Phlegethon, a Nearby 75°-long Retrograde Stellar Stream

We report the discovery of a 75°-long stellar stream in the Gaia DR2 catalog, found using the new STREAMFINDER algorithm. The structure is probably the remnant of a now fully disrupted globular cluster, which lies 3.8 kpc away from the Sun in the direction of the Galactic bulge and possesses highly retrograde motion. We find that the system orbits close to the Galactic plane at Galactocentric distances between 4.9 kpc and 19.8 kpc. The discovery of this extended and extremely low surface brightness stream ( G ∼ 34.3 mag arcsec−2), with a mass of only 2580 140 M , demonstrates the power of the STREAMFINDER algorithm to detect even very nearby and ultra-faint structures. Due to its proximity and length, we expect that Phlegethon will be a very useful probe of the Galactic acceleration field.