ALMA Central molecular zone Exploration Survey (ACES) VI: ALMA Large Program Reveals a Highly Filamentary Central Molecular Zone

Abstract

The Central Molecular Zone (CMZ) of the Milky Way is the way station that primarily controls how much gas flows from the disk of the Galaxy towards the central nucleus. While this region is well documented to have extreme gas properties that clearly distinguish it from the rest of the Galaxy, the properties of the bulk molecular gas at high angular resolution are relatively unexplored. Band 3 data from the ALMA (Atacama Large Millimeter/Submillimeter Array) large program ACES (ALMA CMZ Exploration Survey) reveal the highly filamentary nature of CMZ molecular gas at high resolution (3" or 0.1pc) across the entire CMZ. Visual inspection of these data suggests that there are at least two general classes of elongated structures, which we identify as: i) large-scale (10 pc) filamentary structures (LFs) and ii) a ubiquitous population of small-scale (about 1 pc) filamentary structures (SFs). We present detailed morphological and kinematic properties towards three structures in each category, as well as their association with magnetic fields and the correlation of HNCO 4(0,4)-3(0,3) with other molecular species. Our investigation reveals that these structures are largely coherent in position-position-velocity space. The alignment with the magnetic field structure is mixed, with some parallel, some perpendicular, and some intermediate alignments. We find that LFs likely trace pieces of contiguous CMZ orbital structures and are a manifestation of global CMZ dynamics. The second class, SFs, are pervasive and may be the result of complicated turbulence and shearing dynamics in the CMZ gas flows, as seen in numerical simulations.

Figures (9)

• Figure 1: The ACES HNCO 4(0,4)--3(0,3)~data reveal the ubiquity of molecular filamentary structures in the CMZ on scales from tens to tenths of pc. The central panel shows a peak intensity map of HNCO 4(0,4)--3(0,3)~from ACES and our sample selection in the colored boxes. The top three zoom-ins show the three selected Large-scale Filamentary Structures (LFs) in HNCO, integrated over the velocity range of each individual structure (see Table \ref{['tab:filament_striation_statistics']}). The bottom three zoom-ins show three characteristic regions highlighting the preponderance of Small-scale Filamentary Structures (SFs) in the molecular gas of the CMZ. Within these regions, we select one characteristic SF for quantitative analysis. The zoom-ins on the bottom show HNCO integrated over the velocity range of the selected SFs (see Table \ref{['tab:filament_striation_statistics']}).
• Figure 2: Small-scale filamentary structures (SFs) are ubiquitous in molecular gas in the CMZ as traced with HNCO 4(0,4)--3(0,3)~by ACES. This image shows a zoom-in on the three cyan SF fields displayed in Figure \ref{['fig:overview']}, the M0.8–0.2 ring region Nonhebel2024 (top), the SgrB2 extended region (middle), and the Dust Ridge region (bottom). For each zoom-box, we display the peak intensity over the entire HNCO cube in the top left panel, then step through 5 kms$^{-1}$ intervals with the corresponding moment 0 map of the velocity range displayed in subsequent panels. We highlight the location of SFs 1, 2, and 3 within the map in blue boxes, at the velocity most closely associated with each SF.
• Figure 3: A morphological comparison of the three LFs and SFs is presented in the left panels, which highlights both their similarities (extended filamentary structures coherent in PPV space) and their differences (size scales). The right panels show position-velocity diagrams extracted along the spines (shown as the white line along each filament in left panels, described in Section \ref{['sec:method_morphology']}), and show largely coherent structures in PV space. The left panels show HNCO 4(0,4)--3(0,3)~moment 0 maps (over the velocity ranges listed in Table \ref{['tab:filament_striation_statistics']}) of each structure rotated so that they are all roughly parallel. The position angle of the filament spines with respect to the Galactic plane is given in the top left of each image. The white stars denote the starting position used for the PV diagrams shown in the right panels. The white (black for SF 2) dashed line on the PV plots show the fit used to calculate the filament velocity gradients (Table \ref{['tab:filament_striation_statistics']}). The blue dotted lines in LF3 and SF3 represent alternate slopes (1.0 and 2.6 $km s^{-1} pc^{-1}$ respectively) to the velocity gradients provided in Table \ref{['tab:filament_striation_statistics']}. The SFs show clear contiguity over their short lengths, while the LFs have small gaps in the velocity contiguity over their 10+ pc lengths (most notably LF 1 in the center).
• Figure 4: The LFs identified in this work appear to trace the CMZ orbital streams, while the SFs are embedded within CMZ cloud structures. This image shows the LFs and SFs identified in this work on top of the kinematic decomposition of Mopra HNCO data created by Henshaw2016a (gray background points). The darkness of the gray points is proportional to the peak intensity of each point's spectral component. The closed ellipse orbital model from Walker2025 is shown as a gray line. The colored points correspond to LF 1 (red), LF 2 (orange), and LF 3 (yellow), SF 1 (green), SF 2 (cyan), and SF 3 (magenta). Light blue boxes are included to help identify the location of the SFs. The extraction of PPV points for the LFs and SFs is discussed in Section \ref{['sec:method_kinematic']}. We refer the reader to Walker2025 and related resources for an interactive 3D CMZ viewer.
• Figure 5: A qualitative comparison of POS magnetic field orientations from FIREPLACE (yellow lines) with the LFs shows that LF 1 (top) is mostly perpendicular, LF 2 (middle) is mostly parallel, and LF 3 (bottom) is mixed. Grayscale shows the HNCO 4(0,4)--3(0,3)~moment 0 map (integrated over the velocity extent of the structure) with yellow line segments indicating magnetic field pseudovectors derived from the FIREPLACE survey. The line segment lengths are scaled by percentage polarization. Inset rectangle indicates the length of a pseudovector at the 5% level.