Their results conform to current cosmological models.
“Through a systematic visual inspection campaign, we classify the top candidates as high-likelihood satellites,” the researchers write. “On average, we find 4.0 ± 1.4 high-likelihood candidate satellites per LMC-mass host and 2.1 ± 0.6 per SMC-mass host, which is within the range predicted by cosmological models.”
They also found that their 35 hosts have fewer satellites than Milky-Way mass hosts, and that SMC mass hosts have fewer satellites than LMC mass hosts. This also agrees with Lambda CDM predictions. “Our low-mass hosts also exhibit the trend observed among MW-mass hosts where satellite abundance correlates with host stellar mass, extending this relationship into the dwarf host galaxy mass range,” the researchers explain.
Finding these candidates is just the first phase of the team’s research. Their follow-up work will more strongly confirm their candidate galaxies as actual satellites. It will also determine their masses, their distribution, the amount of gas and dust they contain, and how quickly they form new stars.
“The ultimate goal of ID-MAGE is to create the first statistical view of dwarf satellites around low-mass hosts, substantially extending the range of host masses and environments probed by existing surveys, delivering quantitative constraints for galaxy formation physics in ΛCDM,” the authors explain.
In their conclusion, the authors point out that their ID-MAGE survey already provides important observations into satellite galaxy populations and how they’re influenced by the mass of their host galaxies. “Moving forward, follow-up observations will refine our catalog, enabling detailed analysis of how host mass and environment affect satellite populations. This will lead to a deeper understanding of the galaxy formation and evolution processes in the ΛCDM paradigm,” the authors conclude in their paper.
“Getting the answers will require a lot of resources and telescope time, but the impact will be incredible for understanding the nature of dark matter and galaxy formation at the smallest scale,” said study co-author Burçİn Mutlu-Pakdil, also from Dartmouth College. “Each one of them holds a little clue about the physics of how galaxies form.”