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

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.

Figures (15)

• Figure 1: The black points represent the 47 (41 main sequence + red giant, 6 blue horizontal branch) C-19 member candidates identified in this work. We mark the previously known C-19 member stars that have radial velocity measurements with magenta diamonds. The DESI MWS's footprint from 3 years of observations is shown in grey. We also visualize our fitted C-19 progenitor orbit in MWPotential2014 (§ \ref{['sec:orbit']}; bovy_galpy_2015bovy_galpy_2015), coloured by the galactocentric distance in kpc. The orbit direction is indicated with arrows along its path.
• Figure 2: Chemical and kinematic measurements versus on-sky position ($\phi_1$) for the parameters in the mixture model. We make truncations in these parameters in order to increase the fraction of stream members with respect to the surrounding field; truncated regions are shown in grey. The truncations are informed by the STREAMFINDER (SF; ibata_charting_2021ibata_charting_2023) Gaia DR3 members. The SF members' proper motion values are taken from Gaia DR3, while line-of-sight velocity data either comes from Gaia DR3 or from spectroscopic follow up with the VLT/UVES spectrograph in ibata_charting_2023. Those stars also observed in DESI DR2 are shown as filled green diamonds.
• Figure 3: Histogram showing the distribution of stream membership probability ($P_{\in \text{ stream}}$) for all 2071 stars modelled. The 41 stars with membership probability greater than 50% (grey line) are considered member stars in this work.
• Figure 4: 41 high-probable $(P_{\text{s}} > 50\% , \text{see colourbar})$ member stars in the C-19 stellar stream plus 6 blue horizontal branch members (blue triangle markers) obtained from our box-cuts about the spline tracks. The light grey points are 'low probability' $(P_{\text{s}} > 50\%)$ stars. We plot stars in residual space for $\Delta \mu_\alpha, \Delta\mu_\delta, \text{and } \Delta v_\text{GSR}$. We show the fitted spline tracks from the mixture model in blue for the modelled parameters.The blue error bars along the spline track show the node locations for each spline, and the uncertainties are obtained from the 65% posterior credible intervals. In all panels but metallicity, we show the orbit track (see § \ref{['sec:orbit']}) for a C-19 progenitor within the galpy MilkyWay2014 potential bovy_galpy_2015 in red. Figure \ref{['fig:6_app']} in Appendix \ref{['app:mems']} shows this figure in physical space, rather than as residuals with respect to the spline tracks.
• Figure 5: Absolute and apparent colour-magnitude diagram at the distance of C-19 ($18$ kpc); the apparent magnitude scale shown on the right $y$-axis. The 41 high-probable MS+RGB members are shown with their inferred stream membership probability (see colourbar to the right). We highlight the 6 previously confirmed members radial velocity members with diamond markers (yuan_pristine_2022yuan_pristine_2022; yuan_pristine_2025yuan_pristine_2025), which are all bright giants past the main sequence. We show our dotter_mesa_2016 MS+RGB isochrone at [Fe/H] = $-2.49$ and an age of 13.5 Gyr as the right blue curve; the DESI stars that pass our 0.20 (g-r) magnitude tolerance cut and make it into the mixture model are coloured light blue. The 6 Horizontal branch members from § \ref{['sec:box-cuts']} are shown as blue triangle markers alongside an empirical horizontal branch in blue.