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Target Selection |
Detailed descriptions of the selection algorithms for the different categories of SDSS targets are provided in the series of papers noted below under Target Selection References. Here we provide short summaries of the various target selection algorithms. In the SDSS imaging data output tsObj files
Note the following subtleties:
The following samples are targeted:
Main Galaxy SampleThe main galaxy sample target selection algorithm is detailed in Strauss et al. (2002) and is summarized in this schematic flowchart.
Galaxy targets are selected starting from objects which are detected
in the r band (i.e. those objects which are more than 5σ
above sky after smoothing with a PSF filter).
The photometry is corrected for Galactic extinction using the
reddening maps of
Schlegel, Finkbeiner, and Davis (1998).
Galaxies are separated from stars using the following cut on the difference between
the r-band
PSF rPSF - rmodel >= 0.3
Note that this cut is more conservative for galaxies than the
star-galaxy separation A number of surface brightness cuts are then applied, based on
mu50, the mean surface brightness within the Petrosian half-light
radius petroR50. The most significant cut is mu50
<= 23.0 mag arcsec-2 in r, which already
includes 99% of the galaxies brighter than the Petrosian magnitude
limit. At surface brightnesses in the range 23.0 <=
mu50 <= 24.5 mag arcsec-2, several other
criteria are applied in order to reject most spurious targets, as
shown in the flowchart. Please see the
detailed discussion of these surface brightness cuts, including
consideration of selection effects, in
Section 4.4 of Strauss et al. (2002). Finally, in order to reject
very bright objects which will cause contamination of the spectra of
adjacent fibers and/or saturation of the spectroscopic CCDs, objects
are rejected if they have (1)fiber magnitudes Main galaxy targets satisfying all of the above criteria have the GALAXY bit set in their primTarget flag. Among those, the ones with mu50 >= 23.0 mag arcsec-2 have the GALAXY_BIG bit set. Galaxy targets who fail all the surface brightness selection limits but have r band fiber magnitudes brighter than 19 are accepted anyway (since they are likely to yield a good spectrum) and have the GALAXY_BRIGHT_CORE bit set. Luminous Red Galaxies (LRG)SDSS luminous red galaxies (LRGs) are selected on the basis of color and magnitude to yield a sample of luminous intrinsically red galaxies that extends fainter and farther than the SDSS main galaxy sample. Please see Eisenstein et al. (2001) for detailed discussions of sample selection, efficiency, use, and caveats.
LRGs are selected using a variant of the photometric redshift technique
and are meant to comprise a uniform, approximately volume-limited sample of
objects with the reddest colors in the rest frame. The sample is
selected via cuts in the (g-r, r-i, r)
color-color-magnitude cube. Note that all colors are measured using
model magnitudes The galaxy model colors are rotated first to a basis that is aligned with the galaxy locus in the (g-r, r-i) plane according to:
c&perp = (r-i) + (g-r)/4 + 0.18
Because the 4000 Angstrom break moves from the g band to the r band at a redshift z ~ 0.4, two separate sets of selection criteria are needed to target LRGs below and above that redshift: Cut I for z <~ 0.4
Cut II for z >~ 0.4
Cut I selection results in an approximately volume-limited LRG sample to z=0.38, with additional galaxies to z ~ 0.45. Cut II selection adds yet more luminous red galaxies to z ~ 0.55. The two cuts together result in about 12 LRG targets per deg2 that are not already in the main galaxy sample (about 10 in Cut I, 2 in Cut II). In primTarget, GALAXY_RED is set if the LRG passes either Cut I or Cut II. GALAXY_RED_II is set if the object passes Cut II but not Cut I. However, neither of these flags is set if the LRG is brighter than the main galaxy sample flux limit but failed to enter the main sample (e.g., because of the main sample surface brightness cuts). Thus LRG target selection never overrules main sample target selection on bright objects. QuasarsThe final adopted SDSS quasar target selection algorithm is described in Richards et al. (2002). However, it should be noted that the implementation of this algorithm came after the last date of DR1 spectroscopy. Thus this paper does not technically describe the DR1 quasar sample and the DR1 quasar sample is not intended to be used for statistical purposes (but see below). Interested parties are instead encouraged to use the catalog of DR1 quasars that is being prepared by Schneider et al (2003, in prep.), which will include an indication of which quasars were also selected by the Richards et al. (2002) algorithm. At some later time, we will also perform an analysis of those objects selected by the new algorithm but for which we do not currently have spectroscopy and will produce a new sample that is suitable for statistical analysis. Though the DR1 quasars were not technically selected with the Richards et al. (2002) algorithm, the algorithms used since the EDR are quite similar to this algorithm and this paper suffices to describe the general considerations that were made in selecting quasars. Thus it is worth describing the algorithm in more detail. The quasar target selection algorithms are summarized in this schematic flowchart. Because the quasar selection cuts are fairly numerous and detailed, the reader is strongly recommended to refer to Richards et al. (2002) (link to AJ paper; subscription required) for the full discussion of the sample selection criteria, completeness, target efficiency, and caveats. The quasar target selection algorithm primarily identifies quasars as outliers from the stellar locus, modeled following Newberg & Yanny (1997) as elongated tubes in the (u-g, g-r, r-i) (denoted ugri) and (g-r, r-i, i-z) (denoted griz) color cubes. In addition, targets are also selected by matches to the FIRST catalog of radio sources (Becker, White, & Helfand 1995). All magnitudes and colors are measured using PSF magnitudes, and all quantities are corrected for Galactic extinction following Schlegel, Finkbeiner, and Davis (1998).
Objects flagged by Photo as having either "fatal" errors (primarily those
flagged BRIGHT, SATURATED, EDGE, or BLENDED Objects which pass the above tests are then selected to be quasar targets if they lie more than 4σ from either the ugri or griz stellar locus. The detailed specification of the stellar loci and of the outlier rejection algorithm are provided in Appendices A and B of Richards et al. (2002). These color-selected quasar targets are divided into main (or low-redshift) and high-redshift samples, as follows:
Main Quasar Sample (QSO_CAP, QSO_SKIRT)These are outliers from the ugri stellar locus and are selected in the magnitude range 15.0 < iPSF < 19.1. Both point sources and extended objects are included, except that extended objects must have colors that are far from the colors of the main galaxy distribution and that are consistent with the colors of AGNs; these additional color cuts for extended objects are specified in Section 3.4.4 of Richards et al. (2002). Even if an object is not a ugri stellar locus outlier, it may be selected as a main quasar sample target if it lies in either of these 2 "inclusion" regions: (1) "mid-z", used to select 2.5 < z < 3 quasars whose colors cross the stellar locus in SDSS color space; and (2) "UVX", used to duplicate selection of z <= 2.2 UV-excess quasars in previous surveys. These inclusion boxes are specified in Section 3.5.2 of Richards et al. (2002). Note that the QSO_CAP and QSO_SKIRT distinction is kept for historical reasons (as some data that are already public use this notation) and results from an original intent to use separate selection criteria in regions of low ("cap") and high ("skirt") stellar density. It turns out that the selection efficiency is indistinguishable in the cap and skirt regions, so that the target selection used is in fact identical in the 2 regions (similarly for QSO_FIRST_CAP and QSO_FIRST_SKIRT, below).
High-Redshift Quasar Sample (QSO_HIZ)These are outliers from the griz stellar locus and are selected in the magnitude range 15.0 < iPSF < 20.2. Only point sources are selected, as these quasars will lie at redshifts above z~3.5 and are expected to be classified as stellar at SDSS resolution. Also, to avoid contamination from faint low-redshift quasars which are also griz stellar locus outliers, blue objects are rejected according to eq. (1) in Section 3.4.5 of Richards et al. (2002). Moreover, several additional color cuts are used in order to recover more high-redshift quasars than would be possible using only griz stellar locus outliers. So an object will be selected as a high-redshift quasar target if it lies in any of these 3 "inclusion" regions: (1) "gri high-z", for z >= 3.6 quasars; (2) "riz high-z", for z >= 4.5 quasars; and (3) "ugr red outlier", for z >= 3.0 quasars. The specifics are given in eqs. (6-8) in Section 3.5.2 of Richards et al. (2002).
FIRST Sources (QSO_FIRST_CAP, QSO_FIRST_SKIRT)Irrespective of the various color selection criteria above, SDSS stellar objects are selected as quasar targets if they have 15.0 < iPSF < 19.1 and are matched to within 2 arcsec of a counterpart in the FIRST radio catalog. Finally, those targets which otherwise meet the color selection or radio selection criteria described above, but fail the cuts on iPSF, will be flagged as QSO_MAG_OUTLIER (also called QSO_FAINT). Such objects may be of interest for follow-up studies, but are not otherwise targeted for spectroscopy under routine operations (unless another "good" quasar target flag is set). Other Science TargetsA variety of other science targets are also selected; see also
Section 4.8.4 of Stoughton et al. (2002). With the exception of
brown dwarfs, these samples are not complete, but are
assigned to excess fibers left over after the main samples of
galaxies, LRGs, and quasars have been tiled StarsA variety of stars are also targeted using color selection criteria, as follows:
ROSAT SourcesSDSS objects are positionally matched against X-ray sources from the ROSAT All-Sky Survey (RASS; Voges et al. 1999), and SDSS objects within the RASS error circles (commonly 10-20 arcsec) are targeted using algorithms tuned to select likely optical counterparts to the X-ray sources. Objects are targeted which:
Objects are flagged ROSAT_E if they fall within the RASS error circle but are either too faint or too bright for SDSS spectroscopy. SerendipityThis is an open category of targets whose selection criteria may change as different regions of parameter space are explored. These consist of:
Target Selection References
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