[Paper Review] Line Emission Mapper (LEM): Probing the physics of cosmic ecosystems

Ralph Kraft, Maxim Markevitch+108 more|arXiv (Cornell University)|Nov 17, 2022

Superconducting and THz Device Technology46 citations

TL;DR

LEM is a proposed soft X-ray imaging spectrometer (1–2 eV resolution) with a large grasp to map CGM/IGM emission and study galaxy formation, feedback, and baryon flows. It enables detailed, redshift-discriminated tomography of diffuse gas in and around galaxies and clusters.

ABSTRACT

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.

Motivation & Objective

• Investigate the physics of galaxy formation through the diffuse gas in, around, and between galaxies (CGM/IGM).
• Enable emission-line mapping in the soft X-ray band to constrain feedback processes from stars and black holes.
• Provide a transformative observing capability for a wide range of astrophysical systems via a large-grasp, high-spectral-resolution instrument.
• Facilitate both targeted observations and an all-sky survey to expand discovery space in X-ray astrophysics.

Proposed method

• Use a large microcalorimeter array/IFU operating in the 0.2–2 keV band with 1–2 eV spectral resolution.
• Employ a grazing-incidence Si mirror design to achieve a 30′×30′ field of view and ~10′′ angular resolution.
• Adopt a TES microcalorimeter detector with ~13,806 absorber pixels and central 1 eV resolution, plus 2 eV resolution hydras for extended coverage.
• Provide nondispersive imaging spectroscopy to separate faint CGM/IGM signals from bright Milky Way foreground using Doppler/redshift information.
• Leverage a shallow all-sky survey to identify optimal deep CGM/IGM fields and expand discovery space.

Experimental results

Research questions

• RQ1 How can high-resolution, non-dispersive X-ray spectroscopy map the CGM and IGM to constrain galaxy formation physics?
• RQ2 To what extent can 1–2 eV resolution disentangle extragalactic emission from Milky Way foreground in the 0.2–2 keV band?
• RQ3 What do spatially resolved line maps of O vii, O viii, and Fe xvii reveal about gas temperature, density, metallicity, and dynamics in galactic halos?
• RQ4 How will LEM’s measurements of abundances and velocities inform models of feedback, heating, cooling, and chemical enrichment in galaxies and clusters?
• RQ5 What is the scientific return of an all-sky shallow survey for discovery space and target selection for deep CGM/IGM exposures?

Key findings

• LEM’s 2 eV spectral resolution enables separation of faint CGM/IGM emission from the Milky Way foreground, enabling halo mapping at z≥0.01.
• Simulations show LEM can map CGM/IGM emission in O vii, O viii, and Fe xvii out to large radii, constraining feedback physics.
• The large grasp and 1 eV central array enable velocity measurements and resolution of line diagnostics in bright galactic halos and SNRs.
• An all-sky shallow survey with LEM will open broad discovery space and identify optimal deep-field targets for CGM/IGM studies.
• LEM will quantify abundances and enrichment histories in the ICM of groups/clusters and map gas motions out to R200.
• The mission design aims for a 2032 launch, leveraging XRISM/Athena heritage to advance soft X-ray line emission studies.