Untangling the Galaxy I: Local Structure and Star Formation History of
  the Milky Way
  Gaia DR2 provides unprecedented precision in measurements of the distance and
kinematics of stars in the solar neighborhood. Through applying unsupervised
machine learning on DR2's 5-dimensional dataset (3d position + 2d velocity), we
identify a number of clusters, associations, and co-moving groups within 1 kpc
and $|b|<30^\circ$ (many of which have not been previously known). We estimate
their ages with the precision of $\sim$0.15 dex. Many of these groups appear to
be filamentary or string-like, oriented in parallel to the Galactic plane, and
some span hundreds of pc in length. Most of these string lack a central
cluster, indicating that their filamentary structure is primordial, rather than
the result of tidal stripping or dynamical processing. The youngest strings
($<$100 Myr) are orthogonal to the Local Arm. The older ones appear to be
remnants of several other arm-like structures that cannot be presently traced
by dust and gas. The velocity dispersion measured from the ensemble of groups
and strings increase with age, suggesting a timescale for dynamical heating of
$\sim$300 Myr. This timescale is also consistent with the age at which the
population of strings begins to decline, while the population in more compact
groups continues to increase, suggesting that dynamical processes are
disrupting the weakly bound string populations, leaving only individual
clusters to be identified at the oldest ages. These data shed a new light on
the local galactic structure and a large scale cloud collapse.
