  How many low-mass stars, brown dwarfs and free-floating planets are in the
Milky Way? And how are they distributed in our Galaxy? Recent studies of Milky
Way interlopers in high-redshift observations have revealed a 150-300 pc thick
disk of these cool stars with 7% of the M-dwarfs in an oblate stellar halo. One
can use the High Latitude Survey with the Roman Space Telescope to search for
Galactic ultracool dwarfs (spectral classes M, L, T, and Y) to accurately model
the 3D structure and the temperature and chemical evolution of the Milky Way
disk in these low-mass (sub)stellar objects.
  Accurate typing has been shown to work on HST grism and photometric data
using machine learning techniques. Such an approach can also be applied to
Roman photometry, producing accurate photometric typing to within two subtypes.
The High Latitude Survey provides enough statistical power to model the Milky
Way structural components (thin and thick disks and halo) for M-, L- and
T/Y-dwarfs. This approach has the benefit to allow us to constrain
scale-lengths, scale-heights and densities, as well as the relative position of
our Sun with respect to the disk of dwarf stars of our Milky Way. The total
number of each brown dwarf type can be used to infer both the low-mass end of
the Galaxy-wide Initial Mass Function (IMF) for the first time, the formation
history of low-mass stellar and substellar objects, and the fraction of
low-mass stars in the halo, a statistic that can test cold dark matter
structure formation theories.
