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Redshifts and Classifications

For each spectrum, we estimate a redshift and perform a classification into STAR, GALAXY, QSO or UNKNOWN. In addition, we define subclasses for some of these. Here we describe the redshift and classification methods. The software used is called idlspec2d and is publicly available in our software repository.

The essential strategy for redshift fitting is to perform, at each potential redshift, a least-squares fit to each spectrum given the uncertainties, using a fairly general set of models, for galaxies, for stars, for cataclysmic variables, and for QSOs. The best fit model and redshift is chosen as the reported parameters for the object. The fits are applied without regard to the target category of the object (so that if an object targeted as a galaxy turns out to be a star, we can identify it as such). We describe the galaxy-template redshift analysis in detail here, and describe the differences of other template class analyses relative to the galaxy case.

In detail, for each spectroscopic plate, the fits are done to the spectra, with some pixels masked as untrustworthy as follows. The spreduce1d module in idlspec2d reads the calibrated spectrum flux vectors, associated inverse-variance vectors, and wavelength baseline from the spPlate file written by the two-dimensional extraction procedures. In addition to masking bad pixels within each spectrum, zero weight is given to pixels at wavelengths where the residual reduced chi-squared of the sky-subtracted sky spectra exceeds 3, and to pixels where the brightness from a sky line exceeds the sum of the extracted object flux plus ten times its associated error.

The galaxy class is defined by a rest-frame principal-component analysis (PCA) of 480 galaxies observed on SDSS plate number 306, MJD 51690, which is used to define a basis of 4 "eigenspectra" corresponding to the four most significant modes of variation in the PCA analysis. The redshifts of the galaxy PCA training sample are established by fitting each spectrum with a linear combination of two stellar template spectra and a set of narrow Gaussian profiles at the wavelengths of common nebular emission lines. The stellar template spectra used in this procedure are obtained from the first two components of a PCA analysis of 10 velocity standard stars observed on SDSS plate 321, MJD 51612. The galaxy PCA training sample redshifts are verified by visual inspection.

For all spectra, a range of trial galaxy redshifts is explored from redshift -0.01 to 1.00. Trial redshifts are separated by 138 km/s (i.e., two pixels in the reduced spectra). At each trial redshift, the galaxy eigenbasis is shifted accordingly, and the error-weighted data spectrum is modeled as a minimum-chi-squared linear combination of the redshifted eigenspectra, plus a quadratic polynomial to absorb low-order calibration uncertainties. The chi-squared value for this trial redshift is stored, and the analysis proceeds to the next trial redshift. The trial redshifts corresponding to the 5 lowest chi-squared values are then redetermined locally to sub-pixel accuracy, and errors in these values are determined from the curvature of the chi-squared curve at the position of the minimum.

QSO redshifts are determined for all spectra in similar fashion to the galaxy redshifts, but over a larger range of exploration (z = 0.0333 to 7.00) and with a larger initial velocity step (276 km/s). The QSO eigenspectrum basis is defined by a PCA of 412 QSO spectra with known redshifts. Star redshifts are determined separately for each of 32 single sub-type templates (excluding CV stars) using a single eigenspectrum plus a cubic polynomial for each subtype, over a radial velocity range from -1200 to +1200 km/s. Only the single best radial velocity is retained for each stellar subtype. Because of their intrinsic emission-line diversity, CV stars are handled differently than other stellar subtypes, with a 3-component PCA eigenbasis plus a quadratic polynomial, over a radial velocity range of from -1000 to +1000 km/s.

Once the best 5 galaxy redshifts, best 5 QSO redshifts, and best stellar sub-type radial velocities for a given spectrum have been determined, these identifications are sorted in order of increasing reduced chi-squared, and the difference in reduced chi-squared between each fit and the next-best fit with a radial velocity difference of greater than 1000 km/s is computed. The model spectra for all fits are redetermined, and used to compute statistics of the distribution of data-minus-model residual values in the spectrum for each fit. Both the spectra and the models are integrated over the SDSS imaging filter band-passes to determine the implied broadband magnitudes.

The combination of redshift and template class that yields the overall best fit (in terms of lowest reduced chi-squared) is adopted as the pipeline measurement of the redshift and classification of the spectrum. Several warning flags can be set so as to indicate low confidence in this identification, which are documented in the online data model. The most common flag is set to indicate that the change in reduced chi-squared between the best and next-best redshift/classification is less than 0.01, which indicates a poorly determined redshift.

At the best galaxy redshift, the stellar velocity dispersion is also determined. This is done by computing a PCA basis of eigenspectra from the ELODIE stellar library (Prugniel & Soubiran 2001), convolved and binned to match the instrumental resolution and constant-velocity pixel scale of the reduced SDSS spectra, and broadened by Gaussian kernels of successively larger velocity width ranging from 100 to 850 km/s in steps of 25 km/s. The broadened stellar template sets are redshifted to the best-fit galaxy redshift, and the spectrum is modeled as a least-squares linear combination of the basis at each trial broadening, masking pixels at the position of common emission lines in the galaxy-redshift rest frame. The best-fit velocity dispersion is determined by fitting locally for the position of the minimum of chi-squared versus trial velocity dispersion in the neighborhood of the lowest gridded chi-squared value. Velocity-dispersion error estimates are determined from the curvature of the chi-squared curve at the global minimum, and are set to a negative value if the best value occurs at the high-velocity end of the fitting range. Reported best-fit velocity-dispersion values less than about 100 km/s are below the resolution limit of the SDSS spectrograph and are to be regarded with caution.

Flux values, redshifts, line-widths, and continuum levels are computed for common rest-frame ultraviolet and optical emission lines by fitting multiple Gaussian-plus-background models at their observed positions within the spectra. The initial-guess emission-line redshift is taken from the main redshift analysis, but is subsequently re-fit nonlinearly in the emission-line fitting routine. All lines are constrained to have the same redshift except for Lyman-alpha. Intrinsic line-widths are constrained to be the same for all emission lines, with the exception of the hydrogen Balmer series, which is given its own line-width as a free parameter, and Lyman-alpha and NV 1214, which each have their own free line-width parameters. Known 3:1 line flux ratios between the members of the [OIII] 5007 and [NII] 6583 doublets are imposed. When the signal-to-noise of the line measurements permits doing so, spectra classified as galaxies and QSOs are sub-classified into AGN and star-forming galaxies based upon measured [OIII]/Hβ and [NII]/Hα line ratios, and galaxies with very high equivalent width in Hα are sub-classified as starburst objects. See the spectro catalogs page for details on the line ratio criteria.

The output of the redshift and classification pipeline is stored in three files for each spectroscopic plate observation. The spZbest file contains the detailed results for the best-fit redshift/classification of each spectrum, and includes the best-fit model spectrum that was used to make the redshift measurement. The spZall file contains parameters from all the next-best identifications, without the full representation of the associated model spectra (although these can be reconstructed from template files and reported coefficients). The spZline file contains the results of the emission-line fits for each object.