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Emission and absorption line fitting

Spectro1D fits spectral features at three separate stages during the pipeline. The first two fits are fits to emission lines only. They are done in the process of determining an emission line redshift and these are referred to as foundLines. The final fitting of the complete line list, i.e. both emission and absorption lines, occurs after the object's classification has been made and a redshift has been measured. These fits are known as measuredLines. In all cases a single Gaussian is fitted to a given feature, therefore the quality of the fit is only good where this model holds up.

The first line fit is done when attempting to measure the object's emission line redshift. Wavelet filters are used to locate emission lines in the spectrum. The goal of these filters is to find strong emission features, which will be used as the basis for a more careful search. The lines identified by the wavelet filter are stored in the specLine table as foundLines, i.e., with the parameter category set to 1. They are stored without any identifications, i.e., they have restWave = 0.

Every one of these features is then tentatively matched to each of a list of candidate emission lines as given in the line table below, and a system of lines is searched for at the position indicated by the tentative matching. The best system of emission lines (if any) found in this process is used to calculate the object's emission-line redshift. The lines from this system and their parameters are stored in the specLine table as foundLines, i.e., with the parameter category set to 1. These lines are identified by their restWave as given in the line table below.

The final line fitting is done for all features (both emission and absorption) in the line list below, and occurs after the object has been classified and a redshift has been determined. This allows for a better continuum estimation and thus better line fits. This latter fit is stored in the specLine table with the parameter category set to 2.

Types of line fits stored in spSpec files
Type of fit category restWave
"Found" emission lines from wavelet filter 1 0
"Found" emission lines from best-fit system to wavelet detections 1 restWave from line list
"Measured" emission and absorption lines according to the object's classification and best redshift 2 restWave from line list

For almost all purposes we recommend the use of the measuredLines (category=2) since these result from the most careful continuum measurement and precise line fits.

Details of continuum fitting and line measurements

Parameter Notes

All of the line parameters are measured in the observed frame, and no correction has been made for the instrumental resolution.

Continuum Fitting

The continuum is fit using a median/mean filter. A sliding window is created of length 300 pixels for galaxies and stars or 1000 pixels for quasars. Pixels closer than 8 pixels(560km/s) for galaxies and stars or 30 pixels (2100 km/s) for QSOs to any reference line are masked and not used in the continuum measurement. The remaining pixels in the filter are ordered and the values between the 40th and 60th percentile are averaged to give the continuum. The category=1 lines are fit with a cruder continuum which is given by a fifth order polynomial fit which iteratively rejects outlying points.

Reference Line List

The list of lines which are fit are given as an HTML line table below. Note that many times a single line in the table actually represents multiple features. Since the line fits are allowed to drift in wavelength somewhat, the exact precision of the lines are not important. The wavelength precision does become important for the emission line determination. To improve the accuracy of the emission-line redshift determination for QSOs, the wavelength for many of the lines listed here are not the laboratory values, but the average values calculated from a sample of SDSS QSOs taken from Vanden Berk et al. 2001 AJ 122 .

Line Fitting

Every line in the reference list is fit as a single Gaussian on top of the continuum subtracted spectrum. Lines that are deemed close enough are fitted simultaneously as a blend. The basic line fitting is performed by the SLATEC common mathematical library routine SNLS1E which is based on the Levenberg-Marquardt method. Parameters are constrained to fall within certain values by multiplying the returned chi-squared values by a steep function. Any lines with parameters falling close to these constraints should be treated with caution. The constraints are: sigma > 0.5 Angstrom, sigma < 100 Angstrom, and the center wavelength is allowed to drift by no more than 450 km/sec for stars and galaxies or 1500 km/sec for QSOs, except for the CIV line which is allowed to be shifted by as much as 3000 km/sec.

Testing the results

There are a number of ways that the line fitting can fail. If the continuum is bad the line fits will be compromised. The median/mean filtering routine will always fail for white dwarfs, some A stars as well as late-type stars. In addition is has trouble for galaxies with a strong 4000 Angstrom break. Likewise the line fitting will have trouble when the lines are not really Gaussian. The Levenberg-Marquardt routine can fall into local minima, which can happen when there is self-absorption in a QSO line or both a narrow and broad component for example. One should always check the chi-squared values to evaluate the quality of the fit.

Reference line list

restWaveLine
1857.40AlIII_1857
8500.36CaII_8500
8544.44CaII_8544
8664.52CaII_8665
1335.31CII_1335
2326.00CII_2326
1908.73CIII_1909
1549.48CIV_1549
4305.61G_4306
3969.59H_3970
6564.61Ha_6565
4862.68Hb_4863
4102.89Hd_4103
3971.19He_3971
3889.00HeI_3889
1640.40HeII_1640
4341.68Hg_4342
3798.98Hh_3799
3934.78K_3935
6707.89Li_6708
1215.67Lya_1215
5176.70Mg_5177
2799.12MgII_2799
5895.60Na_5896
2439.50NeIV_2439
3346.79NeV_3347
3426.85NeVI_3427
6529.03NI_6529
6549.86NII_6550
6585.27NII_6585
1240.81NV_1241
1305.53OI_1306
6302.05OI_6302
6365.54OI_6366
3727.09OII_3727
3729.88OII_3730
1665.85OIII_1666
4364.44OIII_4364
4932.60OIII_4933
4960.30OIII_4960
5008.24OIII_5008
1033.82OVI_1033
3836.47Oy_3836
4072.30SII_4072
6718.29SII_6718
6732.67SII_6733
1397.61SiIV_1398
1399.80SiIV_OIV_1400