Why Is 21-Cm Radiation One of the Best Ways to Map

question 50

Multiple Choice

Why is 21-cm radiation one of the best ways to map the spiral arms of the Milky Way?

• A) The molecular hydrogen gas that produces this emission is concentrated in the spiral arms.
• B) These photons, which are produced by neutral hydrogen, are not blocked by the dense clouds of gas and dust in the disk.
• C) The emission is produced by supernovae, which are concentrated in the spiral arms.
• D) Radio telescopes are easier to operate than optical telescopes, and observations can be made even during the daytime.
• E) Radio photons do not have Doppler shifts, so we can detect clouds of gas without having to worry about the orbital velocities.

---

Definitions:

Sediment

Particulate material, ranging from microscopic to large boulders, transported and deposited by water, wind, ice, or gravity.

River Terraces

Step-like landforms along the sides of valleys and rivers, formed by the erosion of soil and rock and the deposition of sediments.

Alluvial Fans

Fan or cone-shaped deposits of sediment crossed and built up by streams, typically found at the outlet of canyons.

Entrenched Meanders

River meanders that have cut deeply into the landscape, often in a canyon form, showing a history of flowing through a relatively stable channel for an extended period.