  The Line Emission Mapper (LEM) is an X-ray Probe for the 2030s that will
answer the outstanding questions of the Universe's structure formation. It will
also provide transformative new observing capabilities for every area of
astrophysics, and to heliophysics and planetary physics as well. LEM's main
goal is a comprehensive look at the physics of galaxy formation, including
stellar and black-hole feedback and flows of baryonic matter into and out of
galaxies. These processes are best studied in X-rays, and emission-line mapping
is the pressing need in this area. LEM will use a large microcalorimeter
array/IFU, covering a 30x30' field with 10" angular resolution, to map the soft
X-ray line emission from objects that constitute galactic ecosystems. These
include supernova remnants, star-forming regions, superbubbles, galactic
outflows (such as the Fermi/eROSITA bubbles in the Milky Way and their analogs
in other galaxies), the Circumgalactic Medium in the Milky Way and other
galaxies, and the Intergalactic Medium at the outskirts and beyond the confines
of galaxies and clusters. LEM's 1-2 eV spectral resolution in the 0.2-2 keV
band will make it possible to disentangle the faintest emission lines in those
objects from the bright Milky Way foreground, providing groundbreaking
measurements of the physics of these plasmas, from temperatures, densities,
chemical composition to gas dynamics. While LEM's main focus is on galaxy
formation, it will provide transformative capability for all classes of
astrophysical objects, from the Earth's magnetosphere, planets and comets to
the interstellar medium and X-ray binaries in nearby galaxies, AGN, and cooling
gas in galaxy clusters. In addition to pointed observations, LEM will perform a
shallow all-sky survey that will dramatically expand the discovery space.
