  We report the detection of a non-corotating gas component in a bright
unlensed submillimeter galaxy at z=4.3, COSMOS-AzTEC-1, hosting a compact
starburst. ALMA 0.17 and 0.09 arcsec resolution observations of [CII] emission
clearly demonstrate that the gas kinematics is characterized by an ordered
rotation. After subtracting the best-fit model of a rotating disk, we
kinematically identify two residual components in the channel maps. Both
observing simulations and analysis of dirty images confirm that these two
subcomponents are not artificially created by noise fluctuations and beam
deconvolution. One of the two has a velocity offset of 200 km/s and a physical
separation of 2 kpc from the primary disk and is located along the kinematic
minor axis of disk rotation. We conclude that this gas component is falling
into the galaxy from a direction perpendicular to the disk rotation. The
accretion of such small non-corotating gas components could stimulate violent
disk instability, driving radial gas inflows into the center of galaxies and
leading to formation of in-situ clumps such as identified in dust continuum and
CO. We require more theoretical studies on high gas fraction mergers with mass
ratio of 1:>10 to verify this process.
