Tests of eBOSS Target Selection in CFHT-LS W3 Field


Photo of Adam Myers
Adam Myers
University of Wyoming


A survey designed to improve target selection of galaxies and quasars for the future eBOSS survey of SDSS-IV

Finding Targets

An object whose ANCILLARY_TARGET2 value includes one or more of the bitmasks in the following table was targeted for spectroscopy as part of this ancillary target program. See SDSS bitmasks to learn how to use these values to identify objects in this ancillary target program.

Program (bit name) Bit number Target Description Number of Fibers
FAINT_HIZ_LRG 30 Candidate galaxy in the redshift range 0.6 < z < 0.9, defined by color cuts 682
QSO_EBOSS_W3_ADM 31 Candidate quasar, defined by one of the complex color cuts described below and recorded in the W3bitmask parameter of the original targeting file 3,510


As a test of target selection algorithms to be used in the SDSS’s upcoming eBOSS survey, six plates were dedicated to a selection of LRGs and quasars at high density over a region of sky overlapping the survey area of the Canada-France-Hawaii Telescope Legacy Survey (CFHTLS)’s W3 imaging footprint.

The quasar component of this ancillary science program was designed to target the CFHTLS W3 region to construct various samples of quasars useful for eBOSS testing. The goals were to find as many quasars as possible to r ~ 22 with a variety of methods, while also testing typical quasar selection using data to the depth of CFHT, so as to compare selection using CFHT-depth data to that using SDSS-depth data.

Target Selection

Targets selected as potential galaxies in the redshift range 0.6 < z < 0.9 were denoted FAINT_HIZ_LRG. These objects were selected in a similar manner to the HIZ_LRG targets from Exploring z > 0.6 LRGs from SDSS and WISE ancillary science program, but at fainter magnitudes and with a new tuning of color cuts. Targets were required to have the following characteristics:

  • 20 < z < 20.5
  • zfib2 < 22.2 || ifib2 < 22.5
  • (r – i) > 0.98
  • (r – W1) > 2(r – i)

Quasar targets, which were assigned the QSO_EBOSS_W3_ADM target class, were selected from photometry from CHFTLS, SDSS, and WISE, as well as variability data from PTF. Five selection techniques were applied, and all were assigned the same target bit. The specific selection technique used to select a given quasar is recorded in the W3bitmask parameter of the original targeting file, hosted on this site along with the matching README file.

Each of the quasar target selection criteria is explained below. The bit numbers quoted here do not refer to bits in ANCILLARY_TARGET2, but rather to W3bitmask in the original target file.

Bit 0: W3 color box selection

These objects were selected from the CFHTLS W3 co-added catalog, available at VizieR. The objects were restricted in CFHT magnitudes to g < 22:8. Stars were excised with the following color cuts (using CFHT photometry):

(g – r) – 0.5(u – g) < 0.2 || (g – r) + 0.7(u – g) < 0.6

The targets were also required to be classified as point sources by SDSS, and to have SDSS r magnitudes in the range 17 < r < 22.

Bit 1: SDSS XDQSOz selection

These objects were selected using the XDQSOz selection of Bovy et al. (2012) based on SDSS photometry. Point sources with 17 < r < 22 were required to have a probability of being a quasar greater than 0.2.

Bit 2: SDSS-WISE selection

This selection technique used WISE forced photometry at SDSS source positions (Lang 2014; Lang, Hogg, & Schlegel 2016). A stacked flux was created in SDSS gri [mopt; with a relative (g,r,i) weighting of (1,0.8,0.6)], and a stacked flux wise WISE W1 and W2 [mwise; with (W1,W2 weights of (1,0.5)].

Objects were selected with the following characteristics:

  • 17 < mopt < 22
  • (g – i) < 1.5
  • moptmwise > (g – i) + 3.0

Extended sources were allowed; the sample was restricted to sources with a difference between SDSS PSF and model magnitudes of less than 0.1.

Bit 3: CFHTLS variability selection

Using three years of observation in the one-square-degree field D3, objects were selected based on the variability measured in their light curves.

Objects were selected on Χ2 and structure function parameters A and Γ (Palanque-Delabrouille 2011) averaged over the three bands gri. Two selections were applied using colors c1 and c3 defined as in Fan (1999):

  • c1 ≡ 0.95*(u-g) + 0.31*(g-r) + 0.11*(r-i)
  • c3 ≡ -0.39*(u-g) + 0.79*(g-r) + 0.47*(r-i)

The first selection used only CFHT information, such that:

  • A > 0.08
  • χ2 > 10.0
  • Γ > 0.3
  • c3 < 0.6 – 0.33c1
  • g < 23.0

The second selection used both SDSS and CFHT information, requiring that the object be pointlike in SDSS, and also that:

  • A > 0.08
  • χ2 > 10.0
  • Γ > 0.2
  • g < 22.0

Bit 4: PTF variability selection

Using light curves computed from PTF R-band imaging linked to SDSS r with a color correction, quasar candidates were again selected by variability.

All structure function or color-term parameters are defined as above in Bit 3. Objects were required to have:

  • A > 0.05
  • χ2 > 10.0
  • Γ > 0.1

In addition, the objects were limited to g < 22.5 and had to pass either of the following two criteria based on
SDSS photometry: a color and magnitude cut with r > 18 and c3 < 1.0 – 0.33c1; or a color and morphology cut requiring the object to be classified as pointlike by SDSS and to have a probability of being a quasar greater than 0.1 according to the XDQSO algorithm.


Bovy, J., et al. 2012, ApJ, 749, 41

Fan, X. 1999, AJ, 117, 2528

Lang, D. 2014, AJ, 147, 108

Lang, D., D. W., & Schlegel, D. J. 2016, AJ, 151, 36

Palanque-Delabrouille, N., et al. 2011, A&A, 530, 122