Molecular Gas Feeding the Circumnuclear Disk of theGalactic Center

Hsieh, Pei-Ying; Koch, Patrick; Ho, Paul; Kim, Woong-Tae; Tang, Ya-Wen; Wang, Hsiang-Hsu; Yen, Hsi-Wei; Hwang, Chorng-Yuan

Academia Sinica Institute of Astronomy and Astrophysics (ASIAA); East Asian Observatory (EAO); Department of Physics & Astronomy, Seoul National University; European Southern Observatory (ESO);
Department of Physics and Institute of Theoretical Physics, The Chinese University of Hong Kong; Institute of Astronomy, National Central University
Hsieh et al. 2017, ApJ, 847, 3 (arXiv: 1708.08579)

The Formation of Molecular Streamers
The origin of the 2 pc circumnuclear disk (CND) in the Galactic center (GC)
has remained unclear in spite of decades of intensive study (e.g. Herrnstein &Ho 2002). The CND is a ring-like molecular structure rotating with respect tothe supermassive black hole SgrA*. The CND, being the closest molecularreservoir in the GC, is critical for the understanding of the feeding of thenucleus. The replenishment of the CND itself, therefore, is an important
problem. Previous NH3(3,3) observations carried out with the Very Large
Array (VLA) have detected several streamers (contours in the top left image;
Herrnstein & Ho 2002). The 20 km/s cloud (hereafter 20 MC) and the 50 km/s
cloud (hereafter 50 MC) lying 20 pc south of SgrA*, appear tomorphologically connect to the CND with several NH3(3,3) streamers. The
20/50 MC are seen in our new NRO 45-m CS(J=2−1) data (gray; Hsieh et al.
2016). The NH3(3,3) streamers are also detected in the high resolution SMA
HCN HCN(J=4−3) map (contours in the bottom left image). However, thephysical link between the streamers and the 20/50 MC are limited by thesmall field of view and missing flux of the interferometric maps. In our highresolution CS(J=2-1) map, we found clear evidences of the origins of thestreamers in the longitude–velocity diagrams (lv-diagrams) (top right image).
The origin of the streamer surrounding the CND might be from the 50 MC. A
part of the 50 MC shows elongation in the lv-diagrams from b= −0.067 andseems to be stretched toward the streamers. The stretching of substructuresin the 50 MC suggests that the gas is being tidally disrupted and dragged tothe inner orbit with progressively higher velocities up to -120 km/s. This tidal
stretching is also clearly seen in the CS(J=5-4) data (bottom right image)(Hsieh al al. 2015) (more information see Hsieh et al. 2017)

Phase Plots - Rotation and Inward Motion

However, the convention lv-diagrams are not suitable for a comparison because one fixed longitude corresponds to different radii of the ring. Therefore, in order to clarify the kinematics ofthe connection between the streamers and the 20/50 MC, we show position-velocity diagrams drawn on trajectories along the ring (phase plot; e.g., Jackson et al. 1993). The phase plots
extracted at intrinsic radii of 5, 4, 3.5, and 2.5 pc (red elliptical annulus shown in the MOM0 maps) are presented for both the CS(J=2-1) and HCN(J=4-3) data. Starting position (0°) of thetrajectory is labeled by the dot, and the phase is counterclockwise. Relative to the lower-velocity components of the 20/50 MC, high-velocity components are seen in both CS and HCN withmaximum velocities of -120 km/s and 120 km/s, which correspond to the streamers and the CND from outer (4, 5 pc) to inner (3.5, 2.5 pc) radii, respectively. Pure Keplerian (blue curve) andKeplerian plus infall models are overlaid on the phase plots with corresponding radii from left to right panels (inclination=65°, PA=6°). We find that the gross features of streamers cannot be
described by a pure Keplerian motion, where the max/min values are at θ =0 or 180° if the gas was subject to pure rotation. In fact, we find an infall motion (Vinfall = 20–70 km/s) is needed toexplain the gross features of the streamers. The loci of the max/min Vrad are at θ = −20 and 160° and indicate that there should be non-negligible radial gas motions, which we found toamount to 20–70 km/s. This range in infall motion defines a band in the phase plot that appears to capture the extended emission of the streamers. Besides, we also find that the fitted
curves connect the inner edges of the 50 MC, as it is shown in the lv-diagrams. We further find that the western streamers seem to intersect with the CND with increasing velocities, whichsuggests replenishing gas for the CND.

Gas Replenishment of the CND though Streamers
Three-color image is shown in the left panel. The blue color
shows the CS(J=2−1) data, the red color shows the SMAHCN(J=4−3) data, and the green color shows the SgrA West in the6-cm. Right: schematic of the configuration of the 20/50 MC, theCND, SgrA East, and the streamers. Our data show that theclumps are ripped away from the 50 MC and form the streamers.
The streamers can be described by a Keplerian rotation and bythe infall motion. The western streamer intersects with the CND
as shown in the phase plots, supplying replenishing gas for theCND. If the accretion rate is constant over 105 years, then thelower limits of the accretion-to-mass fractions onto the CND
are in the range of 0.02%–2%. An accurate measurement of
the CND mass will be essential to evaluate the lifetime of theCND.