Special Target Selection

The aperture bias plates targeted normal Legacy-type targets (marked APBIAS). The exact target algorithms used are stored in the PRIMTARGET and SECTARGET plates. However, these plates are not part of the normal Legacy survey and cannot be used for statistical analysis. The plates were observed using multiple slightly-offset pointings in the different exposures, in an attempt to study the effects of fiber aperture bias. The resulting spectra were combined together in the reductions. For this reason, the resulting spectra are not calibrateable and we have marked all these plates as "bad." However, redshifts are successfully recovered for a number of the objects on these plates.

The commissioning plates were early plates targeting the stellar locus. Point sources on the stellar locus were selected from the photometry (marked COMMISSIONING_STAR). In particular, grids of width 0.04 magnitude in the (u-g),(r-i) and (r-i),(i-z) color planes were set down, and where they existed, a single star was selected in each grid. The process was iterated until about 600 targets on each plate had been assigned. These plates are not part of any broader stellar survey, and cannot be used for statistical analysis.

The disk plates targeted stars in the thin and thick disk of our Galaxy, to study kinematics. We targeted a complete sample of bright, red stars in fifteen plates using three adjacent pointings. It provides an in situ sample of thin and thick disk stars with radial velocities, proper motions, and spectroscopic metallicity determinations, densely sampling a single line of sight out to 2 kpc above the Galactic plane. The plates are near (l = 123°, b = -63°), target point sources, and the color and flux cuts are: i < 18.26, i-z > 0.2 (marked DISKSTAR)

F stars are numerous in the Galaxy, have sharp spectral features allowing accurate radial velocities to be measured, are approximate standard candles if the stars are on the main sequence, and are of high enough luminosity that they can be seen to great distances. This program aims to use F star radial velocities to understand the kinematics of the outer parts of the Milky Way. This program was in the Southern Galactic Cap Equatorial Stripe, but there are similar targets in other programs in the vicinity of M31, and the Perseus cluster. This program selected point sources with -0.3 < (g-r) < 0.3 and 19.0 < g < 20.5 (marked FSTAR).

This plate targeted M-stars in the Hyades cluster (marked HYADES_MSTAR). Most fibers on the plate are extremely low signal-to-noise, but there are ten high signal-to-noise spectra of M-star targets. Note that their spectra are somewhat contaminated by emission lines from their cluster environment.

This program is part of a group of programs that selected samples of low-redshift galaxies deeper than the Legacy Main sample, based on photometric redshifts. Extended sources were selected to satisfy the following cuts i_Petro ≤ 20, i_Petro + (r-i)_model ≥ 17.75, and photometric redshift z_p ≤ 0.17-0.19, where the last condition was chosen plate-by-plate to give enough targets to match the available number of fibers. These targets are marked LOWZ_ANNIS.

This program is part of a group of programs that selected samples of low-redshift galaxies deeper than the Legacy Main sample, based on photometric redshifts. Extended sources were selected to satisfy the following cuts: an EDR photometric redshift from Csabai et al. (2003) > 0.003, r_Petro < 20, and estimated M_r > -18. These targets are marked LOWZ_LOVEDAY.

This program carried out a survey of low-redshift galaxies to 2 magnitudes fainter than the SDSS main sample limit in order to add more low-luminosity galaxies to the sample. It also included a deeper sample of LRGs and Brightest Cluster Galaxies. The exposures here were taken much deeper than ordinary SDSS exposures. For the low-redshift galaxies, we used photometric redshifts derived from second-order polynomial fits to observed Petrosian r magnitudes and model colors, with separate fits done in bins of model g-r color. For Chunks 45, 52, and 62, we used the SDSS EDR photometry and spectroscopy then available to derive photometric redshifts, while for Chunks 74 and 97, we were able to derive improved photometric redshifts using catalog-coadded Stripe 82 SDSS photometry, combined with all available SDSS redshift data on the Southern Equatorial Stripe as of 11 July 2003. This included much of the data taken for the express purpose of calibrating the photometric redshift relation in the SDSS photometric system. Galaxies were then chosen for observations based on their photometric redshift z_p and Petrosian magnitude r_Petro. In particular, the aim was to target as complete a sample as possible for 17.77 ≤ r < 19.0 and true redshift below 0.15, and sparse samples to higher redshifts, as well as at fainter magnitudes 19.0 ≤ r < 19.5. The specific target categories, in order of highest to lowest priority for fiber assignment, are listed in the table below. As there are many more such targets than available fibers, the available targets were sampled sparsely. The sparse sampling fraction values were chosen to get reasonable distributions of objects over the target categories and to keep approximately similar target distributions from chunk to chunk, which resulted in somewhat different sampling fractions for each of the five chunks in which this algorithm was used.

Sparse sampling fractions

magnitudes	redshift	chunk45	chunk52	chunk62	chunk74	chunk97
17.77 ≤ r < 19.0	0.00 ≤ zp < 0.15	1.0	0.85	0.7	1.0	1.0
17.77 ≤ r < 19.0	0.15 ≤ zp < 0.20	0.15	0.1275	0.105	0.15	0.3
17.77 ≤ r < 19.0	0.20 ≤ zp < 0.25	0.15	0.1275	0.105	0.15	0.3
19.0 ≤ r < 19.5	0.00 ≤ zp < 0.15	0.25	0.2	0.15	0.25	0.3
19.0 ≤ r < 19.5	0.15 ≤ zp < 0.20	0.25	0.16	0.1	0.25	0.3
19.0 ≤ r < 19.5	0.20 ≤ zp < 0.25	0.25	0.18	0.15	0.25	0.3
17.77 ≤ r < 19.0	0.25 ≤ zp	0.015	0.015	0.015	0.17	0.3
17.77 ≤ r < 19.0	zp < 0.0	0.65	0.65	0.65	0	0
19.0 ≤ r < 19.5	0.25 ≤ zp	0.005	0.005	0.005	0.15	0.3
19.0 ≤ r < 19.5	zp < 0.00	1.0	1.0	1.0	0	0

A caveat to note is that chunk45 used the star/galaxy separation criteria of the SDSS photometric pipeline to select galaxies, and this resulted in noticeable contamination of stars in several of the chunk45 plates located at lower galactic latitudes. The other chunks used the star/galaxy cut employed by the SDSS main sample target selection algorithm (r_PSF - r_model ≥ 0.3 or 0.24, depending on the version of the photometric pipeline), which is more conservative for selecting galaxies. In these same plates, we targeted LRGs with z > 0.25 which satisfied the Cut I criteria. These spectra serve two purposes: first, to obtain higher signal-to-noise ratio velocity dispersion measurements; and second, to better constrain the spectra of galaxies around the discontinuity in the targeting algorithm at z ~ 0.4 (the distinction between Cut I and Cut II; see Eisenstein et al. 2001). In detail, in these plates some Cut II galaxies were also included (except in chunk97). The Cut I galaxies targeted for deep observations are marked DEEP_GALAXY_RED; the Cut II galaxies targeted for deep observations are marked DEEP_GALAXY_RED and DEEP_GALAXY_RED_II. Finally, in chunk74 and chunk97, these plates also targeted brightest galaxies in clusters. While LRGs are often the brightest galaxies in their clusters, they need not be. The MaxBCG method described by Bahcall et al. (2003) searches for galaxies with the apparent magnitudes and colors of LRGs, together with a red sequence of fainter ellipticals in the vicinity (cf., Gladders & Yee 2000). The BCG program targeted BCG candidates found with this method, with estimated redshifts in the range 0.4 < z < 0.7. The MaxBCG algorithm was run on photometry derived from co-adding the detections (at the catalog level) of multiple scans of the Southern Equatorial Stripe. Such targets are marked BCG.

Near the direction of M31 we targeted F-stars and low-redshift quasars. Low-redshift quasars were targeted on the M31 imaging data using the standard quasar selection algorithm (Richards et al. 2002), but excluding the high-redshift candidates selected from the griz cube (marked QSO_M31). The confirmed quasars can be used to probe gas in the halo of M31. The plates used for this program also included F-star targets similar to those in the fstars program (marked FSTAR). The M31 (e.g., Zucker et al. 2004a and Zucker et al. 2004b) imaging data are available in DR8.

The main-sequence turnoff program was designed to study the kinematics and metallicities of high-latitude thick disk and halo stars. One of the plates overlaps with the area of the disk program described above and uses the extra color cut i-z>0.2 to avoid overlap with that program. One set of spectra is near (l,b) = (64°,-45°), and imposes the conditions that r < 19.15 and g-r < 0.8. The other set is near (l,b) = (114°,-62°) and imposes the additional condition that i-z > 0.2. In both cases, they targets are marked MSTURNOFF.

This program targeted low mass stars in Orion, selected by Peregrine McGehee. There were three classes of low-mass starts selected for, M1 to M3 (marked ORION_EARLY), greater than M3 (marked ORION_LATE) and candidate brown dwarfs (marked ORION_BD). Early M stars needed to satisfy the cuts i < 19.5, r-i > 1, and i-z > 0.6. Late M stars needed to satisfy the cuts i < 19.5, r-i > 1.2, i-z > 1.2 (and anything classified as late M-star was not classified as an early M star). Candidate brown dwarfs needed to satisfy the cuts i < 19.0, r-i > 1.8, i-z > 1.8 (and anything classified as a candidate brown dwarf was not classified as an M star). Additionally, standard targets were chosen here (marked SPECIAL_FILLER, with detailed target selection flags in PRIMTARGET).

This program targeted galaxies and F-stars around the Perseus-Pisces cluster. The F-star selection was the same as that in the fstars program. Imaging scans were taken centered roughly on the Perseus cluster at z ~ 0.018, and were used to target galaxies with a slightly deeper version of the Legacy Main Sample target selection. The double exposure plates 1665, 1666 targeted, and obtained redshifts for, approximately 400 galaxies as faint as r_fiber = 18.8 in a region centered on the cluster. The majority of the galaxies are associated with the cluster, although there are 50 objects in a background overdensity at z ~ 0.05 (Brunzendorf & Meusinger 1999). The Perseus-Pisces imaging scans covering this area are available in DR8.

This program included galaxies likely to be at higher redshifts than the Main sample for the purpose of calibrating photometric redshift techniques. The SDSS five-band photometry goes substantially fainter than does the spectroscopy, allowing photometric redshifts for vastly more objects than have spectroscopy (see, for example, Csabai et al. 2003). Calibrating the photometric redshift relation requires a training sample exploring the same range of apparent magnitudes and colors as the objects for which photometric redshifts will eventually be derived. The SDSS LRG sample (Eisenstein et al. 2001) obtains spectra for red faint (r < 19.5) galaxies; photometric redshifts of this relatively uniform population (e.g., Eisenstein et al. 2003) are fairly robust (e.g., Padmanabhan et al. 2005). The photoz program aimed to create a corresponding sample of faint blue galaxies. The basic set of cuts applied were:

• 0.40 + 0.6(u-g) < g-r < 1.7 - 0.1(u-g)
• -0.5 < u-g < 3.0
• 0 < g-r < 1.8
• -0.5 < r-i < 1.5
• -1 < i-z < 1.5
• 18.0 < u < 24.0
• 18.0 < g < 21.5
• 17.8 < r < 19.5
• 16.5 < i < 20.5
• 16.0 < z < 20.0
• σ_u < 0.6
• σ_g,r,i,z < 0.25

For objects that satisfied the above cuts, the quantity exp[c((g-r) - (0.40 +0.6(u-g)))] was calculated; if it was larger than a random number chosen between zero and one, the object was targeted for spectroscopy (marked PHOTOZ_GALAXY). The coefficient c=0.1411 was chosen to obtain an appropriate density of targets. Note that plates 672 and 809 have the same center, and some of the same objects were inadvertently observed twice. Additionally, standard targets were chosen here (marked SPECIAL_FILLER, with detailed target selection flags in PRIMTARGET).

These plates were pre-BOSS plates meant to be used in testing throughput and target selection issues. Roughly speaking, the luminous red galaxy targets satisfied an early version of the LRG target selection, using preliminary calibrations of the photometry (marked PREBOSS_LRG). The quasar targets were blue point sources roughly in the color range of quasars at redshifts 2 to 3 (marked PREBOSS_QSO).

These plates were pre-MARVELS plates used for stellar pre-selection. The stars included were ones selected as suitable candidates for potential MARVELS followup (marked PREMARVELS). A number of these exposures were very short, due to the fact that we targeted rather bright stars.

The reddening program observed stars (marked TAURUS_STAR) in the Taurus and Orion regions for the purposes of testing the dust reddening law. In addition, some galaxies in these fields were targeted as well (marked TAURUS_GALAXY).

The southern program was executed on the Equatorial stripe in the Southern Galactic Cap, or Stripe 82 in SDSS parlance. It consisted of a number of different targeting classes that in fact evolved over time, as described here. In particular, there were three different chunk (or "tile run") defined over time within this program: chunk22 in 2001, chunk48 in 2002, and chunk73 in 2003. The targeting classes and details of the targeting criteria often vary among these chunks.

Complete Legacy target sample (SOUTHERN_COMPLETE flag)

This program is designed to create a region of the sky where the regular target sample is close to 100 percent complete. The legacy target set is as complete as we can make it. However, over most of the survey area, the subset for which we actually obtain a spectrum is only a bit more than 90%. This incompleteness has several causes, including the fact that two spectroscopic fibers cannot be placed closer than 55 arcseconds on a given plate, possible gaps between the plates, fibers that fall out of their holes, and so on. This program aimed to observe the remaining 10% of galaxy targets in order to have a region of sky with truly complete galaxy spectroscopic coverage. This is particularly important for studies of galaxy pairs, which are by definition strongly affected by the 55-arcsecond rule. The target selection algorithm is simple: all galaxies selected by the algorithm described in Strauss et al. 2002 in the Southern Equatorial Stripe, minus those galaxies that actually have a successfully measured redshift in routine targeting. Many of these galaxies are included in chunk22, where they comprised a filler sample. They were included as a primary target set in chunk48 and chunk73. All such targets are flagged SOUTHERN_COMPLETE. To recover the exact reason for their initial targeting, it is necessary to consult the original values of the PRIMTARGET and SECTARGET flags must be used to distinguish the two, using the target bit definitions of LEGACY_TARGET1.

Double-lobed radio sources (STRAIGHT_RADIO and BENT_RADIO flags)

In chunk48 we also targeted a small number of known double-lobed radio sources. Quasar target selection targets unresolved optical counterparts to FIRST radio sources, while "serendipity" target selection (see the EDR paper, Stoughton et al. 2002) selects FIRST sources with extended optical counterparts. This works fine for compact or core-dominated radio sources, but is less effective for double-lobed radio sources, in which the optical counterpart is associated with a point between the two. An algorithm was developed to find these double-lobed sources, including allowing for the more challenging case of bent double sources. In particular, pairs of FIRST sources separated by 90" or less without SDSS optical counterparts were identified. Given the distance d between the centers of these two sources, a rectangle is drawn centered at the midpoint between the sources, with dimensions 0.57d, 1.33d, with the short axis parallel to the line connecting the two sources. This box size was chosen empirically to include the core of a sample of bent double sources compiled by Elizabeth Blanton (see Blanton et al. 2001). Optical counterparts with r < 19.8 that fell into the box were selected as bent double counterparts. A subset of those objects, which fell into a 12 arcsec square centered on the midpoint between the sources, were selected as straight double counterparts. The straight double-lobed sources are marked STRAIGHT_RADIO, while the bent double-lobed sources are marked BENT_RADIO.

Extended Legacy target selection: Main, LRG, and QSO samples (SOUTHERN_EXTENDED flag)

In chunk22, observed in Fall 2001, we extended the main and LRG survey target selection algorithms for some areas of Stripe 82. In particular, the main galaxy sample (Strauss et al. 2002) was modified only slightly, by removing the cut on objects with half-light Petrosian r band surface brightness μ_50,r below 24.5 mag/arcsec². This adds less than one object per square degree. In these plates, the LRG sample was extended to probe about 0.3 mag fainter than for the ordinary survey selection described by Eisenstein et al (2001). In particular, the following cuts were applied, which yielded about 40 extra objects/deg²:

• r < 19.5
• r < 13.4 + c_||/0.3
• μ_50,r < 25
• |c_||| < 0.4

for Cut I, and

• r < 19.5
• g-r > 1.65 - c_⊥
• μ_50,r < 25
• r_PSF-r_model > 0.3

for Cut II, with

• c_|| = 0.7(g-r) + 1.2(r-i-0.18)
• c_⊥ = (r-i) - (g-r)/8

Finally, the quasar target selection algorithm (Richards et al. 2002) was extended in the following two ways: to fainter flux limits, and to broader color cuts. First, for objects selected from the ugri color cube, the magnitude limit was changed from i = 19.1 to 19.9, while for the griz color cube (where high-redshift quasars are selected), we changed the limit from i = 20.2 to 20.4. Second, we increased the range of colors that QSO targets could have. As Richards et al. (2002) describe, there are regions outside the stellar locus that are heavily contaminated by hot white dwarfs, M dwarf-white dwarf pairs (Smolcic et al. 2004), and other non-quasars. These regions are explicitly excluded from quasar target selection in the main survey; however, in the extension, they are allowed back in. Similarly, there is a region of color space where z ~ 2.7 quasars intersect the stellar locus. In the Legacy survey, objects falling in this region are sparse-sampled to 10% (to reduce the number of stars); in the extension, all objects falling in the mid-z box defined in Richards et al. (2002) are targeted. All sets of targets are marked SOUTHERN_EXTENDED (this is a case where the original values of the PRIMTARGET and SECTARGET flags must be used to distinguish the two, using the target bit definitions of LEGACY_TARGET1.

Faint LRG sample (FAINT_LRG flag)

The LRG Cut II sample aims for a flux-limited sample of LRGs with redshifts roughly between 0.40 and 0.55. We also experimented with an extension of this cut, going substantially fainter. Objects that passed the following cuts were marked FAINT_LRG:

• 17.5 < i_deV < 20
• i_Petro < 19.1
• 0.5 < g-r < 3.0
• 0.0 < r-i < 2.0
• c_|| [ = 0.7(g-r) + 1.2(r-i-0.18)] > 1.6
• c_⊥ [ = (r-i) - (g-r)/8 ] > 0.5

Here, deV refers to deVaucouleurs model magnitude, Petro to Petrosian magnitude, and all colors are based on model magnitudes (see the magnitudes algorithms page).

Faint quasar sample (FAINT_QSO flag)

In chunk48 the quasar target selection was further modified for the faint quasar targets on the southern plates. In particular, the following are the changes over the standard quasar target selection algorithm described in Richards et al. (2002):

• The magnitude limit is set to i= 20.1, rather than i = 19.1 for the ugri color cube;
• The magnitude limit for optical counterparts to FIRST sources is set to i = 20.65, rather than i = 19.1;
• The standard quasar target selection algorithm requires that the estimated PSF magnitude errors in u and g both be less than 0.1 for UV excess sources (u-g<0.6). This limit is now set to 0.2.
• For UV excess objects with 19.1 ≤ i < 20.1, we add a requirement that g - r < 0.7 to minimize stellar contamination.
• Objects in the "mid-z box" are excluded altogether.
• In addition to the usual "distance from the stellar locus"algorithm used to target quasars, in the main sample there are hard color cuts used to select high-redshift quasars in the griz color cube. These color cuts are not used in the Southern targeting.
• Finally, there were additional color cuts to reject unphysical objects affected by bad CCD columns; we required g-r>-0.5, r-i>-0.5, and i-z>-0.6.

Such targets are marked FAINT_QSO.

High proper motion stars (HIPM flag)

For chunk73, a sample of high proper motion stars was defined using proper motions determined using the methods of Munn et al. (2004), where a consistent astrometric solution was found matching USNO-B data (Monet et al. 2003) with SDSS. There were two cuts:

1. A simple cut of proper motion μ > 100 mas/y
2. Cuts using reduced proper motion H_r defined by:H_r = r + 5 log₁₀ μ + 5 The sample was defined by: g-i < 2 H_r > 16 2 < g-i < 2.375 H_r > 8 + 4 (g-i) 2.375 < g-i H_r > 17.5

Stars selected in this manner are marked HIPM.

Spectra of Everything (ALL_PSF* flags)

As part of an exploration of the full stellar locus, as well as a search for unusual objects of all sorts, we carried out a survey of all stellar objects ("Spectra of Everything"), which was used as a filler for a series of so-called merged program plates, which included a mixture of mostly extragalactic targets. In 2002 (chunk48), this included a random sampling of all point sources with clean photometry (see the discussion of fatal and non-fatal flags in Richards et al. 2002) with extinction-corrected i-band PSF magnitudes brighter than 19.1. Not surprisingly, the vast majority of the targets were chosen from the densest core of the stellar locus in color-color space. These target were flagged ALLPSF In 2003 (chunk73), in an attempt to put greater weight on the wings of the stellar locus, we revised the selection algorithm slightly. We defined a distance from the ridge of the stellar locus, by asking for the median and standard deviation u-g, g-r, and i-z of stars in narrow bins of r-i. Having tabulated these, we calculated a crude χ²-like quantity: L = 1/3 Σ_u-g,g-r,i-z ((color-median color)/(standard deviation))² 75% of all stars had L < 1. We gave all stars with L > 1 highest priority (and marked them ALLPSF_NONSTELLAR), and for L < 1, the priority decreased smoothly with L (and they were marked ALLPSF_STELLAR). These spectra were used for a determination of the completeness of the quasar target selection algorithm (Vanden Berk et al. 2005). The overwhelming majority of these objects are confirmed to be stars; only 10 of the 19,543 of the Spectra of Everything targets analyzed in that paper are quasars not previously targeted as such.

u-band galaxies (U_PRIORITY, U_EXTRA, U_EXTRA2 flags)

This program is designed to create a deeper u-band flux-limited sample of galaxies than is possible using the normal Main sample. The main galaxy target selection is carried out in the r band, and is flux-limited at r = 17.77. The bluest galaxies have u-r ~ 0.6, thus a magnitude-limited in the u band corresponds to u ~ 18.4. In order to explore the u-band luminosity density of the universe, and explore the recent history of star formation in the universe, we carried out a u-band survey of galaxies in the SDSS. Baldry et al. (2005) describe the sample in detail and present the resulting u-band luminosity function. In the selection criteria below, we use a (dereddened) "Pseudo-Petrosian" u-band magnitude u_select = u_model - r_model + r_Petro. Note that the objects targeted in this program do not include objects with spectroscopic observations already in hand. Thus one needs to combine objects from various programs to define a complete sample. See the discussion in Baldry et al. (2005) for determination of the completeness of the resulting sample. For chunk48, the flux limit was at u = 19.8; for chunk73 we included those galaxies but at lower priority targeted a slightly fainter set as well. The exact cuts and resulting bits in SPECIAL_TARGET1 are described in the table below.

Chunks	SPECIAL_TARGET1 flag	Selection
chunk48, chunk73	U_PRIORITY	uselect < 19.8 gPetro < 20.5 17.5 < rPetro < 20.5 μ50,r < 24.5 rPSF-rmodel > 0.2
chunk73	U_EXTRA	uselect < 20.3 or umodel < 19.8 or uPetro < 19.5 gPetro < 19.5 17.3 < rPetro < 20.7 μ50,r < 24.7 rPSF-rmodel > 0.15
chunk73	U_EXTRA2	uselect < 20.3 gPetro < 20.5 17.5 < rPetro < 20.5 μ50,r < 24.5 rPSF-rmodel > 0.2

Variable sources (VARIABLE_HIPRI and VARIABLE_LOPRI flags)

In chunk73 we also targeted some sources selected for variability. The Southern Equatorial Stripe was imaged many times in the course of the SDSS, allowing the study of photometric variability. For this targeting class (executed before the major SNe campaigns of SDSS-II), variable sources were selected for follow-up spectroscopy using pairs of observations of unique unsaturated point sources with i < 21. We required that the changes in the g and r bands exceeded 0.1 mag, and were at least 3 sigma significant (using error estimates computed by the photometric pipeline). The time difference between the two observations varied from 56 days to 1212 days. This target selection produced targets over the Southern Equatorial stripe with a surface density of about 10 objects deg^-2. The majority of these targets have colors consistent with z < 2 (UV excess) quasars and RR Lyrae stars, classifications confirmed by the spectroscopy. We prioritized targets with i < 19.5, which we mark with VARIABLE_HIPRI. Targets with 19.5 ≤ i < 21 we mark with VARIABLE_LOPRI.

The taurus program targeted stars in Taurus program, mainly selected to be M-dwarfs or potential brown dwarfs using similar cuts to those in the orion program (marked TAURUS_STAR).