SEGUE Target Selection

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 r_fiber < 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 i_fiber < 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

Color cuts:

g < 20.3

-1 < g-r < -0.2

-1 < u-g < 0.7

u-g + 2(g-r) < -0.1

Cool White dwarf selection

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 r_fiber < 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 i_fiber < 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