What components contribute to emission from an active galactic nucleus?

• A. Star formation in the host galaxy only
• B. The accretion disk and relativistic jets
• C. Only the black hole itself
• D. The dark matter halo

Answer: (B)

The emission from an active galactic nucleus is driven by energy released as matter spirals into a supermassive black hole, and this shows up mainly from two central regions: the accretion disk and the relativistic jets. The accretion disk is a hot, flat structure of gas that radiates thermally, producing bright light that spans from optical to ultraviolet wavelengths. In many AGN, some of the energy also powers highly collimated jets that blast material outward at near-light speeds; the particles in these jets emit strongly through non-thermal processes, especially in radio and can extend to higher energies.

Star formation in the host galaxy adds light on a larger, galactic scale, but it isn’t the primary source of the AGN’s central brightness. The black hole itself doesn’t emit radiation unless material is falling in, and the dark matter halo doesn’t produce visible emissions. So the combination of the accretion disk and the relativistic jets best explains the observed emission from an active galactic nucleus.