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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 3 map of the Milky Way

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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

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

Pan-STARRS 3π map of the Milky Way (Slater et al. 2014)

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The Milky Way halo

Disk Sun

Stellar halo: diffuse

ρ ∝ r−α; α = 2.8 − 2.9

∼ 0.5 %

Density profile: Local halo fraction:

e.g. Juric et al. 2008; Fukushima et al. 2019

Dark mater halo: dominate the total mass

M ∼ 1012M⊙

Wang et al. 2015

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The Milky Way stellar halo

✤Long dynamical time ➡ The process

  • f hierarchical merging for its

formation, test for the ΛCDM model

Font et al. 2006

The only galaxy halo where position, velocity, chemical abundance can be measured for individual stars

✤The present-day structure of

dark matter halo ➡ particle nature of dark matter

Bonaca et al. 2014

✤Chemical evolution ➡ Production

  • f elements in stars, supernovae,

AGB, etc

Illustration of the first (Pop III) stars

Credit: NAOJ

This talk: Complexity of the Milky Way stellar halo, open questions, prospects with wide-field (NIR) surveys

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Wide field photometric surveys of the stellar halo

3D distribution of stars down to main-sequence stars

  • Overdensity in the stellar halo
  • Substructures close to the Galactic disk (e.g. Monoceros

ring)

  • Discovery of new dwarf satellite galaxies by SDSS, DES, etc.

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

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The inner and outer halo revealed by SDSS

  • Inner halo

◇Prograde rotation： 0<VΦ<50 km/s ◇[Fe/H] ≅ -1.6 ◇Flattened: axial ratio ~ 0.6

  • Outer halo

◇Retro-grade rotation:

  • 70<VΦ<-40 km/s

◇[Fe/H] ≅ -2.2 ◇Spherical: axial ratio of ~0.9

(http://www.sdss.org/news/ releases/20071212.dblhalo.html)

SDSS photometry + spectroscopy (SEGUE) for ~ 20000 stars: Carollo et al. 2007

The duality of the stellar halo implies that the two halos have formed by different mechanisms

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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 ~ 108-109 M◎ at ~ 10 Gyrs ago (z~1.8)

Gaia

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Chemical difgerence diversity in halo stars revealed by SDSS/APOGEE

Hayes et al. 2018, see also Hawkins et al. 2015 High-[Mg/Fe] Low-[Mg/Fe]

Fornax dSph LMC Sgr, M54

Low-[Mg/Fe] stars may have been accreted from dwarf galaxies (e.g. Fornax dSph)

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Chemical signatures of galaxy accretion

Group of stars that share similar orbital velocities identified by LAMOST

Zhao et al. 2018

Candidates of accreted stars identified by LAMOST and followed up by Subaru/HDS (Aoki et al. )

Zhao et al. 2015

Lower [Mg, Ca/Fe] ratios for a given [Fe/H] than the bulk

  • f halo stars

Xing et al. 2019, Nature Astronomy

Low-Mg High-Eu

LAMOST J1124+4535 A star showing unusual Mg and Eu abundance LAMOST

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Cold stellar streams

Grillmair et al. 2016

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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 ΛCDM prediction Top: stellar density, middle: line-of-sight velocity dispersion, bottom: line-of-sight velocity

No subhalos Subhalos with different masses

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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

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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

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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

  • uter 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

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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

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Key questions for 2020s

  • Substructures characterized by spatial, kinematics and chemical distribution out to ~ 15kpc by

main sequence ➡ Cosmological simulations predict substructures are even more numerous beyond the present limit

  • Disk-halo interface to put constraints on the impact of Sagittarius merger to the formation of the

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

  • Mass, shape, and lumpiness of the dark matter halo through stellar streams ➡ Velocity

measurements (spectroscopy+astrometry) are essential to constrain dark matter halo properties.

  • The chemical characterization of the stellar halo ➡ Understanding chemical enrichtment history of

the building blocks of the stellar halo

  • Search for chemically pristine stars ➡ Deeper photometric surveys will provide constraints on

low-mass first (Population III) stars (e.g.,Ishiyama et al. 2015), comparison with supernova nucleosynthesis

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Surveys of the Galactic metal-poor stars in 2020’s

Mapping of outer halo , discovery of substructures Parallax and proper motion

  • f distant stars beyond the

Gaia limit (G~20) Kinematics and chemistry of the outer halo Detailed chemical abundance

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HSC

  • Wide-field imaging with 8m telescope
  • Complementary to LSST in 2020’s
  • Surveys with narrow-band filter ➡

Distribution of chemical elements in the

  • uter 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)

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PFS

  • The only instrument capable of wide-

field spectroscopic measurements of distant stars, dwarf galaxies, M31 halo, the Milky Way outer disk/halo, streams

  • Medium resolution mode: near-IR

calcium triplet absorption lines and

  • ther weaker lines ➡ Line-of-sight

velocities (σ ~ a few km/s), [Fe/H], [α/Fe] (σ < 0.3 dex)

4m limit PFS limit

M31 RGB

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Cold stellar streams as a probe of dark matter halo

Deep spectroscopy and proper motion measurements ➡ Constraints on the mass profile and lumpiness of dark matter halo

Mock CMD for Pal5 stream PFS field-of-view Shallow (g<20) Deep (g<22)

➡ less affected by disk/bulge

Spectroscopy with PFS + Precise astrometry from WFIRST, Euclid

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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

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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, 12C/13C 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)

Ishigaki et al. 2014; Frebel & Norris 2015

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Summary

  • In the last decade, wide-field surveys have revolutionized our view of the outskirt of the

Milky Way

  • 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

abundances are limited by

  • 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

missions (Euclid, WFIRST) will be powerful and unique in 2020