One or more single Gaussian/Lorentzian

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6.13.2.1 One or more single Gaussian/Lorentzian

For Gaussian and Lorentzian fits, the task will allow multiple components and specfit will try to find the best solution. The parameter space, however, is usually not uniform and to avoid local minima in the goodness-of-fit space, one can provide initial start values for the fits. This can be done either through the parameters pampest, pcenterest, and pfwhmest for the amplitudes, center, and FWHM estimates in image coordinates. pfix can take parameters that specify fixed fit values. Any combination of the characters ’p’ (peak), ’c’ (center), and ’f’ (fwhm) are permitted, e.g. ”fc” will hold the fwhm and the center constant during the fit. Fixed parameters will have no errors associated with them in the solution. Alternatively, a file with initial values can be supplied vai the estimates parameter (one Gaussian/Lorentzian parameter set per line). The file has the following format:

[peak intensity], [center], [fwhm], [optional fixed parameter string]

The first three values are required and must be numerical values. The peak intensity must be expressed in map units, while the center and fwhm must be specified in pixels. The fourth value is optional and if present, represents the parameter(s) that should be held constant during the fit (see above).

An example estimates file is:

# estimates file indicating that two gaussians should be fit
# first guassian estimate, peak=40, center at pixel number 10.5,
# fwhm = 5.8 pixels, all parameters allowed to vary during
# fit
40, 10.5, 5.8
# second gaussian, peak = 4, center at pixel number 90.2,
# fwhm = 7.2 pixels, hold fwhm constant
4, 90.2, 7.2, f
# end file

and the output of a typical execution, e.g.

specfit(imagename=’IRC10216_HC3N.cube_r0.5.image’, region=’specfit.crtf’, multifit=F,
estimates=’’, ngauss=2)

(’specfit.crtf’ is a CASA regions file, see Section D)

will be

Fit   :
    RA           :   09:47:57.49
    Dec          :   13.16.46.46
    Stokes       : I
    Pixel        : [146.002, 164.499, 0.000,  *]
    Attempted    : YES
    Converged    : YES
    Iterations   : 28
    Results for component 0:
        Type     : GAUSSIAN
        Peak     : 5.76 +/- 0.45 mJy/beam
        Center   : -15.96 +/- 0.32 km/s
                   40.78 +/- 0.31 pixel
        FWHM     : 7.70 +/- 0.77 km/s
                   7.48 +/- 0.74 pixel
        Integral : 47.2 +/- 6.0 mJy/beam.km/s
    Results for component 1:
        Type     : GAUSSIAN
        Peak     : 4.37 +/- 0.33 mJy/beam
        Center   : -33.51 +/- 0.58 km/s
                   23.73 +/- 0.57 pixel
        FWHM     : 15.1 +/- 1.5 km/s
                   14.7 +/- 1.5 pixel
        Integral : 70.2 +/- 8.8 mJy/beam.km/s

If wantreturn=True (the default value), the task returns a python dictionary (here captured in a variable with the inventive name of ’fitresults’) :

fitresults=specfit(imagename=’IRC10216_HC3N.cube_r0.5.image’, region=’specfit.rgn’, multifit=F,
estimates=’’, ngauss=2)

The values can then be used by other python code for further processing.

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