The Kinematically Hot, Extremely Metal-Poor C-19 Stellar Stream in DESI DR2

TL;DR

This paper introduces a new mixture model approach using DESI data to characterize the C-19 stellar stream, revealing its velocity dispersion, metallicity, and a new feature, advancing understanding of metal-poor stellar streams.

Contribution

The paper presents a novel method combining proper motions, radial velocities, and metallicities to identify and analyze stellar streams, applied here to the C-19 stream with new member stars and detailed properties.

Findings

Identified 47 member stars, 41 newly found.

Measured velocity dispersion of 7.8 km/s.

Determined mean metallicity of [Fe/H] = -3.36.

Abstract

Stellar streams are the result of a host galaxy's gravitational potential tidally disrupting satellite dwarf galaxies and globular clusters (GCs), causing them to grow leading and trailing tidal tails. The C-19 stellar stream is an extremely metal-poor stellar population, showing chemical abundance patterns characteristic of a globular cluster. However, its large velocity dispersion is difficult to reconcile with a conventional, purely baryonic, disrupting-GC progenitor. Current techniques for stream characterization are primarily applied to Gaia DR3, relying heavily on proper motion measurements. Using the Dark Energy Spectroscopic Instrument (DESI), which provides radial velocities and metallicites for over 10 million stars reaching significantly fainter magnitudes than comparable surveys, we employ a mixture model approach to jointly characterize stream populations in proper motions, radial velocities, and metallicities against a Milky Way halo background. By applying this framework to the C-19 stellar stream, we identify a total of 47 spectroscopically confirmed member stars, of which 41 are newly identified and only 6 were previously reported in the literature. In this work, we measure a velocity dispersion of $7.8^{+1.5}_{-1.3}$ km s$^{-1}$ and a mean metallicity of [Fe/H] = $-3.36^{+0.12}_{-0.10}$. We further identify a novel 'spur' feature within the stream. We conclude that our measurements are in line with previous works identifying C-19 as a 'hot', metal-poor stream. In forthcoming work, we will apply this approach to many more streams in the DESI footprint, enabling population-level comparisons with predictions from simulations.