3D dynamics of the Orion cloud complex -- Discovery of coherent radial
  gas motions at the 100-pc scale
  We present the first study of the 3D dynamics of the gas in the entire
southern Orion cloud complex. We used the parallaxes and proper motions of YSOs
from Gaia DR2 as a proxy for gas distance and proper motion, and the gas radial
velocities from archival CO data, to compute the space motions of the different
star-forming clouds in the complex, including subregions in Orion A, Orion B,
and two outlying cometary clouds. From the analysis of the clouds' orbits in
space and time, we find that they were closest about 6 Myr ago and are moving
radially away from roughly the same region in space. This coherent 100-pc scale
radial motion supports a scenario where the entire complex is reacting to a
major feedback event, which we name the Orion-BB (big blast) event. This event,
which we tentatively associate with the recently discovered Orion X stellar
population, shaped the distribution and kinematics of the gas we observe today,
although it is unlikely to have been the sole major feedback event in the
region. We argue that the dynamics of most of the YSOs carry the memory of the
feedback-driven star formation history in Orion and that the majority of the
young stars in this complex are a product of large-scale triggering, which can
raise the star formation rate by at least an order of magnitude, as for the
head of Orion A (the Integral Shape Filament). Our results imply that a
feedback, compression, and triggering process lies at the genesis of the Orion
Nebula Cluster and NGC2023/2024 in Orion B, thus confirming broadly the
classical feedback-driven scenario proposed in Elmegreen & Lada (1977). The
space motions of the well-known young compact clusters, $\sigma$ Orionis and
NGC 1977, are consistent with this scenario. A momentum estimate suggests that
the energy of a few to several supernovae is needed to power the coherent 3D
gas motion we measure in this paper.
