How to answer this question, 21-cm emission line

[jqmhelios]

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.

 

[ohwilleke]

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?

 

[Bosko]

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...

Only the stars that are on this circle move either straight towards us or straight away from us.