Probing physics of galaxy formation with a wide-field X-ray microcalorimeter

One of the key drivers of galaxy evolution is the warm (10^5.5–6.5 K), extremely dilute gas residing within the galactic virial halos, known as Circumgalactic Medium (CGM). This gas provides material for star formation and accretion onto the central galactic black hole and recycles ejecta from the galactic inner region. Further afield lies the Intergalactic Medium (IGM), the ultimate depository of matter and energy that escaped from galaxies over cosmic time. CGM and IGM constitute the majority of the baryonic matter in the present-day Universe, but remain largely unexplored because they are so difficult to observe — or even detect. This dramatically limits our understanding of how galaxies grow, which is one of the key scientific challenges identified by the Astro2020 Decadal Survey. We present a mission concept, Light Element Mapper (LEM), optimized to explore the X-ray emission from CGM and IGM. A fast silicon-shell mirror and a microcalorimeter array with a ~2 eV spectral resolution sensitive in the 0.3-2 keV band, covering a 30×30' field of view with 15" pixels, will be able to map the CGM of a nearby galaxy in one pointing, which is beyond the reach of any of the planned missions. Indeed, LEM’s “grasp” — the ability to map faint extended sources such as IGM, GCM, or the Fermi bubbles — will be 20 times that of Athena’s microcalorimeter array. In contrast to stacking or absorption line studies, LEM will determine the physical properties and spatial distribution of the CGM in individual galaxies by mapping the X-ray line emission from OVII and OVIII, the dominant CGM species. This capability is the last frontier for the field of galaxy evolution and large scale structure formation. It would directly address one of the Astro2020 Decadal Survey’s Discovery Areas, “Mapping the Circumgalactic Medium and Intergalactic Medium in Emission.”