Simulated background-subtracted WFI spectrum of a 5 × 10¹³ M_⊙ group at z = 0.5, 1.0, 1.5, 2.0, 2.5 (from top to bottom), at an off-axis angle of 10′, and an exposure time of 80 ks. First panel (from the left): galaxy group spectra with the background subtracted. Second panel: extracted location of galaxy group in ATHENA FoV. Third panel: zoom-in view of the galaxy groups (box size 6′×6′); the blue circle represents r₅₀₀; the red circle is 2.0 × r₅₀₀; the annulus is used for background subtraction. Fourth panel: detected AGNs are excluded with a PSF corresponding to a 90% encircled energy fraction (99% for the bright ones outside r₅₀₀).