Presentation #110.17 in the session LEM.
Exploring the circumgalactic medium (CGM) of galaxies is a fundamental challenge in modern astrophysics and has been identified as a key area in the Astro2020 Decadal report. The Line Emission Mapper (LEM) Probe concept fits into the astrophysics landscapes of the 2020s and 2030s and will explore the complex and comprehensive physics of the formation of structure. Thanks to its large effective area (1600 cm2 at 0.5 keV) and 1-2 eV spectral resolution, LEM will explore the CGM in emission around Milky Way-type galaxies out to their R500 radius. However, the large field-of-view of LEM also offers the possibility to study the CGM of galaxies in absorption even beyond the R500 radius. Traditional absorption studies aim to detect absorption lines from various ions imprinted on the spectrum of a bright background source. However, these studies require extremely bright background sources, such as quasars, which are rare, thereby limiting the galaxies that can be probed and only allowing the study of the CGM in a pencil beam. Large area, high spectral resolution X-ray microcalorimeters offer a new approach to study the CGM in absorption. We demonstrate that the cumulative X-ray emission from cosmic X-ray background (CXB) sources can be used to probe the CGM in absorption. By using column density maps from hydrodynamical simulations and building realistic LEM mock images, we simulate the X-ray spectrum of CXB sources and the large-scale CGM, and probe the detectability of absorption lines in the outskirts of galaxies. We find that in reasonably long exposure times, the OVII and OVIII emission lines are detectable at a statistically significant level around massive galaxies and also establish that the CGM of Milky Way-type galaxies can be detected even beyond the virial radius when co-adding data from multiple galaxies. We place these results in context and discuss how the proposed absorption studies will advance our understanding about structure formation.