Counting the Milky Way’s Hidden Satellites: The DELVE Census

Astronomers have long suspected that the Milky Way is surrounded by many more small galaxies, called satellites, than we can see. These faint galaxies are important because they tell us about how galaxies form and about the mysterious substance called dark matter. In this new paper, Chin Yi Tan and collaborators present the first results of the DELVE Milky Way Satellite Census, which aims to create the most complete count of these small companions to our galaxy.

Why Satellites Matter

The standard model of cosmology predicts that the Milky Way should be surrounded by hundreds, if not thousands, of small dark matter clumps, some of which should host stars and become visible as satellite galaxies. But for many years, astronomers could only find a few dozen of these galaxies. This mismatch is known as the “Missing Satellites Problem.” Tan and the team emphasize that this problem may not mean the model is wrong, it may simply mean we haven’t looked deep enough or carefully enough to spot the faintest galaxies.

Gathering the Data

To perform their census, the researchers combined data from three large sky surveys: the Dark Energy Survey (DES), the DECam Local Volume Exploration survey (DELVE), and Pan-STARRS1 (PS1). Together, these surveys cover nearly the entire sky away from the Milky Way’s bright disk. By applying strict data cuts and geometric masks to avoid crowded or messy regions, the team ensured that their search focused on places where small galaxies could be reliably found.

How to Find a Faint Galaxy

Satellite galaxies are detected as tiny clumps of stars grouped together on the sky. To identify them, Tan and collaborators used two different search algorithms: one based on maximum likelihood statistics and another based on simpler density maps. By requiring that a candidate galaxy be found by both methods, they minimized the risk of including false positives. They also tested their methods by injecting artificial “fake” galaxies into the data to see how well their algorithms could recover them. This process allowed them to measure the efficiency of their search and understand what kinds of galaxies they might be missing.

What the Census Found

From this careful analysis, the team recovered 49 known satellites within their survey footprint. They also showed that their detection limits excluded some of the faintest or most crowded systems, ensuring the sample was pure even if not complete. Using their models of detection efficiency, they predicted that the Milky Way likely hosts around 265 satellite galaxies within 300,000 light-years, with some uncertainty on that number. They also noted that the satellites are not evenly spread across the sky; instead, some clustering appears near the Large Magellanic Cloud, which itself may have brought in a retinue of smaller galaxies.

Implications for Cosmology

This work shows that the Milky Way’s satellite population is broadly consistent with the predictions of cosmological models once observational limitations are taken into account. The team’s results provide new measurements of the luminosity function (how many galaxies there are at each brightness) and the relationship between galaxy size and brightness. These measurements are crucial for testing theories about dark matter and galaxy formation in the early universe.

Looking Ahead

The DELVE census establishes a rigorous framework for counting satellites with uniform methods. While the census is not yet complete, some faint galaxies likely remain undiscovered, it provides the cleanest picture to date of our galaxy’s companions. With future surveys like the Rubin Observatory’s Legacy Survey of Space and Time, astronomers expect to push the census even further, potentially discovering dozens more of the Milky Way’s hidden neighbors.

Source: Tan