Evidence for a Milky Way Tidal Stream Reaching Beyond 100 kpc

TL;DR

This paper reports the discovery of a distant Milky Way tidal stream beyond 100 kpc using RR Lyrae stars, providing new insights into the galaxy's outer halo and challenging existing models of the Sagittarius stream.

Contribution

It presents the first extensive analysis of a stellar stream beyond 100 kpc, combining multiple surveys and revealing discrepancies with current models of known streams.

Findings

Detection of a new outer halo tidal stream beyond 100 kpc

Inconsistencies with existing Sagittarius stream models

Possible association with globular cluster NGC 2419

Abstract

We present the analysis of 1,207 RR Lyrae found in photometry taken by the Catalina Survey's Mount Lemmon telescope. By combining accurate distances for these stars with measurements for ~14,000 type-AB RR Lyrae from the Catalina Schmid telescope, we reveal an extended association that reaches Galactocentric distances beyond 100 kpc and overlaps the Sagittarius streams system. This result confirms earlier evidence for the existence of an outer halo tidal stream resulting from a disrupted stellar system. By comparing the RR Lyrae source density with that expected based on halo models, we find the detection has ~8 sigma significance. We investigate the distances, radial velocities, metallicities, and period-amplitude distribution of the RR Lyrae. We find that both radial velocities and distances are inconsistent with current models of the Sagittarius stream. We also find tentative evidence for a division in source metallicities for the most distant sources. Following prior analyses, we compare the locations and distances of the RR Lyrae with photometrically selected candidate horizontal branch stars and find supporting evidence that this structure spans at least 60 deg of the sky. We investigate the prospects of an association between the stream and unusual globular cluster NGC 2419.