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Probing the formation of the Milky Way with WFMOS

Investigating the formation of the Milky Way outer halo through precise measurements of basic quantities for individual stars. This study aims to address key questions about the accretion history, mass distribution, and chemical enrichment of the halo.

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Probing the formation of the Milky Way with WFMOS

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  1. Miho Ishigaki1 , Masashi Chiba1, Wako Aoki2, Lang Zhang3 Tohoku University NAOJ National Astronomical Observatories, Chinese Academy of Science Probing the formation of the Milky Way with WFMOS

  2. Outline • Background • Chemical abundance of the Milky Way outer halo with Subaru/HDS • Issues to be addressed with WFMOS • Future prospects with WFMOS

  3. The Milky Way - Laboratory of galaxy formation in the Universe - Precise measurements of basic quantities for individual stars are possible for nearby stars. • Positions and 3-D velocity components • Mass distribution out to several kpc • Identification of clumpy groups in a coordinate/velocity space Signature of recent accretion events • Distance (+metallicity) • Age • Chemical abundances of various elements (Fe, Mg, Si, Zn, Ba, etc…) • Chemical enrichment histories

  4. Recent pictures of the formation of the Milky Way halo • Theories • Hierarchical formation of galaxies -> Some fraction of the stellar halo have been accreted from smaller sub systems (e.g. dwarf galaxies) • Observations • Various substructures • Over density, stellar streams, … -> Direct evidence of recent accretion events • The inner/outer halo (Carollo et al. 2007) • Different formation mechanism is needed for the inner and the outer halo “Virgo overdensity” Juric et al. 2008

  5. How did the Milky Way halo form? Questions to be answered: • What fraction of the halo have been accreted? • When did majority of accretion events occur? • What is a typical mass of the accreted systems? • How did star formation proceed within an accreted progenitor WFMOS (+GAIA)

  6. Probing the Milky Way outer halo with Subaru/HDS • Aim: Investigating a systematic difference in chemical abundance patterns in the outer halo • Considered elements: • Alpha-elements: Mg, Si, … • Fe-peak elements: Cr, Ni, … • Neutron capture elements: Y, Ba The outer halo

  7. Observations with Subaru/HDS • Sample selection; Zmax>5kpc, [Fe/H]<-1, V<12 • High-resolution spectroscopy with Subaru/HDS • Spectral coverage of 4000-7000Å • S/N>100 • 2003/2: 26objects (Aoki+), 2005/5:3 objects (Inoue+), 2008/6-7: 28objects (Ishigaki+, service obs.) Total: ~60 outer halo stars

  8. Kinematics of the sample Highly- prograde Inner halo Zmax-Vφ relation Outer halo The sample includes Stephens & Boesgaard 2002, Gratton 2003 Highly- retrograde Zmax=5 kpc

  9. [Mg/Fe]-[Fe/H]

  10. [Mg/Fe]-Zmax Zmax=5 kpc

  11. Comparison of [Mg/Fe] with nearby dwarf spheroids • The outer halo [Mg/Fe]-[Fe/H] relation is similar to the nearby dSphs.

  12. Interpretations for lower [Mg/Fe] Lanfranchi & Matteucci 2003 tSF短 • Building block of the outer halo could be… • Systems that are lack of massive Type II SNe (IMF) • Systems in which a star formation timescale (tSF) is longer. [Mg/Fe] tSF長 [Fe/H]

  13. Constraints from other elements • Zn is largely produced in energetic SNe • [Zn/Fe] is slightly lower for the outer halo as observed in the nearby dSphs Inner halo Outer halo Dsph data from Shetrone et al. 2001

  14. Implications from present study • The outer halo stars exhibit distinct abundance ratios in alpha-elements, Zn • For -2<[Fe/H]<-1, the outer halo [Mg/Fe] values are partially overlapped with those observed for nearby dSphs However, the fraction of their contributions, properties of progenitors, etc. could not be constrained with the present sample size.

  15. Issues to be addressed with WFMOS • Present sample is restricted to a small number of bright stars (V<12) Multi object spectroscopy up to V<17 • Systematic errors caused by different instruments, analysis methods, etc could eliminate intrinsic trends/scatters. A homogeneous data set • Sample selection bias could introduce artificial trends Maximizing a completeness of the sample

  16. Future prospects with WFMOS • Identification of groups in abundance space • Measurements of various elements simultaneously to make constraints on progenitors • A spatial distribution (gradients, overdentisy) of each elements Font et al. 2006 Comparable with theoretical predictions of galaxy formation for an accreted fraction, an accretion time, progenitor masses, etc.

  17. Summary • Detailed chemical abundances (+ stellar dynamics) are useful to probe how the Milky Way halo formed. • The present study using the Subaru/HDS implies that the Milky Way outer halo exhibits distinct abundance ratios in certain elements. However, the key issues for the halo formation remain unresolved because of the small sample size (~60 stars). • A large homogeneous data set with enough accuracy taken with WFMOS is essential to obtain a stronger constrains on the key issues for the Milky Way formation.

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