Piercing the Milky Way: an all-sky view of the Orphan Stream (1812.08172v2)

Abstract: We use astrometry, broad-band photometry and variability information from the Data Release 2 of ESA's Gaia mission (GDR2) to identify members of the Orphan Stream (OS) across the whole sky. The stream is traced above and below the celestial equator and in both Galactic hemispheres, thus increasing its visible length to ~ 210 degrees equivalent to ~150 kpc in physical extent. Taking advantage of the large number of RR Lyrae stars in the OS, we extract accurate distances and proper motions across the entire stretch of the tidal debris studied. As delineated by the GDR2 RR Lyrae, the stream exhibits two prominent twists in its shape on the sky which are accompanied by changes in the tangential motion. We complement the RR Lyrae maps with those created using GDR2 Red Giants and the DECam Legacy Survey Main Sequence Turn-Off stars. The behavior of the OS track on the sky is consistent across all three tracers employed. We detect a strong non-zero motion in the across-stream direction for a substantial portion of the stream. Such a misalignment between the debris track and the streaming velocity cannot be reproduced in a static gravitational potential and signals an interaction with a massive perturber.

Overview of the Orphan Stream Study

The paper "Piercing the Milky Way: an all-sky view of the Orphan Stream," provides a comprehensive analysis of the Orphan Stream (OS), utilizing data from ESA's Gaia Data Release 2 (GDR2) alongside other observational platforms. The Orphan Stream is a stellar tidal stream in the Milky Way, believed to originate from a disrupting satellite galaxy or globular cluster, yet its progenitor remains unidentified. This paper traces the OS across an unprecedented span of 210 degrees on the sky, equivalent to about 150 kpc, revealing its complexity and depth through astrometry, photometry, and variability metrics.

Key Findings and Methodology

• All-Sky Mapping: Utilizing Gaia's precision in astrometry and photometry, the paper maps the OS across the entire sky, extending its known domain significantly. The researchers analyze RR Lyrae stars, known for their utility in distance determination due to their well-calibrated period-luminosity relationship, allowing accurate measurements across the stream's path.
• Stream Morphology: The paper reveals two prominent twists or curvatures within the stream, noticeable when comparing RR Lyrae maps to those developed using Red Giants and Main Sequence Turn-Off stars from Gaia and the Dark Energy Camera Legacy Survey (DECaLS).
• Kinematic Behavior: Importantly, the paper detects a non-zero motion in the across-stream direction for a portion of the OS, suggesting an interaction with a massive perturber. Such misalignment between the debris track and streaming motion cannot be explained by a static gravitational potential, indicating the influence of external forces.
• Progenitor Characteristics: The number of RR Lyrae stars identified suggests a progenitor that was likely a classical dwarf galaxy rather than a globular cluster, with luminosity estimates placing it amongst the brighter Milky Way satellite galaxies.
• Theoretical Implications: The unexpected kinematic and morphological features imply perturbations due to dark matter subhalos or other massive structures in the Milky Way, affecting the standard models of stream dynamics. The paper also raises questions about the potential presence of additional stellar populations previously unrecognized within the stream.

Implications and Future Directions

The findings provide valuable insights into the dynamics and composition of the Milky Way halo, offering potential constraints on its gravitational potential and dark matter distribution. The paper invites further investigation into the OS's kinematic anomalies, advocating the need for advanced modeling that can account for dynamic gravitational interactions.

Future research could benefit from targeted spectroscopic campaigns to measure the radial velocities and chemical abundances of stars in different sections of the stream, potentially clarifying the nature of the perturbing mass and refining models of the Milky Way’s gravitational environment. Moreover, the exploration of similar streams with Gaia and next-generation surveys might illuminate the broader context of tidal disruption phenomena and Galactic evolution.

Conclusion

This paper presents a detailed and expansive view of the Orphan Stream, leveraging multiple data sources to decode its intricate structure and behavior. While the progenitor remains elusive, the paper advances our understanding of stellar streams and the complex interactions governing galactic dynamics. The transformative era of Gaia has ushered in new possibilities for unraveling the mysteries of the Milky Way's halo and the enigmatic forces at play within it.