SEGUE
SEGUE

SEGUE Target Selection


Example color-color diagram of SEGUE target selection categories.

Current version of the target selection algorithm is v4.2.

Previous versions: v4.0 v3.4 v3.3 v3.2 v3.1 v3.0 v2.0

The plate list table denotes which version of the target selection was used for each plate. Notes at the beginning of each version of the target selection highlight changes from previous version. In addition, there are occasional special target selection cuts utilized for specific plates/targets.

Special targets


Target Categories

  • White Dwarf
  • Cool White Dwarf
  • A/BHB
  • F turnoff, subdwarf
  • F/G stars
  • G stars
  • K giant
  • Low Metallicity
  • K dwarf
  • Main sequence - White dwarf pair
  • M sub-dwarf
  • L, T Brown dwarf
  • AGB candidate

    Overview

    SEGUE has as one of its goals a kinematic and stellar population study of the high-latitude thick disk and halo of the Milky Way. The halo is sampled sparsely with a series of seven deg2 tiles both in the SEGUE imaging stripes and the main Legacy survey area, with centers separated by roughly 10 deg. Each such tile is sampled with two pointings, split at r=17.8, approximately the median target magnitude.

    Half of the ~1180 targets on each line of sight are selected using straightforward color-color and color-magnitude cuts designed to sample across the main sequence from g-r = 0.75 (K-dwarfs at T eff ~ 5000K) through the main sequence turnoff. To this sample we add metal-poor main sequence turnoff stars selected by their blue ugr colors, essentially an ultraviolet excess cut that is extremely efficient at separating the halo from the thick disk near the turnoff. At r = 19.5, the average star that makes this selection is at a heliocentric distance of 10 kpc for [Fe/H] = -1.54. To reach to greater distances, we use the strength of the Balmer jump to select field blue horizontal branch (BHB) stars in the ugr color-color diagram. The halo BHB sample extends to distances of 40 kpc, and we select all available BHB candidates in our high-latitude fields.

    The most distant spectroscopic target category in SEGUE is halo red giants. We select halo giant candidates by the photometric offset in the ugr color-color diagram caused by their ultraviolet excess and weak MgH relative to foreground disk dwarfs, augmented by a 3σ proper motion cut using a recalibrated version of the USNO-B catalog. Spectroscopic identification of true giants using the methodology in has shown that the giant selection is roughly 50% efficient at g < 17, the current limit to which we can reliably identify giants in the spectra. The halo giant sample identified in this way reaches distances of 40 kpc from the sun.

    The spectroscopic selection also includes smaller categories of rare but interesting objects. These include cool white dwarfs selected with the recalibrated USNO-B reduced proper motion diagram, which can be used to date the age of the Galactic disk, and high proper motion targets from the SUPERBLINK catalog, which have uncovered some of the most extreme M subdwarfs known and aided in the calibration of their metallicity scale using common proper motion pairs.


    v4.2 Target Selection

    Notes:

    • All ugriz magnitudes listed below are dereddened PSF mags objc_flag[2] check:
      No bright, no edge, no bad interp, no bad deblends
    • For selections which involve u band, we require that the u flags indicate a u band detection.
    • The 'regular SDSS bright end cut' is:
      No spectra of objects with g,r or i fiber mag < 14.5 allowed.
    • For SEGUE v2_0 we cut on r only at rfiber < 14.5. This resulted in fiber warnings for red stars where
      r-i > 0 → i < 14.5.
      The spectra were ok however and scattered light was not harmful to adjacent fibers on the CCDs.
    • For SEGUE v3_0 we cut on ifiber < 14.2. This allows bright blue things to get spectra and still avoids putting very red bright stars on the plate to scatter.
    • Also, proper motions have been included for objects which have matches to the USNO-B catalog and are used in the K giant and Cool White dwarf selection categories.
    • Low Latitude Algorithms implemented:
      • AGB type objects at low latitude
      • K III type objects at low latitude
      • Bluetip objects at low latitude
      • AMNH high proper motion objects at low latitude
    • Principle component color definitions:
       
      l = -0.436u + 1.129g - 0.119r - 0.574i + 0.1984
       
      and is a photometry metalicity indicator for stars in the color range 0.5 < g-r < 0.8.
      Reference: Lenz, D., Newberg, H., Rosner, R., Richards, G. & Stoughton, C. 1998 ApJS 119, 121
       
      s = -0.249u + 0.794g - 0.555r + 0.234
       
      P1(s) = P1 = 0.91(u-g) + 0.415(g-r) -1.280
       
      Reference: Helmi et al 2003

    White dwarf selection

    (bit -2146959360 → 0x80080000)
    Color cuts:
    g < 20.3
    -1 < g-r < -0.2
    -1 < u-g < 0.7
    u-g + 2(g-r) < -0.1
    Weighting: magslope 1.2
    Target count Goal: 25 per plate pair

    Cool White dwarf selection

    (bit -2147352576 → 0x80020000)
    Color cuts:
    14.5 < r < 20.5
    -2 < g-i
    g-i < 1.7 (if no neighbor with g > 22 within 7 arcsec)
    otherwise g-i < 0.12
    Require reliable proper motion (μ) measurement:
    • one-to-one match to USNO_B
    • good proper motion fit (small rms in each coordinate)
    • be detected in all 5 epochs in USNO-B
    • no neighbor brighter than g = 22 within 7 arcsec (to avoid blending problems)
      OR
      blue enough (g-i < 0.12) that target is likely white dwarf
    Cuts on reduced proper motion Hg = g + 5*log(μ) + 5
    Hg > 17.5
    Hg > 16.05 + 2.9*(g-i)

    Weighting: magslope 1.2
    Target count Goal: 10 per plate pair (but allow overflow if they are present)

    A/BHB selection

    (bit -2147475456 → 0x80002000)
    Color cuts:
    g < 20.5
    0.8 < u-g < 1.5
    -0.5 < g-r < 0.2
    u detection present
    Also compute color-gravity index:
    v = 0.283(u-g) - 0.354(g-r) + 0.455 (r-i) + 0.766 (i-z)
    Weighting: weight on the 'v' parameter so object with v color of 0 is 4 times as likely to get fiber as v of 0.4.
    Target count Goal: 150 per plate pair (get ~100 per pair)

    F turnoff, sub-dwarf selection

    (bit -2146435072 → 0x80100000)
    Color cuts:
    -0.7 < P1(s) < -0.25
    0.4 < u-g < 1.4
    0.2 < g-r < 0.7
    g < 20.3
    • P1 is a color perpendicular to the locus for F/G/K stars; more negative P1 indicates lower-metallicity side of locus.
    • Note: there was an s-color cut in versions prior to v3.3, s &ge -0.065, no longer present for v3.3 and beyond.

    Weighting: colorslope+ magnitude weighting P1 min/max slope = 5
    Target count Goal: 200 per plate pair

    F/G stars

    (bit -2147483136 → 0x80000200)
    Color cuts:
    0.2 < g-r < 0.48
    14.0 < g < 20.2

    Unbiased random subsampling of this range of g-r color space
    Target count Goal: 50 per plate pair

    G selection

    (bit -2147221504 → 0x80040000)
    Color cuts:
    14.0 < r < 20.2
    0.48 < g-r < 0.55
    Note: slightly wider g-r color band to get sufficient targets at high |b|.

    Weighting: random
    Target count Goal: 375 per plate pair

    K Giant Selection

    (bit -2147467264 → 0x80004000)
    Color cuts:
    r < 20.2
    0.7 < u-g < 4.0
    0.35 < g-r < 0.8
    0.15 < r-i < 0.6
    l-color > 0.07
    Also: Proper motion < 11 milliarcsec/year and good proper motion match

    Weighting: magslope 2.0
    Target count Goal: 95 per plate pair (expected density 4/sq deg = 28/plate pair)

    Low Metallicity selection

    (bit -2147418112 → 0x80010000)
    Color cuts:
    14 < r < 19.5
    -0.5 < g-r < 0.75
    0.6 < u-g < 3.0
    l-color > 0.135
    require detection in u band

    Weighting: magslope 1.8 and faint plate targets sorted by magnitude to 'pack' them, so that brightest ones targeted before fainter ones.
    Target count Goal: 150 per plate pair

    K dwarf selection

    (bit -2147450880 → 0x80008000)
    Color cuts:
    14.5 < r < 19.0
    0.55 < g-r < 0.75

    Selection: Random subsample
    Target count Goal: 95 per plate pair

    Main Sequence - White Dwarf pairs

    (bit -2147479552 → 0x80001000)

    Note that for this category only: colors are NOT dereddened.

    Color cuts (from Potsdam group):
    u-g < 2.25
    g-r > -0.2
    g-r < 1.2
    r-i > 0.5
    r-i < 2.0
    g-r > -19.78*(r-i) + 11.13
    g-r < 0.95*(r-i) + 0.5
    i-z > 0.5 for r-i > 1.0
    i-z > 0.68*(r-i)-0.18 for r-i &le 1.0
    15 < g < 20

    Target goal: 5 per plate pair, maximum of 10

    M sub-dwarf selection

    (bit -2143289344 → 0x80400000)
    Color cuts (based on West et al):
    g-r > 1.6
    0.95 < r-i < 1.3
    14.5 < r < 19.0

    Selection: magslope 0.9
    Target count Goal: < ~5 per plate pair

    Plus:
  • Lepine Shara AMNH high proper motion stars
  • Goal 25-30 per plate pair
  • Proper motion selected.

    L,T brown-dwarf selection

    (bit -2145386496 → 0x80200000)
    Color cuts:
    z < 19.5
    u > 21
    g > 22
    r > 21
    i-z > 1.7
    Note: for brown dwarf selection we require additional flag checks on i and z band as suggested by X.Fan.
    Target count Goal: < 5 per plate pair

    AGB candidate selection

    (bit -2139095040 → 0x80800000)
    Color cuts:
    14.0 < r < 19.0
    2.5 < u-g < 3.5
    0.9 < g-r < 1.3
    s-color < -0.06
    detection in u band

    Selection: magslope 1.4
    Target count Goal: 10 per plate pair


    Special Targets

    Plates 2336, 2337:
    Special K giant selection

    Approximately 200 fibers allocated to objects with

  • 0.35 < g-r < 0.9
  • proper motion consistent with zero.
  • No selection boundary parallel to the stellar locus imposed.

    Another 200 fibers allocated to random subsample of stellar locus with

  • 0.2 < g-r < 0.48

    These two plates are at (l,b) = (130, -23), (150,+26) respectively, and likely contain Monoceros Giants.


    v4.0 Target Selection

    Summary of changes from v3.4:

  • Low Latitude Algorithms implemented:
    • AGB type objects at low lat
    • K III type objects at low lat
    • Bluetip objects at low lat
    • AMNH high proper motion objects at low lat

    v3.4 Target Selection

    Summary of changes from v3.3:

  • MSWD colors are now NOT dereddened psf mags (previously were dereddened).
  • 'l-cut' for Low metallicity backed off to 0.135 from 0.15, which allows more in, as too many bright candidates were not getting fibers.

    v3.3 Target Selection

    Summary of changes from v3.2:

    A/BHB

  • new color cuts (H. Newberg algorithm)
    • g < 20.5, 0.8 < u-g < 1.5, -0.5 < g-r < 0.2, u detection present
  • Also: compute color-gravity index:
    v = 0.283(u-g) - 0.354(g-r) + 0.455 (r-i) + 0.766 (i-z)
  • Weighting: weight on the 'v' parameter so object with v color of 0 is 4 times as likely to get fiber as v of 0.4.
  • Target count Goal: 150 per plate pair (get ~100 per pair)

    F turnoff

  • s-color cut is no longer present

    *Addition of new F/G stars category:

  • bit -2147483136
  • 0.2 < g-r < 0.48, 14.0 < g < 20.2, unbiased random subsampling of this range of g-r color space,
  • Target count Goal: 50 per plate pair

    Low Metallicity

  • New for v3_3: We move the 'l' weight to l-color > 0.15 (was > 0.12). This is more restrictive, hoping to land higher fraction of [Fe/H] < -2 objects.

    *Addition of new Main Sequence-White Dwarf pair category:

    (based on color cuts from Potsdam group)

    Note that for this category only: colors are NOT dereddened.

  • bit -2147479552
    • u-g < 2.25
    • g-r > -0.2
    • g-r < 1.2
    • r-i > 0.5
    • r-i < 2.0
    • g-r > -19.78*(r-i) + 11.13
    • g-r < 0.95*(r-i) + 0.5
    • i-z > 0.5 for r-i > 1.0
    • i-z > 0.68*(r-i)-0.18 for r-i <= 1.0
    • 15 < g < 20
  • Target goal: 5 per plate pair, maximum of 10

    v3.2 Target Selection

    Summary of changes from v3.1:

    A/BHB selection

  • CHANGE For v3_2: Sort candidates by color, so that all, or most, blue BHBs g-r < 0 , get a fiber.

    v3.1 Target Selection

    Summary of changes from v3.0 (in bold):

    Cool White Dwarf:

  • Target goal: 10 per plate pair, but allow overflow if they are present.

    K Giant

  • Target Goal: 95 per plate pair (expected density 4/sq deg = 28/plate pair)

    Low Metallicity

  • New for v 3.1: We also weight by l-color to favor the ones closer to the blue edge of the u-g color in the stellar locus.

    K dwarf

  • Target Goal: 95 per plate pair

    M sub-dwarf

  • M sub-dwarfs (color based West et al ) g-r > 1.6, 0.95 < r-i < 1.3, 14.5 < r < 19.0
  • Target count Goal: <~5 per plate pair
  • Selection: magslope 0.9
  • Plus: Lepine Shara AMNH high proper motion stars , goal 25-30 per plate pair, proper motion selected.

    v3.0 Target Selection

    Summary of changes from v2.0:

    • For selections which involve u band, we require that the u flags indicate a u band detection.
    • The 'regular SDSS bright end cut' is: No spectra of objects with g, r or i fiber mag < 14.5 allowed.
    • For SEGUE v2.0 we cut on r only at rfiber < 14.5, this resulted in "whopping" fiber warnings for red stars where r-i > 0 → i < 14.5. The spectra were ok however and scattered light was not harmful to adjacent fibers on the CCDs.
    • For SEGUE v3_0 we are cutting on ifiber < 14.2.
      This allows bright blue things to get spectra and still avoids putting very red bright stars on the plate to scatter.
    • Also, proper motions have been included for objects which have matches to the USNO-B catalog and are used in the K giant and Cool White dwarf selection categories.
    • Significant changes to Cool White Dwarf, M dwarf
    • Addition of Brown dwarf category
    Specific category changes (in bold):

    White Dwarf:

  • g < 20.3 , -1 < g-r < -0.2, -1 < u-g < 0.7
  • u-g + 2(g-r) < -0.1
  • Weighting: magslope 1.2
  • Target goal: 25 per plate pair

    Cool White Dwarf:

      Color cuts:
      14.5 < r < 20.5
      -2 < g-i
      g-i < 1.7 (if no neighbor with g > 22 within 7 arcsec)
      otherwise g-i < 0.12
      Require reliable proper motion (μ) measurement:
      • one-to-one match to USNO_B
      • good proper motion fit (small rms in each coordinate)
      • be detected in all 5 epochs in USNO-B
      • no neighbor brighter than g = 22 within 7 arcsec (to avoid blending problems)
        OR
        blue enough (g-i < 0.12) that target is likely white dwarf
    • Weighting: magslope 1.2
    • Target goal: 10 per plate pair

    A/BHB

  • g < 20.5, 0.6 < u-g < 1.4, -0.5 < g-r < 0.2, scolor < -0.065
  • Weighting: magslope 2.0
  • Target goal: 150 per plate pair (get ~ 100 per pair)

    F turnoff, subdwarf

  • -0.7 < p1(s) < -0.25, 0.4 < u-g < 1.4, 0.2 < g-r < 3.0, g < 20.3
  • Weighting: colorslope + magnitude wighting P1 min/max slope = 5
  • Target goal: 200 per plate pair

    G star

  • 14.0 < r < 20.2, 0.48 < g-r < 0.55
  • Note slightly wider g-r color band to get sufficient targets at high |b|.
  • Weighting: random
  • Target goal: 375 per plate pair

    K Giant

  • r < 20.2, 0.7 < u-g < 4.0 , 0.35 < g-r < 0.8, 0.15 < r-i < 0.6, l-color > 0.07
  • proper_motion < 11 milliarcsec/year and good propermotion match
  • Weighting: magslope 2.0
  • Target Goal: 100 per plate pair

    Low Metallicity

  • 14 < r < 19.5, -0.5 < g-r < 0.75, 0.3 < u-g < 3.0, l-color > 0.12
  • also require detection in u band.
  • Weighting: magslope 1.8 and
  • Faint plate targets sorted by magnitude to 'pack' them, so that brightest ones targeted before fainter ones.
  • Target Goal: 150 per plate pair

    K dwarf

  • 14.5 < r < 19.0, 0.55 < g-r < 0.75
  • Selection: Random subsample
  • Target count Goal: 100 per plate pair

    *M sub-dwarf

  • bit -2143289344
  • M sub-dwarfs (color based West et al )
  • g-r > 1.51, 0.95 < r-i < 1.3, 14.5 < r < 19.0
  • Target count Goal: <~25 per plate pair
  • Selection: magslope 0.9

    *L,T brown-dwarf selection

  • bit -2145386496
  • z < 19.5, u > 21, g > 22, r > 21, i-z > 1.7
  • for Brown dwarf selection we require additional flag checks on i and z band as suggested by X. Fan.
  • Target count Goal: < 5 per plate pair

    AGB candidate selection

  • 14.0 < r < 19.0, 2.5 < u-g < 3.5, 0.9 < g-r < 1.3, s-color < -0.06
  • detection in u band required.
  • Selection: magslope 1.4
  • Target count Goal: 10 per plate pair

    v2.0 Target Selection

    All magnitudes are dereddened PSF mags
    Flags: No bright, no edge, no bad interp, no bad deblend

    White dwarf selection

  • bit -2146959360
  • g < 20.3 , -1 < g-r < -0.2, -1 < u-g < 0.5
  • Weighting: colorslope
  • Target count Goal: 25 per plate pair

    Cool White dwarf selection

  • bit -2147352576
  • 15 < r < 20, -0.1 < g-r < 1.1, g-r > 2.4(r-i) + 0.5, i-z < 0
  • Weighting: colorslope
  • Target count Goal: 10 per plate pair

    A/BHB selection

  • bit -2147475456
  • g < 20.5, 0.5 < u-g < 1.4, -0.8 < g-r < 0.2, scolor < -0.065
  • Weighting: colorslope
  • Target count Goal: 150 per plate pair

    F turnoff, sub-dwarf selection

  • bit -2146435072
  • -0.7 < p1(s) < -0.3, 0.4 < u-g < 1.7, -0.3 < g-r < 3, g < 20.3
  • Weighting: colorslope+ magnitude weighting
  • Target count Goal: 150 per plate pair

    G selection

  • bit -2147221504
  • 14.2 < r < 20.2, 0.50 < g-r < 0.55
  • Weighting: random
  • Target count Goal: 375 per plate pair

    K Giant Selection

  • bit -2147467264
  • r < 20.2, 0.7 < u-g < 4.0 , 0.40 < g-r < 1.2, 0.15 < r-i < 0.6, l-color > 0.1
  • proper_motion < 15 arcsec/century
    (this should have been 1.5 arcsec/cent)
  • Weighting: colorslope
  • Target count Goal: 100 per plate pair

    Low Metallicity selection

  • bit -2147418112
  • r < 20.2, -0.5 < g-r < 0.9, 0.3 < u-g < 3.0, l-color > 0.15
  • Weighting: colorslope
  • Target count Goal: 150 per plate pair

    K dwarf selection

  • bit -2147450880
  • 14.5 < r < 19.5, g-r > 0.7, r-i < 0.8
  • Selection: Random
  • Target count Goal: 125 per plate pair

    M dwarf selection

  • bit -2145386496
  • Three selections in one:
    • regular M dwarfs
    • high velocity M dwarfs (ppm based),
    • M sub-dwarfs (color based)
  • regular M dwarfs: r-i > 0.3, 14.5 < r < 19.0 (N=12)
  • sub-dwarf cands: u-g > 1.8, g-r > 0.8, 0.5 < r-i , 14.5 < r < 19 (N=15)
  • high-velocity M dwarfs: r-i > 0.3, 14.5 < r < 19.3
  • velocity proxy >~ 100 km/s (25)
  • Selection: colorslope (Mdwarfs), high velocity weighted colorslope, colorslope (SubDwarfs)
  • Target count Goal: 50 per plate pair (sum of three sub-categories)

    AGB candidate selection

  • bit -2139095040
  • 14.5 < r < 19.5, 2.5 < u-g < 3.5, 0.9 < g-r < 1.3, s-color < -0.07
  • Selection: colorslope
  • Target count Goal: 15

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