A New Map of Our Galactic Neighborhood: The DECam Field of Streams
In their recent study, Peter Ferguson and Nora Shipp present a new, richly detailed view of our galaxy’s outer structure, or "stellar halo," using data from a powerful instrument called the Dark Energy Camera (DECam). This updated “Field of Streams” gives astronomers a visual guide to the Milky Way’s stellar debris—the leftovers from smaller galaxies and star clusters that have been pulled apart by our galaxy’s gravity over billions of years. The research builds on earlier surveys and highlights just how much progress has been made in understanding the Milky Way’s formation and structure.
Historical Context
The study begins by explaining how wide-area sky surveys have transformed our understanding of the Milky Way. Starting with the Sloan Digital Sky Survey’s original “Field of Streams” in 2006, astronomers began noticing many faint trails of stars—known as stellar streams—stretching across the sky. These streams are remnants of smaller galaxies or star clusters that got too close to the Milky Way and were torn apart by its gravity. Such features not only tell us about the history of our galaxy but also help scientists measure its mass and probe the mysterious presence of dark matter.
Data Sources
To create the updated map, Ferguson and Shipp use imaging from two major surveys—the Dark Energy Survey (DES) and the DECam Legacy Survey (DECaLS)—along with supplemental data from Gaia, a European space observatory. All these surveys used DECam, which is mounted on the Blanco 4-meter telescope in Chile. The DES focused on the southern sky with very high-quality observations, while DECaLS filled in more of the sky but with slightly less depth. Together, they cover about 18,700 square degrees, which is nearly half the sky.
Matched-Filter Technique
The authors wanted to find specific types of stars—old and metal-poor ones—that are most likely to trace out ancient structures in the halo. To do this, they used a method called “matched filtering,” which selects stars based on how they appear in color and brightness. They designed a filter to pick out stars similar to those in globular clusters and dwarf galaxies, across three distance ranges: nearby (10–15 kpc), intermediate (15–26 kpc), and far (26–100 kpc). By coloring the selected stars blue, green, or red based on how far away they are, they built a three-color image that reveals structures across the galaxy’s outskirts.
Major Features Revealed
In the resulting map—shown in their Figure 1—many well-known features leap out. The Sagittarius stream, one of the most prominent and well-studied, wraps around the galaxy like a ribbon. Within the DES region, a host of stellar streams are visible, including Elqui, Tucana III, and Phoenix, each with its own unique story. Some of these streams, like Aliqa Uma and Atlas, show signs of disturbance, hinting at the dynamic forces shaping them. Others connect to famous structures like the Orphan-Chenab stream, originally discovered in Sloan data.
Data Processing and Visualization
The map also includes the locations of nearby dwarf galaxies and globular clusters, which appear as concentrated points or circles. Interestingly, the image reveals areas where data quality differs—for example, the DES region stands out because of its more uniform and deeper imaging. To make the image clearer and reduce confusion from unrelated stars, the authors applied several processing techniques, including corrections for survey depth variations, masking of regions dominated by disk contamination, and morphological operations to fill small gaps.
Future Prospects
Looking ahead, the study highlights that this impressive view is just the beginning. The upcoming Legacy Survey of Space and Time (LSST), led by the Vera C. Rubin Observatory, will soon start scanning the sky with even greater sensitivity. LSST will extend this kind of work to new areas and new depths, promising to uncover even more of the faint structures surrounding our galaxy and further unravel its complex formation history.
Source: Ferguson
