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Distant Halo Giants


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

Finding Targets

An object whose ANCILLARY_TARGET1 value include one or more of the bitmasks in the following table was targeted for spectroscopy as part of this ancillary target program. See SDSS-III bitmasks to learn how to use these values to identify objects in this ancillary target program.

(bit name)
Bit Number Target density
Survey Area
RED_KG 48 0.75 10,000
RVTEST 49 ~0.75 ~50


Rare giants in the outer halo of the Milky Way are selected using a targeting strategy tested in SEGUE2 (the SDSS-III counterpart to SEGUE; Yanny et al. 2009, Rockosi et al., in prep.). 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 RED_KG will enable measurements of substructure of the Milky Way in position-velocity phase-space, a signature of the hierarchical assembly of the stellar halo. The data will 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 Details

The following object selection criteria were used:

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

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.

Primary contact

Connie Rockosi
UC Santa Cruz and UCO/Lick Observatory
crockosi -at-

Other contacts

Heather Morrison, Paul Harding


Yanny, B., et al., 2009, AJ, 137, 4377, doi:10.1088/0004-6256/137/5/4377
Xue, X.-X., Rix, H-W., Yanny, B., Beers, T.C., Bell, E.F., Zhao, G., Bullock, J.S., Johnston, K.V., Morrison, H., Rockosi, C., Koposov, S.E., Kang, X., Liu, C., Luo, A., Lee, Y.S., Weaver, B.A., 2011, ApJ, 738, 79, doi:10.1088/0004-637X/738/1/79