How to answer this question, 21-cm emission line

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TL;DR Summary: Completely clueless on how to answer this question, came up as part of an assessed lab script

Why does 21-cm emission only give the rotation curve of the Galaxy at galactocentric radii less than that of the Solar system?

The lab script asking this question made zero clarification on how to begin answering it, as have internet searches. Completely clueless.
 
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jqmhelios said:
TL;DR Summary: Completely clueless on how to answer this question, came up as part of an assessed lab script

Why does 21-cm emission only give the rotation curve of the Galaxy at galactocentric radii less than that of the Solar system?

The lab script asking this question made zero clarification on how to begin answering it, as have internet searches. Completely clueless.
Reading this, it sounds like there is context or language omitted from this question (possibly discernible from the overall total package and not the specific question) that is necessary to understand what is being asked.

For example, is "the Galaxy" the Milky Way galaxy, or any particular arbitrary galaxy that is being observed?

Which way is the radio-telescope pointed?
 
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jqmhelios said:
TL;DR Summary: Completely clueless on how to answer this question, came up as part of an assessed lab script

Why does 21-cm emission only give the rotation curve of the Galaxy at galactocentric radii less than that of the Solar system?

The lab script asking this question made zero clarification on how to begin answering it, as have internet searches. Completely clueless.
You need to provide more details...

Does that measurement test the speed of the stars?

Are spectral lines of hydrogen used?

Does the measurement only take place if the star is moving either straight towards us or straight away from us? ( Earth - Solar System)

If this above is the case, then it is a simple geometric problem.

Only stars that are closer to the galactic center than the Sun can be caught in position when they are moving straight towards us or straight away from us.

I need to draw this to explain...
galaxy_rotation_H_I_line.png

Only the stars that are on this circle move either straight towards us or straight away from us.
 
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1. What is the 21-cm emission line?

The 21-cm emission line refers to a spectral line in the radio frequency range that is emitted by neutral hydrogen atoms. It is produced when the electron in a hydrogen atom transitions between two energy levels, resulting in the emission of a photon with a wavelength of 21 centimeters.

2. How is the 21-cm emission line used in astronomy?

Astronomers use the 21-cm emission line to study the distribution and dynamics of neutral hydrogen gas in galaxies and the interstellar medium. By measuring the Doppler shift of the 21-cm line, astronomers can determine the velocity of the hydrogen gas and map its distribution in space.

3. What information can be obtained from studying the 21-cm emission line?

Studying the 21-cm emission line can provide valuable information about the structure, kinematics, and evolution of galaxies. It can also help astronomers study the formation of stars and galaxies, as well as the large-scale structure of the universe.

4. How do astronomers detect the 21-cm emission line?

Astronomers use radio telescopes to detect the 21-cm emission line. These telescopes are equipped with receivers that are sensitive to radio waves at the frequency corresponding to the 21-cm line. By scanning the sky with these telescopes, astronomers can detect and map the emission of neutral hydrogen gas.

5. What are some current research topics related to the 21-cm emission line?

Current research topics related to the 21-cm emission line include studying the early universe, probing dark matter and dark energy, and understanding the processes of galaxy formation and evolution. Astronomers are also using the 21-cm line to study the properties of gas in the Milky Way and other galaxies.

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