The discussion revolves around the concept of dispersion measure in astrophysics, exploring its theoretical meaning, units of measurement, and potential confusion with related terms such as velocity dispersion. Participants seek clarification on the definition and application of dispersion measure, particularly in the context of pulsars and electron density.
Participants express differing views on the interpretation of dispersion measure and its units, with some seeking clarification while others provide explanations. The distinction between dispersion measure and velocity dispersion remains a point of contention, with no consensus reached.
Participants highlight potential ambiguities in terminology and the importance of context in understanding the discussion. The relationship between different units and their implications for calculations in astrophysics is also noted.
Simon Bridge said: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
Simon Bridge said: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.
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 ;)shounakbhatta said: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?