DESI maps C-19, an extremely metal-poor Milky Way stellar stream

Using the Mayall 4-meter telescope at Kitt Peak National Observatory, an international team of astronomers has observed C-19—an extremely metal-poor stellar stream in the Milky Way’s halo. Results of the observational campaign, published March 11 on the arXiv pre-print server, provide crucial insights into the properties of this stellar stream.

Disrupted and stretched

Stellar streams are remnants of dwarf galaxies or globular clusters (GCs) that once orbited a galaxy but have been disrupted and stretched out along their orbits by tidal forces of their hosts. Observations show that many stellar streams are elongated debris of tidally disrupted globular clusters.

Studies of galactic stellar streams could answer some crucial questions about the Milky Way. For instance, they could help us understand the large-scale mass distribution of the galactic dark matter halo. Moreover, the investigation of stellar streams could confirm whether or not our galaxy contains low-mass dark matter subhalos.

Extremely metal-poor stellar stream

With a mean metallicity of below -3.0 dex, the stellar stream C-19 is the most metal-poor stellar population discovered to date. It has a size of about 650 light years and stretches across an arc of over 100 degrees in the sky. The mass of C-19 is estimated to be within the range of 40,000–50,000 solar masses. The stream is located some 58,700 light years away from Earth.

In order to shed more light on the properties and origin of C-19, a group of astronomers led by Nasser Mohammed of the University of Toronto in Canada, has employed Dark Energy Spectroscopic Instrument (DESI) to explore this stellar stream. DESI is a new-generation multi-object survey spectrographic facility operating on the Mayall 4-meter telescope.

“Using DESI, which provides radial velocities and metallicities 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,” the researchers explain.

Properties and uncertain origin

First of all, Mohammed’s team assigned membership probabilities to 2,071 stars, and then designated 41 stars with probabilities of over 50% as high-probable members of C-19. It turned out that 36 of these 41 stars are new spectroscopically confirmed member stars of this stream. Moreover, they identified six blue horizontal branch candidate member stars.

The observations found that C-19 has a velocity dispersion of 7.8 km/s, which is larger when compared with typical stellar streams of GC origin. The metallicity of C-19 was measured to be approximately -3.36 dex, therefore confirming its extremely metal-poor nature.

The study also identified a spur-like feature in the structure of C-19, spatially offset (by about 1,000 light years) from the main stream track. This “spur,” which is approximately 3,000 light years in length, is a kinematically related group of stars that deviates from the expected thin, linear track of the stream.

All in all, the presence of the “spur,” along with a high velocity dispersion of C-19, indicate that this stream is kinematically hot. However, the collected data do not allow to conclusively determine the progenitor of C-19 as its low-metallicity points to a globular cluster, but the spur feature suggests a dwarf galaxy origin.

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