Distant Halo Giant Stars


Photo of Connie Rockosi
Connie Rockosi
Santa Cruz and UCO/Lick Observatory


Spectra of class K giant stars in the outer halo of the Milky Way

Finding Targets

An object whose ANCILLARY_TARGET1 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 Number of Unique Primary Objects
RED_KG 48 Stars near the tip of the red giant branch in color-color space 10,468 7,515
RVTEST 49 Stars with previous SDSS spectra, used to test the reproducibility of velocity measurements 84 11


Rare giants in the outer halo of the Milky Way are selected using a targeting strategy tested in SEGUE-2 (Yanny et al. 2009). Observations of the new BOSS targets are expected to increase the number of known halo stars beyond 60 kpc by a factor of ten.

The full sample of RED_KG stars enables measurements of substructure of the Milky Way in position-velocity phase-space, a signature of the hierarchical assembly of the stellar halo. The data thus provide tests of CDM predictions for galaxy assembly in a part of the Galaxy expected to be almost entirely composed of debris from recent accretions (Xue et al. 2011). The sample will also enable dynamical mass estimates for the Milky Way out to 150 kpc.

Target Selection

The following object selection criteria were used:

  • 17 < gfib2 < 19.5
  • Color box, with (uPSF – gPSF, gPSF – rPSF) corners of (0.8, 2.35), (0.8, 2.65), (1.4, 3.0), and (1.4, 3.9)

The first two selection criteria identify stars near the tip of the red giant branch in the region of the (u – g) / (g – r) diagram where giants separate from the locus of foreground dwarfs (Yanny et al. 2009). In addition, it is required that:

  • Total proper motion be < 11 mas/year, zero within three standard deviations of the proper motion errors (to reject nearby stars)
  • The 3″ fiber magnitude must be i > 16.5 (an additional mechanism to reject bright targets)

A smaller sample of targets is found on the commissioning plates. Denoted RVTEST, this sample targets stars previously observed spectroscopically as a test of the reproducibility of velocity measurements.


Xue, X.-X., et al. 2011, ApJ, 738, 79, doi:10.1088/0004-637X/738/1/79

Yanny, B. et al. 2009, AJ, 137, 4377, doi:10.1088/0004-6256/137/5/4377