  The Fermi Large Area Telescope (Fermi-LAT) has been widely used to search for
Weakly Interacting Massive Particle (WIMP) dark matter signals due to its
unparalleled sensitivity in the GeV energy band. The leading constraints for
WIMP by Fermi-LAT are obtained from the analyses of dwarf spheroidal galaxies
within the Local Group, which are compelling targets for dark matter searches
due to their relatively low astrophysical backgrounds and high dark matter
content. In the meantime, the search for heavy dark matter with masses above
TeV remains a compelling and relatively unexplored frontier. In this study, we
utilize 14-year Fermi-LAT data to search for dark matter annihilation and decay
signals in 8 classical dwarf spheroidal galaxies within the Local Group. We
consider secondary emission caused by electromagnetic cascades of prompt gamma
rays and electrons/positrons from dark matter, which enables us to extend the
search with Fermi-LAT to heavier dark matter cases. We also update the dark
matter subhalo model with informative priors respecting the fact that they
reside in subhalos of our Milky Way halo aiming to enhance the robustness of
our results. We place constraints on dark matter annihilation cross section and
decay lifetime for dark matter masses ranging from $10^3$ GeV to $10^{11}$ GeV,
where our limits are more stringent than those obtained by many other
high-energy gamma-ray instruments.
