Into the Darkness: Classical and Type II Cepheids in the Zona Galactica
  Incognita
  The far side of the Milky Way's disk is one of the most concealed parts of
the known Universe due to extremely high interstellar extinction and point
source density toward low Galactic latitudes. Large time-domain photometric
surveys operating in the near-infrared hold great potential for the exploration
of these vast uncharted areas of our Galaxy. We conducted a census of distant
classical and type II Cepheids along the southern Galactic mid-plane using
near-infrared photometry from the VISTA Variables in the V\'ia L\'actea survey.
We performed a machine-learned classification of the Cepheids based on their
infrared light curves using a convolutional neural network. We have discovered
640 distant classical Cepheids with up to ~40 magnitudes of visual extinction,
and over 500 type II Cepheids, most of them located in the inner bulge.
Intrinsic color indices of individual Cepheids were predicted from sparse
photometric data using a neural network, allowing their use as accurate
reddening tracers. They revealed a steep, spatially varying near-infrared
extinction curve toward the inner bulge. Type II Cepheids in the Galactic bulge
were also employed to measure robust mean selective-to-absolute extinction
ratios. They trace a centrally concentrated spatial distribution of the old
bulge population with a slight elongation, consistent with earlier results from
RR Lyrae stars. Likewise, the classical Cepheids were utilized to trace the
Galactic warp and various substructures of the Galactic disk, and to uncover
significant vertical and radial age gradients of the thin disk population at
the far side of the Milky Way.
