Miho Ishigaki1 , Masashi Chiba1, Wako Aoki2, Lang Zhang3

Tohoku UniversityNAOJNational Astronomical Observatories, Chinese Academy of Science

BackgroundChemical abundance of the Milky Way outer halo with Subaru/HDSIssues to be addressed with WFMOSFuture prospects with WFMOS

Precise measurements of basic quantities for individual stars are possible for nearby stars.Positions and 3-D velocity componentsMass distribution out to several kpcIdentification 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

TheoriesHierarchical formation of galaxies -> Some fraction of the stellar halo have been accreted from smaller sub systems (e.g. dwarf galaxies)ObservationsVarious substructuresOver density, stellar streams, -> Direct evidence of recent accretion eventsThe inner/outer halo (Carollo et al. 2007)Different formation mechanism is needed for the inner and the outer halo Virgo overdensityJuric et al. 2008

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 progenitorWFMOS (+GAIA)

Aim: Investigating a systematic difference in chemical abundance patterns in the outer haloConsidered elements:Alpha-elements: Mg, Si, Fe-peak elements: Cr, Ni, Neutron capture elements: Y, Ba The outer halo

Sample selection; Zmax>5kpc, [Fe/H]

Zmax-V relationThe sample includesStephens & Boesgaard 2002, Gratton 2003

The outer halo [Mg/Fe]-[Fe/H] relation is similar to the nearby dSphs.

Building block of the outer halo could beSystems that are lack of massive Type II SNe (IMF)Systems in which a star formation timescale (tSF) is longer.

Lanfranchi & Matteucci 2003[Fe/H]tSFtSF

Zn is largely produced in energetic SNe[Zn/Fe] is slightly lower for the outer halo as observed in the nearby dSphs

The outer halo stars exhibit distinct abundance ratios in alpha-elements, Zn For -2

Present sample is restricted to a small number of bright stars (V

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

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.

Subaru/HDSouter haloWFMOSWFMOS**i*

*outer haloSubaru/HDS*HDS*HDSlog(Zmax)VphiZmax>5kpc outer halo*[Fe/H][Mg/Fe]-2