Discovery of Multiple Ultra-Broad-Velocity Molecular Features Associated with the W44 Molecular Cloud

TL;DR

This paper reports the discovery of multiple ultra-broad-velocity molecular features near W44, interpreted as high-velocity plunges of compact objects, providing evidence for interactions between halo clusters and molecular gas.

Contribution

It introduces the discovery of multiple Petit-Bullets with distinct V-shaped velocity structures, suggesting a new mechanism involving compact objects plunging into dense gas.

Findings

Eight Petit-Bullets with broad velocity widths identified

Features suggest high-velocity plunges of compact objects

Main Bullet likely formed by an isolated black hole

Abstract

We report the discovery of multiple compact molecular features exhibiting extremely broad velocity widths toward the W44 molecular cloud. ALMA CO $J$=3--2 data reveal eight ``Petit--Bullets'' surrounding the previously known ``Bullet.'' Each Petit--Bullet shows a distinct V-shaped structure in position--velocity space, reminiscent of the Y-shaped morphology of the Bullet, suggesting a common origin. These features are interpreted as the result of high-velocity plunges of compact gravitational objects into dense molecular gas. The spatial and kinematic properties of the Petit--Bullets suggest that the plunging material was not a single object but rather a small cluster of compact bodies. A virial mass of $1.0\!\times\! 10^{5}\, M_\odot$ inferred from their velocity dispersion is comparable to that of typical globular clusters. Momentum analysis further implies that the main Bullet likely formed by an isolated black hole. These findings provide new evidence for dynamical interactions between halo clusters and disk molecular gas.