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High-speed molecular cloudlets around the Galactic Center supermassive black hole

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Authors : Goicoechea, Javier R. ; Pety, J. ; Chapillon, E. ; Cernicharo, J. ; Gerin, M. ; Herrera, C. ; Requena-Torres, M. A. ; Santa-Maria, M. G

Abstract : We present 1"-resolution ALMA observations of the
circumnuclear disk (CND) and the environment around SgrA*. The images
unveil the presence of small spatial scale CO (J=3-2) molecular
"cloudlets" within the central pc of the Milky Way, moving at high
speeds, up to 300 km/s along the line-of-sight. The CO-emitting
structures show intricate morphologies : extended and filamentary at
high negative-velocities (v_LSR < -150 km/s), more localized and clumpy
at extreme positive-velocities (v_LSR > +200 km/s). Based on the
pencil-beam CO absorption spectrum toward SgrA* synchrotron emission,
we also present evidence for a diffuse gas component producing
absorption features at more extreme negative-velocities (v_LSR < -200
km/s). The CND shows a clumpy spatial distribution. Its motion requires
a bundle of non-uniformly rotating streams of slightly different
inclinations. The inferred gas density peaks are lower than the local
Roche limit. This supports that CND molecular cores are transient. We
apply the two standard orbit models, spirals vs. ellipses, invoked to
explain the kinematics of the ionized gas streamers around SgrA*. The
location and velocities of the CO cloudlets are inconsistent with the
spiral model, and only two of them are consistent with the Keplerian
ellipse model. Most cloudlets, however, show similar velocities that
are incompatible with the motions of the ionized streamers or with gas
bounded to the central gravity. We speculate that they are leftovers of
more massive, tidally disrupted, clouds that fall into the cavity, or
that they originate from instabilities in the inner rim of the CND and
infall from there. Molecular cloudlets, all together with a mass of
several 10 M_Sun, exist around SgrA*. Most of them must be short-lived :
photoevaporated by the intense stellar radiation field, blown away by
winds from massive stars, or disrupted by strong gravitational shears.

Journal : Astronomy&Astrophysics A&A