Column density NH

• NH … number of H atoms along line-of-sight
• NH = few atoms / cm³ x few kpc = 10²²/cm²
• Compton-thick: ~ 1024 cm² – Circinus, NGC1068
• Nuclear regions with high density
• Mergers – major accretion events for SMBH
• Fe, O, He, H
• absorb everything except hard X-ray, re-radiate thermally in IR
• Measure NH assuming local abundances from X-ray spectrum