The Study of the Milky Way CGM with the Line Emission Mapper

The Line Emission Mapper (LEM) is a proposed NASA probe class mission which will combine ~ 1600 cm2 effective area at 0.5 keV (2600 cm2 at 1 keV) with microcalorimeter 2 eV spectral resolution and 15” spatial resolution over 30’ by 30’ field of view in the soft X-ray band (0.2-2 keV). The unprecedented grasp for a spectroscopic mission makes it possible to build sensitive maps for very large sky areas over short periods of time, and an all-sky survey is currently considered as part of the mission.

LEM will map the velocities of the inner regions of the Milky Way CGM, and in particular, the expansion of the Fermi/eROSITA bubbles, believed to be evidence of feedback from either the SMBH or star-forming regions in the Galactic Center. It will map the temperature structure of the inner CGM across the sky and along the line of sight using lines of the various ion species - something only a calorimeter can do, in the presence of multiple temperature components and solar wind charge exchange emission on each line of sight. The MW observations will complement the studies of CGM in other galaxies (a critical science driver of LEM) where LEM will map the outer halos but have limited insight into the interface between the disk and the halo, where the exchange of mass and metals with the outer CGM takes place.