# Butterfly in a Cocoon, Understanding the origin and morphology of Globular Cluster Streams: The case of GD-1

## TL;DR

This study reveals a diffuse, extended stellar component around the GD-1 globular cluster stream, suggesting an extragalactic origin and providing a new morphological marker to distinguish between in-situ and accreted streams.

## Contribution

The paper uncovers a secondary diffuse component around GD-1, indicating its likely formation outside the Milky Way and proposing a new morphological feature for origin identification.

## Key findings

- Detection of a 100 pc wide diffuse component around GD-1 at >5σ confidence.
- Synthetic streams from dark matter sub-halos show similar morphological features.
- The cocoon component may distinguish between in-situ and ex-situ globular cluster streams.

## Abstract

Tidally disrupted globular cluster streams are usually observed, and therefore perceived, as narrow, linear and one-dimensional structures in the 6D phase-space. Here we show that the GD-1 stellar stream ($\approx$ 30 pc wide), which is the tidal debris of a disrupted globular cluster, possesses a secondary diffuse and extended stellar component ($\approx$ 100 pc wide) around it, detected at >5$\sigma$ confidence level. Similar morphological properties are seen in synthetic streams that are produced from star clusters that are formed within dark matter sub-halos and then accrete onto a massive host galaxy. This lends credence to the idea that the progenitor of the highly retrograde GD-1 stream was originally formed outside of the Milky Way in a now defunct dark satellite galaxy. We deem that in future studies, this newly found $cocoon$ component may serve as a structural hallmark to distinguish between the in-situ and ex-situ (accreted) formed globular cluster streams.