Procedure for Generating Fits

To generate a fit for a spectrum generate_total_fit() is called. This function controls the identification of line complexes, the fit of a series of absorption lines for each appropriate species, checks of those fits, and returns the results of the fits.

Finding Line Complexes

Line complexes are found using the find_complexes() function. The process by which line complexes are found involves walking through the array of flux in order from minimum to maximum wavelength, and finding series of spatially contiguous cells whose flux is less than some limit. These regions are then checked in terms of an additional flux limit and size. The bounds of all the passing regions are then listed and returned. Those bounds that cover an exceptionally large region of wavelength space will be broken up if a suitable cut point is found. This method is only appropriate for noiseless spectra.

The optional parameter complexLim (default = 0.999), controls the limit that triggers the identification of a spatially contiguous region of flux that could be a line complex. This number should be very close to 1 but not exactly equal. It should also be at least an order of magnitude closer to 1 than the later discussed fitLim parameter, because a line complex where the flux of the trough is very close to the flux of the edge can be incredibly unstable when optimizing.

The fitLim parameter controls what is the maximum flux that the trough of the region can have and still be considered a line complex. This effectively controls the sensitivity to very low column absorbers. Default value is fitLim = 0.99. If a region is identified where the flux of the trough is greater than this value, the region is simply ignored.

The minLength parameter controls the minimum number of array elements that an identified region must have. This value must be greater than or equal to 3 as there are a minimum of 3 free parameters that must be fit. Default is minLength = 3.

The maxLength parameter controls the maximum number of array elements that an identified region can have before it is split into separate regions. Default is maxLength = 1000. This should be adjusted based on the resolution of the spectrum to remain appropriate. The value correspond to a wavelength of roughly 50 angstroms.

The splitLim parameter controls how exceptionally large regions are split. When such a region is identified by having more array elements than maxLength, the point of maximum flux (or minimum absorption) in the middle two quartiles is identified. If that point has a flux greater than or equal to splitLim, then two separate complexes are created: one from the lower wavelength edge to the minimum absorption point and the other from the minimum absorption point to the higher wavelength edge. The default value is splitLim =.99, but it should not drastically affect results, so long as the value is reasonably close to 1.

Fitting a Line Complex

After a complex is identified, it is fitted by iteratively adding and optimizing a set of Voigt Profiles for a particular species until the region is considered successfully fit. The optimizing is accomplished using scipy’s least squares optimizer. This requires an initial estimate of the parameters to be fit (column density, b-value, redshift) for each line.

Each time a line is added, the guess of the parameters is based on the difference between the line complex and the fit so far. For the first line this just means the initial guess is based solely on the flux of the line complex. The column density is given by the initial column density given in the species parameters dictionary. If the line is saturated (some portion of the flux with a value less than .1) than the larger initial column density guess is chosen. If the flux is relatively high (all values >.9) than the smaller initial guess is given. These values are chosen to make optimization faster and more stable by being closer to the actual value, but the final results of fitting should not depend on them as they merely provide a starting point.

After the parameters for a line are optimized for the first time, the optimized parameters are then used for the initial guess on subsequent iterations with more lines.

The complex is considered successfully fit when the sum of the squares of the difference between the flux generated from the fit and the desired flux profile is less than errBound. errBound is related to the optional parameter to generate_total_fit(), maxAvgError by the number of array elements in the region such that errBound = number of elements * maxAvgError.

There are several other conditions under which the cycle of adding and optimizing lines will halt. If the error of the optimized fit from adding a line is an order of magnitude worse than the error of the fit without that line, then it is assumed that the fitting has become unstable and the latest line is removed. Lines are also prevented from being added if the total number of lines is greater than the number of elements in the flux array being fit divided by 3. This is because there must not be more free parameters in a fit than the number of points to constrain them.

Checking Fit Results

After an acceptable fit for a region is determined, there are several steps the algorithm must go through to validate the fits.

First, the parameters must be in a reasonable range. This is a check to make sure that the optimization did not become unstable and generate a fit that diverges wildly outside the region where the fit was performed. This way, even if particular complex cannot be fit, the rest of the spectrum fitting still behaves as expected. The range of acceptability for each parameter is given in the species parameter dictionary. These are merely broad limits that will prevent numerical instability rather than physical limits.

In cases where a single species generates multiple lines (as in the OVI doublet), the fits are then checked for higher wavelength lines. Originally the fits are generated only considering the lowest wavelength fit to a region. This is because we perform the fitting of complexes in order from the lowest wavelength to the highest, so any contribution to a complex being fit must come from the lower wavelength as the higher wavelength contributions would already have been subtracted out after fitting the lower wavelength.

Saturated Lyman Alpha Fitting Tools

In cases where a large or saturated line (there exists a point in the complex where the flux is less than .1) fails to be fit properly at first pass, a more robust set of fitting tools is used to try and remedy the situation. The basic approach is to simply try a much wider range of initial parameter guesses in order to find the true optimization minimum, rather than getting stuck in a local minimum. A set of hard coded initial parameter guesses for Lyman alpha lines is given by the function get_test_lines(). Also included in these parameter guesses is an an initial guess of a high column cool line overlapping a lower column warm line, indictive of a broad Lyman alpha (BLA) absorber.