Characterizing the GD-1 Stream with DESI DR2 Data: Thin Stream and Hot Cocoon

TL;DR

This paper presents a comprehensive spectroscopic analysis of the GD-1 stellar stream using DESI DR2 data, revealing its detailed structure, a cold thin component, a hot cocoon, and implications for dark matter substructure.

Contribution

It provides the largest homogeneous spectroscopic catalog of GD-1, updates its phase-space structure, and identifies a hot cocoon component consistent with dark matter heating.

Findings

Identified a cold thin stream with velocity dispersion 2.49 km/s and width 0.23°.

Detected a hot cocoon component with 30% of members and velocity dispersion 6.13 km/s.

Observed a large proper motion dispersion indicating line-of-sight distance spread.

Abstract

GD-1 is among the longest, coldest stellar streams in the Milky Way, making it an ideal target for probing dark matter substructure through dynamical heating. We present a catalog of 608 spectroscopically confirmed GD-1 members from the first three years of Dark Energy Spectroscopic Instrument (DESI) observations. This constitutes the largest homogeneous spectroscopic sample of GD-1, doubling the number of members previously available only through heterogeneous compilations combining multiple surveys with different systematics. Using these data, we derive updated stream tracks in sky position, proper motion, and radial velocity that extend over $100^\circ$ of the stream. We apply a Gaussian mixture model to decompose the stream into a dynamically cold thin component ($\sigma_V = 2.49\pm 0.28$ km s$^{-1}$, width $= 0.23\pm0.01^\circ$) and a kinematically hot cocoon ($\sigma_V = 6.13\pm0.75$ km s$^{-1}$, width $= 2.18\pm0.17^\circ$). The cocoon contains $\sim30\%$ of members and its velocity dispersion is consistent with $\sim11$ Gyr of heating by cold dark matter subhalos. We also detect a large proper motion dispersion ($41.36\pm4.98$ km s$^{-1}$) along the stream direction in the cocoon component. This feature indicates a significant line-of-sight distance spread in the cocoon, and its origin will be further explored in a forthcoming paper. These measurements demonstrate the power of DESI spectroscopy for characterizing the multi-component phase-space structure of stellar streams and constraining small-scale dark matter substructure.