A galaxy’s Spectral Energy Distribution (SED) shows how its energy is emitted at different wavelengths of light, from X-rays to radio waves. It helps astronomers understand the galaxy’s properties, including its stars, gas, dust, and activity in its nucleus.

Key Components of a Galaxy’s SED

A typical galaxy SED has contributions from multiple sources:

  1. Stars – Emit most of their light in the ultraviolet (UV), optical, and near-infrared (NIR).

    • Young, hot stars → strong UV emission.

    • Older, cooler stars → peak in visible or NIR.

  2. Gas (Nebulae) – Emits specific wavelengths due to ionized elements.

    • Emission lines (e.g., Hα at 656 nm) appear in spectra of star-forming regions.

  3. Dust – Absorbs UV/optical light and re-emits it in the infrared (IR).

    • A warm dust peak appears in the mid- to far-IR.

  4. Active Galactic Nucleus (AGN) – If present, an AGN contributes:

    • X-ray and ultraviolet from a hot accretion disk.

    • Infrared from dust heated by the AGN.

    • Radio waves from jets (in some cases).

  5. Cold Hydrogen and Molecular Gas – Emits in radio waves (e.g., 21-cm line for neutral hydrogen, CO lines for molecular gas).

Why Do We Study SEDs?

  • Determine a galaxy’s age, mass, and star formation rate.

  • Identify dust content and its role in reprocessing light.

  • Classify galaxies (e.g., star-forming vs. quiescent, AGN presence).

  • Investigate galaxy evolution over cosmic time.

SED Shapes & Galaxy Types

  • Star-forming galaxies → Strong UV, optical, and IR peaks.

  • Elliptical galaxies → Weak UV, mostly optical/NIR light from old stars.

  • AGN-hosting galaxies → Additional power-law-like emission from the nucleus.

Understanding a galaxy’s SED helps us piece together its history, composition, and activity, making it a crucial tool in modern astrophysics!

Read the review article on galaxy spectral energy distribution by Iyer et al. (2025). Start with the Abstract, Glossary, Introduction, and Figures. Then read the entire paper thoroughly. Look up new terms as needed, and reread difficult sections multiple times. These concepts are new, and you are not expected to understand everything right away. However, over time, many of these ideas will become clearer.