Formation of the Milky Way based on chemodynamical analysis of - PowerPoint PPT Presentation

Formation of the Milky Way based on chemodynamical analysis of metal-poor stars - Prospects with wide-field surveys of the Galactic outskirts - Miho N. Ishigaki Tohoku University, Astronomical Institute Pan-STARRS 3π map of the Milky Way (Slater et al. 2014) 研究会「我が国の ( 近 ) 赤外線広視野観測サイエンスの戦略と展望」 ２０１９年７月１ − ２日、国立天文台三鷹キャンパス

The Milky Way halo Disk Sun Stellar halo: diffuse Density profile: Local halo fraction: e.g. Juric et al. 2008; Fukushima et al. 2019 Dark mater halo: dominate the total mass Wang et al. 2015 M ∼ 10 12 M ⊙ ρ ∝ r − α ; α = 2.8 − 2.9 ∼ 0.5 %

The Milky Way stellar halo Bonaca et al. 2014 This talk: Complexity of the Milky Way stellar halo, open Credit: NAOJ first (Pop III) stars Illustration of the AGB, etc of elements in stars, supernovae, nature of dark matter dark matter halo ➡ particle abundance can be measured for individual stars The only galaxy halo where position, velocity, chemical Font et al. 2006 formation, test for the ΛCDM model of hierarchical merging for its questions, prospects with wide-field (NIR) surveys ✤ Long dynamical time ➡ The process ✤ The present-day structure of ✤ Chemical evolution ➡ Production

Wide field photometric surveys of the stellar halo 3D distribution of stars down to main-sequence stars ring) SDSS tomography of the stellar halo X [pc] Pan-STARRS map of outer disk (Slater et al. 2014) Ivezic et al. 2012 The stellar halo within ~ 15kpc contains various substructures, some of which are close to the Galactic plane • Overdensity in the stellar halo • Substructures close to the Galactic disk (e.g. Monoceros • Discovery of new dwarf satellite galaxies by SDSS, DES, etc.

The inner and outer halo revealed by SDSS -70<V Φ <-40 km/s The duality of the stellar halo implies that the two halos Carollo et al. 2007 for ~ 20000 stars: spectroscopy (SEGUE) SDSS photometry + have formed by different mechanisms 0<V Φ <50 km/s ● Inner halo ◇ Prograde rotation： ◇ [Fe/H] ≅ -1.6 ◇ Flattened: axial ratio ~ 0.6 ● Outer halo ◇ Retro-grade rotation: ◇ [Fe/H] ≅ -2.2 (http://www.sdss.org/news/ releases/20071212.dblhalo.html) ◇ Spherical: axial ratio of ~0.9

Sky distribution of the Gaia-Enceladus stars selected in the plane of E-Lz: (Helmi et al. 2018) Astrometry with Gaia reveals a merger of Gaia-Enceladus Energy Angular momentum A merger of a galaxy of stellar mass ~ 10 8 -10 9 M ◎ at ~ 10 Gyrs ago (z~1.8) Gaia

Chemical di fg erence diversity in halo stars revealed by SDSS/APOGEE Hayes et al. 2018, see also Hawkins et al. 2015 Fornax dSph High-[Mg/Fe] LMC Low-[Mg/Fe] Low-[Mg/Fe] stars may have been accreted from Sgr, M54 dwarf galaxies (e.g. Fornax dSph)

Chemical signatures of galaxy accretion Candidates of accreted stars identified by LAMOST and Zhao et al. 2015 followed up by Subaru/HDS (Aoki et al. ) LAMOST A star showing unusual Mg and Eu abundance Group of stars that share Low-Mg similar orbital velocities identified by LAMOST LAMOST J1124+4535 High-Eu Lower [Mg, Ca/Fe] ratios for a given [Fe/H] than the bulk Zhao et al. 2018 of halo stars Xing et al. 2019, Nature Astronomy

Cold stellar streams Grillmair et al. 2016

Stellar stream gaps as probes of missing satellites Yoon et al. 2011 Gaps in a simulated stellar stream orbiting in a host halo with the dark matter subhalo mass distribution consistent with the Λ CDM prediction No subhalos Subhalos with different masses Top: stellar density, middle: line-of-sight velocity dispersion, bottom: line-of-sight velocity

On-going merger of Sagittarius dwarf galaxy All M-giants selected by 2MASS + WISE Metal-poor ([M/H]<-0.5) M-giants Li et al. 2016 Sagittarius core Tidal streams

Dominant population in the outer halo Sesar et al. 2017 Distribution of RR-Lyrae stars mapped by Pan-STARRS survey Sesar et al. 2017 The outer halo (D>20kpc) contains numerous Sagittarius debris Interaction between Sgr and the Milky Way disk is largely obscured

Sagittarius impact as the origin of streams at the disk/halo interface Purcell et al. 2011 Laporte et al. 2019 Chemical abundances and kinematics are the key to confirm the scenario that the outer disk streams are actually perturbed parts of the Galactic disk (e.g. Bergmann et al. 2018, Li et al. 2019) A simulation of the Sagittarius merger that have found to have large impact on the disk structure

Wide-field surveys with narrow-band filters Sky Mapper Southern Sky survey with metallicity-sensitive v (violet) filter (Bessel et al. 2011) Discovery of the most Fe-poor ([Fe/H]<-7) star SMSS 0313-6708 (Keller et al. 2014) ➡ Physical properties (e.g. mass) of the first stars, their supernova explosions and their compact remnant Comparison of the observed chemical abundance and the first star’s supernova yield models (Ishigaki et al. 2014) Credit: IPMU

Key questions for 2020s main sequence ➡ Cosmological simulations predict substructures are even more numerous beyond the present limit Milky Way ➡ Lack of spectroscopic and astrometric measurements limit quantitative prediction from theory, Properties of the Sagittarius and their contribution to the field halo population measurements (spectroscopy+astrometry) are essential to constrain dark matter halo properties. the building blocks of the stellar halo low-mass first (Population III) stars (e.g.,Ishiyama et al. 2015), comparison with supernova nucleosynthesis • Substructures characterized by spatial, kinematics and chemical distribution out to ~ 15kpc by • Disk-halo interface to put constraints on the impact of Sagittarius merger to the formation of the • Mass, shape, and lumpiness of the dark matter halo through stellar streams ➡ Velocity • The chemical characterization of the stellar halo ➡ Understanding chemical enrichtment history of • Search for chemically pristine stars ➡ Deeper photometric surveys will provide constraints on

Surveys of the Galactic metal-poor stars in 2020’s Mapping of outer halo , discovery of substructures Parallax and proper motion of distant stars beyond the Gaia limit (G~20) Kinematics and chemistry of the outer halo Detailed chemical abundance

HSC Distribution of chemical elements in the outer halo, identification of chemically pristine stars Ivezic et al. 2012 Cosmological simulation of a stellar halo of a Milky- Way-sized galaxy (Bullock & Johnston 2005) Main sequence (<4m) RR Lyrae (<4m) Main sequence (8m) RR Lyrae (8m) • Wide-field imaging with 8m telescope • Complementary to LSST in 2020’s • Surveys with narrow-band filter ➡

PFS field spectroscopic measurements of distant stars, dwarf galaxies, M31 halo, the Milky Way outer disk/halo, streams calcium triplet absorption lines and other weaker lines ➡ Line-of-sight velocities (σ ~ a few km/s), [Fe/H], [α/Fe] (σ < 0.3 dex) • The only instrument capable of wide- • Medium resolution mode: near-IR 4m limit M31 RGB PFS limit

+ Spectroscopy with PFS Precise astrometry from WFIRST, Euclid Cold stellar streams as a probe of dark matter halo ➡ less affected by disk/bulge Deep spectroscopy and proper motion measurements ➡ Constraints on the mass profile and lumpiness of dark matter halo Mock CMD Shallow Deep for Pal5 stream (g<20) (g<22) PFS field-of-view

Outer disk-halo interface with PFS + astrometry Sheffield et al. 2018 Sun Sgr core Outer disk regions Laporte et al. 2018 Purcell t al. 2011 Line of sight velocity and chemical abundance (PFS) Parallax and proper motion (ULTIMATE, WFIRST) ⬇ Characterization of the outer disk with full 6D phase space + chemistry ➕

Ishigaki et al. 2014; Frebel & Norris 2015 High-resolution spectroscopy of chemically pristine stars with large aperture telescopes • Nucleosynthetic signature of Pop III stars in Fe-poor stars • Measurements of C and O abundances, 12 C/ 13 C isotopic abundance for a large sample of extremely metal-poor stars High-resolution (R>30,000) spectroscopy with large-aperture telescopes (e.g. GMT/G-CLEF, TMT/ HROS)

Summary Milky Way abundances are limited by missions (Euclid, WFIRST) will be powerful and unique in 2020 • In the last decade, wide-field surveys have revolutionized our view of the outskirt of the • Discovery and panoramic mapping of substructures in the stellar halo and the outer disk • The implications of the substructures on galaxy formation, dark matter and chemical • Shallow depth and sky coverage (especially the regions close to the disk plane) • Lack of astrometric and spectroscopic measurements of velocity and chemistry • Wide-field instruments on Subaru (HSC, PFS, ULTIMATE), combined with astrometric space