The Case for the Dual Halo of the Milky Way

Journal Article · Fri Apr 01 04:00:00 EDT 2011 · Submitted to Astrophys.J.

OSTI ID:1012424

Beers, Timothy C; Carollo, Daniela; Ivezic, Zeljko; An, Deokkeun; Chiba, Masashi; Norris, John E; Freeman, Ken C; Lee, Young Sun; Munn, Jeffrey A; Fiorentin, Paola Re; Sivarani, Thirupathi

Based on an analysis of the local kinematics of SDSS DR7 calibration stars, Carollo et al. have resolved the stellar population of the Milky Way halo into at least two components. This result has recently been criticized by Schoenrich et al., who claim that the retrograde signature associated with the outer halo is due to the adoption of faulty distances. We refute this claim, and demonstrate that the Schoenrich et al. photometric distances are themselves flawed because they adopted an incorrect main-sequence absolute magnitude relationship from the work of Ivezic et al.. When compared to the recommended relation from Ivezic et al., which is tied to a Milky Way globular cluster distance scale and accounts for age and metallicity effects, the incorrect relation adopted by Schoenrich et al. yields, on average, 18% shorter distances (independent of metallicity) for stars near the main-sequence turnoff (TO). When the correct relationship is used, the distances assigned by Carollo et al. and Ivezic et al. for low-metallicity dwarfs agree to within 6-10%, depending on the color range considered. We have also compared the Carollo et al. distances with the distances derived from the calibrated isochrones of An et al., and find a similar level of agreement for low-metallicity dwarfs. Schoenrich et al. also point out that stars of intermediate gravity (3.5 {<=} log g < 4.0, based on spectroscopic determinations) are likely misclassified, at least for colors significantly redder than the TO region, with which we concur. We implement a new procedure to reassign luminosity classifications for the TO stars that require it. New derivations of the rotational behavior for the Carollo et al. stars that are most likely associated with the outer halo demonstrate that, when either a sample of exclusively dwarf stars or the full sample of dwarf, TO, and subgiant/giant stars is used, the retrograde signature and high velocity dispersion of the outer-halo population remains, with values similar to those previously derived. An additional test of the reality of the retrograde signature is provided, based exclusively on the observed proper motions of low-metallicity stars. Further evidence for a complex halo comes from inspection of the metallicity distribution function of the Carollo et al. sample as a function of distance from the Galactic plane. We summarize additional lines of evidence for a dual halo, based on different stellar samples from the SDSS and other surveys. We conclude that the overwhelming body of evidence rejects the single-halo interpretation beyond reasonable doubt.

• Research Organization: Fermi National Accelerator Laboratory (FNAL), Batavia, IL
• Sponsoring Organization: DOE Office of Science
• DOE Contract Number: AC02-07CH11359
• OSTI ID: 1012424
• Report Number(s): FERMILAB-PUB-11-187-AE-CD; arXiv eprint number arXiv:1104.2513
• Journal Information: Submitted to Astrophys.J., Journal Name: Submitted to Astrophys.J.
• Country of Publication: United States
• Language: English

Related Subjects

71 CLASSICAL AND QUANTUM MECHANICS
GENERAL PHYSICS
Astrophysics
CALIBRATION
COLOR
DISTRIBUTION FUNCTIONS
DWARF STARS
LUMINOSITY
MILKY WAY
PROPER MOTION
STARS
VELOCITY