Unraveling the GD-1 Stream and Its Mysterious Cocoon: A DESI Perspective
The Milky Way is surrounded by streams of stars, remnants of ancient star clusters and dwarf galaxies that have been torn apart by the galaxy’s gravitational forces. One of the most well-studied of these stellar streams is GD-1, a long and narrow structure stretching across the sky. In this paper, Valluri et al. use data from the Dark Energy Spectroscopic Instrument (DESI) to investigate a puzzling feature surrounding GD-1—a broader, kinematically distinct structure called the cocoon.
Mapping the GD-1 Stream with DESI
The DESI instrument, installed on the Mayall 4-meter telescope at Kitt Peak National Observatory, is designed to collect spectra from millions of celestial objects. During a survey validation phase, DESI collected spectroscopic data from stars in the GD-1 region. The researchers used this data to identify 115 new stars belonging to GD-1, greatly expanding the number of confirmed members. These new observations provide high-precision velocity measurements, revealing new details about how the stream moves through space.
A Closer Look at the Cocoon
One of the study’s key findings is the confirmation of a cocoon—a broad, diffuse structure surrounding the thin GD-1 stream. While the central stream is narrow (about 55 parsecs wide) and relatively kinematically “cold” (having a small velocity dispersion), the surrounding cocoon is significantly wider (about 460 parsecs) and much hotter, with stars moving at a broader range of speeds. The researchers found that the chemical composition of stars in the cocoon and the main stream are remarkably similar, suggesting they share a common origin.
Possible Explanations for the Cocoon
The study considers several possible explanations for how the cocoon might have formed:
Pre-accretion tidal stripping: GD-1's original star cluster may have lost stars before being pulled into the Milky Way, leaving behind a broader distribution of stars.
Debris from a larger parent galaxy: GD-1 may have originated in a dwarf galaxy that was gradually disrupted, spilling stars into both the narrow stream and the surrounding cocoon.
Interactions with dark matter subhalos: The gravitational pull of invisible dark matter structures could have heated the stream, causing it to expand.
Influence of the Sagittarius dwarf galaxy: The gravitational effects of this nearby galaxy might have perturbed GD-1, stretching and dispersing its stars.
Implications and Future Research
Understanding the origin of the GD-1 cocoon could provide important clues about how the Milky Way has evolved and how it interacts with its satellites. If the cocoon was shaped by dark matter subhalos, it could offer direct evidence for the small-scale structure of dark matter. Future DESI observations will help refine these models, potentially uncovering new details about the formation and fate of stellar streams.
Source: Valluri
