What is Dispersion Measure and How is it Used in Astrophysics?

[nenyan]

I read some papers on astrophysics and they discussed dispersion measure.
Is there any theoretic meaning of dispersion measure? And what does the unit pc/cm^3 mean?

 

[Simon Bridge]

The dispersion measure is the total column density of free electrons between the observer and, say, a pulsar.
See also:
http://en.wikipedia.org/wiki/Dispersion_measure#Dispersion_in_pulsar_timing

 

[nenyan]

The dispersion measure is the total column density of free electrons between the observer and, say, a pulsar.
See also:
http://en.wikipedia.org/wiki/Dispersion_measure#Dispersion_in_pulsar_timing

Culd you tell me why the unit is pc/cm^3. How to understand the unit.

 

[nenyan]

1pc/cm^3 = 30.857×10^21 m^−2
So there is 30*10^21 electrons per m^2?

Why we use pc/cm^3 instead of m^-2?

 

[Simon Bridge]

You can use any units you like as like as you are representing the same quantity.
The dimensions are inverse area - astronomers keep the "parsecs" separated out to make calculations using DM, which will involve dividing by the distance to a star (in parsecs), easier to do.

 

[nenyan]

You can use any units you like as like as you are representing the same quantity.
The dimensions are inverse area - astronomers keep the "parsecs" separated out to make calculations using DM, which will involve dividing by the distance to a star (in parsecs), easier to do.

Thank you for your reply.

 

[shounakbhatta]

Hello Nenyan,

Is it speaking of just dispersion or velocity dispersion using 'sigma?' Velocity dispersion is used to measure the mean velocity of galaxy and clusters. So which one it is?

 

[Simon Bridge]

Hello Nenyan,

Is it speaking of just dispersion or velocity dispersion using 'sigma?' Velocity dispersion is used to measure the mean velocity of galaxy and clusters. So which one it is?

There are some clues in the first post that tell you which was meant... i.e the word "measure" instead of "velocity" with the word "dispersion" but that could have been a mistake so looking further ... there's the units asked about in connection to dispersion measure as well (what are the units of velocity dispersion?) ... then there is the fact that he didn't immediately reply with "no no no that's not the one I meant" after my answer ;)

Note: as you know, in English, some nouns can have two words.
The sentence "what is dispersion measure?" is different from "what does dispersion measure?"
In the first case, "measure" is part of the noun.

To avoid this sort of confusion, one places the noun in quotes when asking about it:
"What is 'dispersion measure'?"