PHYSICAL CONTACT BETWEEN THE +20 km s⁻¹ CLOUD AND THE GALACTIC CIRCUMNUCLEAR DISK

Abstract

This paper reports the discovery of evidence for physical contact between the Galactic circumnuclear disk (CND) and an exterior giant molecular cloud. The central 10 pc of our Galaxy has been imaged in the HCN J=1-0, HCO + J=1-0, CS J=2-1, H 13 CN J=1-0, SiO J=2-1, SO N J =2 3 -1 2 , and HC 3 N J=11-10 lines using the Nobeyama Radio Observatory 45 m radio telescope. Based on our examination of the position-velocity maps of several high-density probe lines, we have found that an emission "bridge" may be connecting the +20 km s −1 cloud (M-0.13-0.08) and the negative-longitude extension of the CND. Analyses of line intensity ratios imply that the chemical property of the bridge is located between the +20 km s −1 cloud and the CND. We introduce a new interpretation that a part of the CND may be colliding with the 20 km s −1 cloud and the collision may be responsible for the formation of the bridge. Such collisional events could promote mass accretion onto the CND or into the inner ionized cavity, which may be further tested by proper motion studies.

A common origin for the circumnuclear disc and the nearby molecular clouds in the Galactic Centre

“…We do not detect, in this diagram, any feature similar to the negative-longitude extension (NLE), as visible in the longitude-velocity diagram in Fig. 1 of Takekawa et al (2017). ticular, these authors found 13 H2O masers coincident with submm continuum detected clumps and further 5 gravitationally bound clumps detected only in the submm continuum, for a total of 18 potential protostars.…”

“…8 in Hsieh et al 2017 and Fig. 1 in Takekawa et al 2017) and it is coherent with the orbital motion of the cloud (i.e., the CND is corotating with the +20 km/s cloud). The infalling molecular cloud is occupying the right part of the PV diagram, with a median line-of-sight velocity of roughly +20 km/s, though with a moderately larger spread in velocity, compared to the observations.…”

“…Recent observations might show indications of this being the case (cf. with the bridge in Takekawa et al 2017 and the extW streamer in Hsieh et al 2017). On the other hand, as already discussed in section 4, most of the current models connect the +20 km/s and +50 km/s clouds on the southeast.…”

“…According to some authors, these two clouds orbit around the Galactic Centre at distances of 50-100 pc from the centre of the Galaxy, possibly in a larger belt composed of many molecular clouds (Güsten & Downes 1980;Güsten et al 1981;Zylka et al 1990;Kruijssen et al 2015;Henshaw et al 2016;Kruijssen et al 2019). On the other hand, many authors agree on rather placing them at distances 20 pc, based on their possible interaction with SgrA East (e.g., Genzel et al 1990;McGary et al 2001;Herrnstein & Ho 2005;Lee et al 2008) and on their internal velocity pattern and the presence of several streamers of gas connecting them to the CND (e.g., Okumura et al 1989;Coil & Ho 1999Ferrière 2012;Oka et al 2012;Liu et al 2012;Hsieh et al 2017;Takekawa et al 2017;Tsuboi et al 2018). In particular, the so-called southern streamer seems to continously extend from the +20 km/s cloud to the CND, feeding it and possibly the inner pc of the Galaxy (in particular, Okumura et al 1991;Coil & Ho 1999Montero-Castaño et al 2009).…”

The Fifth Candidate for an Intermediate-mass Black Hole in the Galactic Center

“…• 088. The emission layer distributed over V LSR 10 to 40 km s −1 is from M-0.13-0.08 (also referred to as the 20 km s −1 cloud), which probably lies immediately in front of the circumnuclear disk (e.g., Takekawa et al 2017a). T1 has the broadest velocity width, which reaches up to ∆V ∼ 100 km s −1 in full width at zero intensity, showing a remarkably steep velocity gradient.…”