wiki:u/erica/CF_coldens

Version 9 (modified by Erica Kaminski, 9 years ago) ( diff )

Column Density Maps

Using projections operators in the code for column density, one is performing the integral operation,

which has the rather strange units of computational number density times computational length. We can not just assume that the resultant product will have units of 1/computational length squared. This is because while nscale has units of 1/lscale3, it does not equal 1/lscale3,

but in fact equals,

where X is the mean molecular weight.

Therefore,

This means that we can not simply multiply our projected quantity by lscale-2 to get units of cm-2. Instead, we should multiply it by lscale*nscale. This will give us the correct units for column density, when lscale and nscale are in cgs.

Note, for my setup, lscale is the number of cm in 1 pc, and my nscale is 1. This means that computational density and length can be thought of as being in units of cm-3 and pc. Thus the quantity the code gives for 'projected column density in computational units' is for all intents and purposes, in units of pc/cm3. To convert this to 1/cm2, multiply by the conversion factor 1pc = 3.08567758 × 1018 cm (which is of course equivalent to multiplying by nscale and lscale as stated above).

Also, as an aside, when the scales are all set to 1 in the code, all computational units = cgs. Using scales that DNE 1 is equivalent to scaling the computational quantities away from cgs. When all of the scales = 1, nscale = rhoscale/X means that nscale is essentially Avogadro's number (over the mean molecular weight). Also, other scales in the code behave nicer than nscale in that they are interrelated directly.

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