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

TL;DR

This study reveals eight compact Petit--Bullets around the previously known Bullet in the W44 molecular cloud, each displaying broad V-shaped velocity features in ALMA CO $J=3{-}2$ data. The consistent morphologies and kinematics across Bullet Main and PBs support an extended shooting model in which a small cluster of compact objects plunges into the dense molecular layer, with the Bullet Main likely an isolated black hole and the PBs stellar-mass remnants. Virial and momentum analyses yield a dynamical mass scale of $ ext{M}_ ext{VT} \,\sim\, 1\times10^5\,\text{M}_⊙$, comparable to globular clusters, and imply a halo-origin population with counter-rotating velocities relative to Galactic rotation. The findings suggest dynamical interactions between halo clusters and disk gas, offering a potential new avenue to study compact remnants and their environments, and motivate deeper multi-wavelength follow-ups to test the nature of the plunging objects.

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.

Figures (4)

• Figure 1: (a) Velocity-integrated map and (b) longitude-velocity l--V map of the ASTE CO J = 3--2 data 2017ApJ...834L...3Y. The integration range is $\hbox{$V_{\rm LSR}$}=-90$ to $+60$ km s$^{-1}$, and the intensity is averaged over $b=-0\fdg4716$ to $-0\fdg4722$. The black rectangles indicate the area covered by our ALMA observation. (c) Velocity-integrated map and (d) l--V map of the ALMA CO J = 3--2 data. The integration and averaging ranges in velocity and latitude are identical to those in panels (a) and (b), respectively.
• Figure 2: Velocity channel maps of the Bullet in the CO J = 3--2 line. Each panel shows the integrated intensity over a velocity range of 10 km s$^{-1}$ from $\hbox{$V_{\rm LSR}$} = -90$ to $+60\ \hbox{km s$^{-1}$}$. The number in the upper-left corner of each panel indicates the central velocity in km s$^{-1}$.
• Figure 3: (a) Velocity-integrated CO $J$ =3--2 map of the Bullet over $\hbox{$V_{\rm LSR}$}=-130$ to $+60\ \hbox{km s$^{-1}$}$. Magenta crosses denote the positions of the Petit--Bullets. Black arrows outside the panel indicate the Galactic coordinates where the position--velocity b--V and l--V maps shown in panels (b)--(f) are extracted. (b)--(c) The b--V maps at $l=34\fdg7161$ and $34\fdg7288$ while (d)--(f) The l--V maps at $b=-0\fdg4601$, $-0\fdg4615$ and $-0\fdg4650$, respectively.
• Figure 4: (a) Schematic of the shooting model in which the plunging object moves obliquely with respect to the line of sight. Red arrow indicates the plunge direction. The yellow dashed circle marks the low-velocity accretion region centered at the yellow cross, and the black dot represents the high-velocity peak. (b) $l$--$b$ map viewed along the line of sight, corresponding to the observed morphology for an oblique plunge. (c) Position--velocity diagram cut along the plunge direction (red arrows), showing a distorted Y-shaped feature.