Reconciling 3D Models for the Central 10 parsecs of the Milky Way

Abstract

The construction of an accurate 3D model of the Milky Way center is necessary to understand inflow processes that drive its overall evolution, and to compare our Galactic nucleus to other galaxies' nuclei. A main point of contention is the line-of-sight location of sources observed toward the central 10 pc of the Galaxy, including recent star formation (the Sgr A East supernova remnant and Sgr A HII regions) and copious gas (the 50 and 20 km/s molecular clouds, the Circumnuclear Disk, and the Sgr A West ionized "minispiral" that encircles the central supermassive black hole, Sgr A*). Some models place all of these structures within a radius of 5 pc from Sgr A*, while others place the 20 and 50 km/s clouds at a distance of at least 30 - 50 pc away from Sgr A* along the line of sight. We present new radio and millimeter observations of the molecular gas toward the central ~10 pc, from which we have constructed an alternative 3D model that is consistent with both prior radio observations and orbital gas kinematics. Our model places the 20 km/s cloud, 50 km/s cloud, and Sgr A East more than 10 pc in front of Sgr A*. While this model does not conclusively rule out a connection between the 50 and 20 km/s clouds and the circumnuclear disk, we argue that prior evidence for these connections is tenuous, especially given the complex spatial and kinematic overlap of structures along the line of sight.

Figures (13)

• Figure 1: A multiwavelength composite image highlighting structures seen toward the central 10 pc of the Milky Way. Blue/green is VLA data of emission from the (3,3) transition of NH$_{3}$Mills14, highlighting the molecular gas structure in this region. Red/orange is a 6 cm VLA map from Zhao16 showing thermal (HII regions including the minispiral, outlined in light yellow solid lines) and nonthermal (supernova remnants, outlined in orange) radio continuum structures. Spitzer 8$\mu$m data Stolovy06 is shown in a grey scale. The angle of the Galactic plane is indicated with a grey dotted line at $b=-0.06$$^{\circ}$.
• Figure 2: Left: A reproduction of the top-down model of Kruijssen15, showing the location of the 50 and 20 km/s clouds on orbital streams that place them $>$50 pc in front of Sgr A* along the line of sight. The inner $\sim$ 10 parsec region from the Herrnstein05 model is indicated as a box around Sgr A*. Right: A recreation of the model schematic from Lee08, showing a top-down view of the arrangement of sources first suggested by Herrnstein05. As a scale bar was not originally included in this model, we scale this model to the observed major axis of Sgr A East (8.3 pc).
• Figure 3: Left: An integrated intensity map of the NH$_{3}$ (3,3) line. The location of Sgr A* is shown as a red cross. Overlaid on this map are the fields of view of other data sets discussed in this paper. The field of view of the ALMA $^{12}$CO measurements is shown as a dotted circle. The field of view of the ALMA CND mosaics is shown as a solid rectangle. The field of view of the Herrnstein05 NH$_{3}$ mosaic from Figure \ref{['fig:Fig-old-model']} is shown as a dashed square. Right: A map of the velocity in the NH$_{3}$ (3,3) cube at the location of the peak intensity for each pixel. The locations of the CND and the 'Molecular Ridge' connecting the 50 and 20 km/s clouds are indicated. Overlaid on the map are the 1720 MHz OH masers from Sjouwerman08. The velocities of these masers are displayed with the same color bar as the background image
• Figure 4: Left: A peak intensity map of NH$_{3}$ (3,3) overlaid with a slit of width and length used for extracting position-velocity data. Right: A position-velocity diagram of NH$_{3}$ (3,3) observed toward the central 10 pc. The locations of prominent structure, including the 50 km/s cloud, 20 km/s cloud, CND, and 'Western Streamer' are indicated. Line widths from this diagram should not be assumed to be be representative of intrinsic gas properties, due to the hyperfine structure of the (3,3) line, which is indicated (satellite lines are at $\pm$20.7 and $\pm$28.2 km s$^{-1}$.)
• Figure 5: Left: Contours of NH$_{3}$ (3,3) emission from Figure 1 of Herrnstein05, overlaid on the new NH$_{3}$ (3,3) velocity data shown in Figure \ref{['fig:Fig-velocity']}. Right: Features from Herrnstein05 identified on our new NH$_{3}$ (3,3) velocity data. Gas belonging to the CND is traced with solid black contours. The 80 km/s cloud, which overlaps with the CND, is not outlined. The locations of the 'Western Streamer' and 'Northern Ridge' are delineated with dotted black lines. As they spatially overlap with other gas, we also use colored arrows to show the velocity of the indicated feature at each position. Features that appear to be extensions of the 50 and 20 km/s clouds (the 'E. CND' and 'Southern Streamer', respectively) are outlined with dashed contours.