Spectroscopy of the Extragalactic Universe
Clusters of Galaxies
Cosmic chemical evolution remains a major unsolved problem in astronomy. Galaxy clusters are a powerful laboratory to explore this issue, being large enough to be treated as mini-universes with cosmological fractions of dark and baryonic matter. For clusters with redshift z < 1, there is a remarkable similarity in intracluster medium (ICM) metallicity with a mean value of ~0.3 Z_sol (where Z_sol is the solar metallicity) and only 10–35% scatter. Moreover, there is no evidence that the ICM mean metallicity has evolved over the past ~ 9 Gyr, either from direct measurements of the metal content in high-z clusters, or from indirect inferences based on spatial distributions of metals, despite the fact that clusters have more than doubled in mass over the same time interval. This implies that the ICM was enriched early (i.e., via galactic winds at z ~ 2.5), and then remained unaltered despite > 10 Gyr of accretion of material along cosmic filaments. This surprising conclusion can be reconciled in two different ways: either the cosmic web material that accretes onto clusters has the same metallicity as the ICM; or the accreting cosmic web material is primordial (low metallicity), but this is roughly offset by metal-enriched outflows from galaxies within the cluster.
STROBE-X/XRCA will be the most effcient instrument ever for measuring the bulk metallicity of diffuse, ionized gas, enabling key, complementary measurements that will help solve this puzzle. First, XRCA will detect the Fe K alpha line in distant clusters at z > 2, allowing measurement of ICM bulk metallicity with ~15% accuracy with only a 20ks observation per cluster, 30x faster than XMM and 3x faster than Athena. The eROSITA and SPT-3G surveys will catalog at least 100 such z > 2 clusters by the mid-2020s, and XRCA will measure the metallicity of all of them, observed within ~ 1 Gyr of their initial collapses at z ~ 3. More speculatively, XRCA may be able to identify cosmic filaments within which the metallicity of the warm-hot intergalactic medium (WHIM) can be measured. At present, there are only three X-ray detections of the WHIM, but associating this emission with cosmic filaments has been difficult. The huge area and low particle background of XRCA makes it ideal for detecting faint di↵use emission where the WHIM meets the outskirts of galaxy clusters at moderate redshifts z = 0.2–0.3.
Active Galactic Nuclei
STROBE-X/WFM will be more sensitive than eROSITA to Compton-thick (CT) AGN, thanks to its large collecting area at Fe Kalpha-complex energies. The CT-AGN are especially interesting as they represent the systems where the dominant mode of black hole growth takes place, and are good candidates for precision maser-based mass measurements. AGN at 500 Mpc with 4–10 keV luminosities of 10e43 erg/sec and typical 1 keV equivalent-width lines at 6.4, 6.7, and 6.97 keV should be detectable as emission line sources with WFM in 30 Msec of on-source time, enabling a comprehensive survey for CT-AGN.