Diagonal Ridge pattern of different age populations found in Gaia DR2
  with LAMOST Main-Sequence-Turn-Off and OB type Stars
  We revisit the diagonal ridge feature (diagonal distributions in the $R,
v_{\phi}$ plane) found in $Gaia$ and present timing analysis for it between
Galactocentric distances of $R=7.5$ and 12 \,kpc, using Main-Sequence-Turn-Off
and OB stars selected from the LAMOST Galactic spectroscopic surveys. We
recover the ridge pattern in the $R$--$v_{\phi}$ plane color coded by mean
radial velocity and find this feature is presented from very young (OB stars,
few hundred \,Myr) to very old populations ($\tau$ = 9$-$14 \,Gyr). Meanwhile,
some ridge features are also revealed in the metallicity [Fe/H], [$\alpha$/Fe]
and $v_{z}$ distributions. In the $L_{Z}, v_{\phi}$ plane, one of the ridge
patterns, with constant angular momentum per unit mass, shows variations with
different age populations compared. However, the remaining two ones are
relatively stable, implying there might have two kinds of ridge patterns with
different dynamical origins and evolution.
