This IDL library implements ammonia fitting using the method originally developed in Rosolowsky et al. (2008) and refined in Dunham et al. (2011). The work has been reimplemented in pyspeckit and improvements and refinements are being developed through that channel.

This code relies on mpfit and probably a lot of other things you can find in my core IDL libraries.

nh3fit is a code that forward models the ammonia spectrum with fine structure components given a uniform slab of gas. This version assumes that all the lines are characterized by the same excitation temperature and that the different states are populated according to thermodynamic equilibrium within the ortho- and para-ammonia ladders.

Calling nh3fit uses the main routine nh3fit.pro, which operates on vectors of frequency and main beam brightness temperature (i.e., corrected for atmospheric opacity and main beam efficiency, as is appropriate for a beam-filling slab).

IDL> nh3fit, nu, Tmb, s = output

where nu and Tmb are the frequency in GHz and the main beam temperature in K respectively. These arrays should be the same length. output is then a structure with fields that contain the optimal physical parameters.

If you want to create a model spectrum for overplotting, you call the modelspec routine with the optimized parameters.

IDL> Tmb_bestfit = modelspec(nu, output.model)

Note that the fit can make use of embedded information about a frequency switch in the data (i.e., accounting for the off components of a simple frequency switch). This is controlled by setting the !NH3 system variable with the IDL routine defsysv. The tag freqswthrow sets the magnitude of the throw in MHz (because that's how the GBT defines the throw).