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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Related to How to answer this question, 21-cm emission line

What is the 21-cm emission line?

The 21-cm emission line, also known as the hydrogen line, is a specific radio wavelength (21 centimeters) emitted by neutral hydrogen atoms in space. This occurs when the electron in a hydrogen atom flips its spin direction relative to the proton, releasing energy in the process.

Why is the 21-cm emission line important in astronomy?

The 21-cm emission line is crucial because it allows astronomers to map the distribution and density of neutral hydrogen in the galaxy and beyond. It provides insights into the structure, dynamics, and evolution of galaxies, as well as the large-scale structure of the universe.

How is the 21-cm emission line detected?

The 21-cm emission line is detected using radio telescopes. These telescopes are equipped with sensitive receivers that can pick up the specific radio frequency emitted by neutral hydrogen atoms. Data collected helps in creating detailed maps of hydrogen distribution in space.

What information can be derived from the 21-cm emission line?

From the 21-cm emission line, astronomers can derive information about the velocity, temperature, and density of hydrogen clouds. It also helps in understanding the rotation curves of galaxies, which in turn provides evidence for the presence of dark matter.

Can the 21-cm emission line be used to study the early universe?

Yes, the 21-cm emission line can be used to study the early universe, particularly the period known as the "Cosmic Dawn" and the "Epoch of Reionization." By observing the redshifted 21-cm line, scientists can learn about the formation of the first stars and galaxies, and the reionization of the universe.

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