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The Current View of the Halo System of the Milky Way. Daniela Carollo Macquarie University Research Centre in Astronomy, Astrophysics & Astrophotonics Department of Physics & Astronomy, Macquarie University. IAU General Assembly 2009 Symposium. Aboriginal Art Milky Way Dreaming G. Possum.

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The Current View of the Halo System of the

Milky Way

Daniela Carollo

Macquarie University Research Centre in Astronomy, Astrophysics & AstrophotonicsDepartment of Physics & Astronomy, Macquarie University

IAU General Assembly 2009


Aboriginal Art

Milky Way Dreaming

G. Possum


The Galactic Stellar Halo: Locus of Ancient Stars

Fossil Record of the first generation of stars that formed shortly after the Big-Bang

Two basic information

Their chemical composition

encode information about the

environment in which they

formed during the early

history of the Galaxy

Their motion has an imprints

of the

dynamical history of the Galaxy


The halo system

  • Smooth components: Inner and Outer Halo
  • Substructures
  • Overdensities
  • Globular Clusters

Smooth Halo Components

D.Carollo et al., Nature 2007, Vol. 450, 1020-1025

  • Halo  Halos
  • Inner Halo:
    • Dominant at R < 10-15 kpc
    • Highly eccentric orbits
    • Slightly Prograde
    • Metallicity peak at [Fe/H] = -1.6
  • Outer Halo:
    • Dominant at R > 15-20 kpc
    • More uniform distribution of eccentricity
    • Highly retrograde orbits
    • Metallicity peak around [Fe/H] = -2.2

Full [Fe/H] Range

Extracting the Fractions of Each Galactic Component

Carollo et al. 2010, ApJ, 712, 692

Stellar Fractions,

FIN and FOUT, as a

function of Zmax

The value Zmax ~ 15-20 kpc

is the inversion point!


Inner and Outer Halo Velocity Ellipsoids

Carollo et al. 2010, ApJ, 712, 692

Inner Halo : Ellipsoid dominated by the radial motion

Outer Halo: Ellipsoid less dominated by the radial motion


Power-Law Density Profiles for Inner/Outer Halo

Carollo et al. 2010, ApJ, 712, 692

Outer halo population exhibit a much shallower spatial density

profile than the Inner halo


Axial Ratio for the Inner Halo : c/a ~ 0.6 (flattened distribution)

Axial Ratio for the Outer Halo : c/a ~ 0.9 (nearly spherical distribution)


Why is the Presence of a Dual Halo so Important?

It is not just that it is double…

Members of the two components exhibit:

Different Spatial Distributions

Different Kinematics

Different Chemical Composition (MDFs)

Distinct Astrophysical Origin

Crucial to Understand the Formation and Subsequent Evolution of the Milky Way

Inthe context of the CDM model:

Inner Halo: Dissipative radial merger of few massive sub-Galactic

fragments formed at early stage, mainly in situ stars

Outer Halo: Dissipationless chaotic merging of small sub-systems within

a pre-existing Dark Matter Halo, mainly accreted stars


Inner and Outer Halo in Equatorial Stripe 82

(An et al. , submitted)

Observed photometric MDF is inconsistent with a single [Fe/H] population.


Distinct Halo Populations in the Solar Neighborhood

Nissen & Schuster, 2010

High [a/Fe]MainlyPrograde Orbits

Low [a/Fe]MainlyRetrograde Orbits

High-a pop.: dissipative component of the Galaxy that experienced

a rapid chemical evolution.

Low-a pop.: accreted from dwarf galaxies that had lower star formation rates.

evidence of inner outer halo from segue vertical photometry stripes de jong et al 2010
Evidence of Inner/Outer Halo from SEGUE Vertical Photometry Stripes (de Jong et al. 2010)
  • Metallicity shift as a function of the Galactocentric distance:
  • [Fe/H] ~ -1.6 at R < 15 kpc ; [Fe/H] ~ -2.2 at R > 15-20 kpc
  • Mean stellar mass density exhibit two halo populations with inversion points in the predominance at 15-20 kpc



Disk/Halos Structure from SEGUE Stellar Photometry

(De Jong et al. 2010)

Left Panel

Black dots: total stellar mass density

Red line: density for the thick-disk-like population

Blue line: density for the inner- halo-like population

Green line: density for the outer- halo-like population

Right Panel

Fractional contribution of the individual template populations

Inner-outer halo inversion point

between 15-20 kpc.

In agreement with Carollo et al.




BHB and RR-Lyrae towards the Anticenter: Transition between Inner and Outer Halo

Kinman et al., 2012

Stars become highly retrograde as

the Galactocentric distance


Retrograde orbits dominate for

Galactocentric distances > 12.5 kpc

Transition between inner and

outer halo


The Dual Halo in the High Resolution Simulations

Font et al. 2011

General feature of stellar spheroids of simulated disk galaxies with Milky Way mass:

Metallicity shift in the halo as a function of radius

NB: Metallicities in these simulations are rather arbitrary at present, the important information is that DIFFERENCES are seen as a function of distance


The Dual Halo in the High Resolution Simulations

McCarthy et al., 2012

General feature of stellar spheroids of simulated disk galaxies with Milky Way mass:

Shear exists in the mean rotational velocity between the in situ and the accreted halo component


Distinct Chemical Pattern Between Inner and Outer Halo: Carbon-Enhanced Metal-Poor Stars

  • Stars with [Fe/H] < -2.0 and [C/Fe] > +1.0
  • - At least 20% of ALL stars with [Fe/H] < -2.0 are CEMP
  • - At least 40% of ALL stars with [Fe/H] < -3.5 are CEMP
  • - Most of the stars with [Fe/H] < -4.0 are CEMP
  • including the two most iron-poor stars (Hyper Metal Poor;
  • [Fe/H] < -5.0) are extremely carbon rich, [C/Fe] ~ + 4.0
  • CEMP stars contain useful information on the nature of
  • nucleosynthesis in the early Galaxy.
  • Where does this carbon come from?

Sources of Carbon

  • Intrinsic internal production by low mass stars of extremely low
  • [Fe/H] (Fujimoto et al 2000)
  • Extrinsic production of carbon by stars of intermediate mass (2 < M < 8)
  • during their AGB phase in a binary system (Suda et al. 2004)
  • Faint Supernova Models: extensive mixing and fallback during
  • explosions (Umeda & Nomoto, 2003; Iwamoto et al. 2005)
  • Nucleosynthesis in Rotating Massive Stars (Meynet 2006, and
  • references therein)

CEMP Stars in the Inner and Outer Halo

Carollo D. et al. 2012 ApJ, 744, 195

Black Dots: Global trend of CEMP

stellar fraction vs [Fe/H].

Blue dot-dashed curve: Second order

polynomial fit.

Blue Filled Circles: Expected values

of CEMP fraction in each bin of

[Fe/H] (0.5 dex).

Green Filled Circle: Observed CEMP

fraction for the inner halo.

Red Filled Circle: Observed CEMP

fraction for the outer halo.


CEMP Stars: Summary

  • Observed increases of CEMP stars as [Fe/H] decreases
  • At -2.5 < [Fe/H] < -2.0 :
  • This trend could be confirmed at [Fe/H] < -2.5

CEMP Stars: Implication for Galaxy Formation

  • One of the key element: Barium (Ba)
  • The majority of (~ 80%) CEMP stars exhibit over-abundances

of Barium (CEMP-s stars: Beers & Christlieb 2005)

  • Ba is produced by s-process (n-capture) in AGB stars
  • A small fraction of CEMP stars have normal or low
  • Ba abundances (CEMP-no)
  • CEMP-no: carbon excess perhaps has a different origin
  • CEMP-s stars are seen at [Fe/H] > -3.0
  • CEMP-no stars appear in the lowest metallicity range [Fe/H] < -2.5

CEMP Stars: Implication for Galaxy Formation

At -2.5 < [Fe/H] < -2.0

Is the CEMP-no fraction in the outer halo higher than in

the inner halo?

Work still in progress (Carollo et al. in preparation)

Suggests that CEMP stars in the Outer Halo formed

through different sources. Progenitors were predominantly massive fast rotating stars? Faint supernova?

CEMP stars in the inner halo formed predominantly in a

binary system.


CEMP Stars: connection with Ultra Faint dSphs

CEMP in the SEGUE 1 System

(Norris, Gilmore, Wyse et al., 2010)

[Fe/H] = -3.52, [C/Fe] = +2.3; [Ba/Fe] < -1.0


Segue 1 and Bootes I:

Large Spread in Carbon Abundance

Normalized Flux


CEMP in the Halo


CEMP Stars: connection with Ultra Faint dSphs

Bootes I observed with LRIS on Keck

(Lai et al., 2011, Apj, 738, 51)

R = 2000, analyzed with the SSPP

[C/Fe] also estimated

Milky Way CDF at high |Z|


Connection with galaxies at high redshift: Damped Lya Absorption

Systems (DLAs)

  • Recently a DLAs with [Fe/H] ~ -3 observed at z ~ 2.3 exhibit strong
  • carbon enhancement (Cooke et al. 2011b).
  • All others DLAs at [Fe/H] < -2 show chemical abundance ratios
  • consistent with very metal poor Galactic halo stars (Cooke et al. 2011a).

Suggests possible connections between these high redshift galaxies

and the early building blocks of the Milky Way –like galaxies.



  • The halo system of the Milky Way is extremely complex: a more clear picture is emerging in both observational and theoretical works.
  • The smooth halo has at least two components which exhibit different kinematics, orbital parameters, spatial distribution and chemical composition.
  • The duality of the Galactic halo is emerging in several observational results in which different sample of stars and techniques are used.
  • The duality is dramatically emerging in high resolution simulation of Milky Way galaxies and it is a general property of such simulations.
  • First clear evidence of different chemical pattern in the two components: the CEMP stars fraction
  • DLAs : similar chemical patterns of the Milky Way’s stellar halos

HERMES isa new high-resolution

fiber-fed multi-object spectrometer

on the AAT

spectral resolution 28,000

(also R = 50,000 mode)

400 fibres over  square degrees

4 bands (BGRI) ~ 1000 Å

First light 2013

Main driver: the GALAH survey

(Galactic Archaeology with HERMES)

Team of about 40, mostly from Australian institutions



Chemical Tagging

Use the detailed chemical abundances of stars to tag or

associate them to common ancient star-forming aggregates with similar abundance patterns (eg Freeman & Bland-Hawthorn ARAA 2002)

The detailed abundance pattern reflects the chemical evolution

of the gas from which the aggregate formed.


My main scientific interests in the Galactic Archaeology Survey


The Galactic Halo(s) with HERMES

  • With HERMES ~ 50000 stars in the Halo
  • All these stars will be observed at a resolution of 28000 or more
  • Detailed abundance analysis for:
  • Smooth Halos
  • Substructures

While I’m waiting for the GALAH survey..

  • Test and validate:
  • Data Reduction Pipeline
  • Abundance Analysis Pipeline

The End

The Milky Way from Death Valley