Sagittarius Dwarf Galaxy(HST, NASA, ESA)

Two Distinct Populations inDwarf Spheroidal Galaxies

Nobuo Arimoto (NAOJ, Tokyo)

Two Distinct Ancient Populationsin the Sculpter Dwarf Spheroidal Galaxy
Tolstoy et al. (2004) ApJL 617, 119• The First Result from DART(Dwarf Abundances and Radial velocity Team )E.Tolstoy, M.J.Irwin, A.Helmi, G.Battaglia,P.Jablonka, V.Hill, K.A.Venn, M.D.Shetrone,B.Letarte, A.A.Cole, F.Primas, P.Francois,
N.Arimoto, K.Sadakane, A.Kaufer, T.Szeifert, T.Abeｌ1

CM diagram for the WFI coverage of Scl.

Spatial Distribution of BHB and RHB
Stars in the Sculptor dSph

RHB

BHB2

Radial Metallicity Gradient of RGBs

0.25kpc

Kinematical Properties of Scl dSph Stars

metal-richmetal-poor

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Two Distinct Ancient Populationsin the Sculptor Dwarf Spheroidal Galaxy• The Sculptor dSph contains two distinct stellar
components, one metal-rich, -0.9 > [Fe/H] > 1.7, and one metal-poor, -1.7 > [Fe/H] > -2.8.
• The metal-rich population is more centrally
concentrated than the metal-poor one, and on
average appears to have a lower velocity
dispersion σ= 7 ± 1 km/s, whereas metal-poorstars have σ= 11 ± 1 km/s.

What Mechanism Can Create Two Ancient
stellar Compositions in a Small dSph Galaxy?
• The formation of these dSph galaxies began with an initial
burst of star formation, resulting in a stellar population with amean [Fe/H]<-2. Subsequent supernovae explosions would havebeen sufficient to cause gas and metal loss such that starformation was inhibited until the remaining gas could sink
deeper into the center and begin star formation again (Mori etal. 2002).
• Another possible cause is external influences, such as minor
mergers, or accretion of additional gas at later epoch.
• Events surrounding the epoch of re-ionization strongly
influenced the evolution of these small galaxies and resulted instripping of photo-evaporation of the outer layers of gas in
the dSph galaxy, meaning that the subsequent more metalenhanced star formation occurred only in the central regions(Susa & Umemura 2004).

4Origin of Two Distinct Populations in
Dwarf Spheroidal Galaxies

Hierarchical Growth (DM=287,491, gas=233,280)
Kawata, Arimoto, Cen & Gibson (2006)

Galactic Chemodynamics Code (GCD+)
Kawata & Gibson (2003) MNRAS 340, 908• Three dimensional tree N-body/smoothed particle
hydrodynamics (SPH) code which incorporates• Self-gravity,• Hydrodynamics,• Radiative cooling,• Star formation,• Supernovae feedback,• Metal enrichment by SNeII and SNe Ia,• Mass-loss from intermediate mass stars,• Chemical enrichment history of gas and stars.

5DMGASSTAR
Evolution of the distribution of the dark matter (top), gas density(2nd), and K-band observed frame luminosity (bottom).

densitytemperature

metallicity
Evolution of the distribution of the gas density (top),
the gas temperature (2nd), the iron abundance of gas (3rd).

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No Star Formation at z<5.9 due to re-ionization and/or galactic wind.
Although some minor
mergers are involved, thesystem is forming throughthe smooth accretion.

merging
merging
SNe feedback has a strong
effect on the gas dynamics, andcontinuously blows out the gas
from the system. Continuous gas
accretion, however, leads to further
star formation but with low rate.

Metallicity Distribution
Sculptor dSph

r<0.25 kpc
r>0.25 kpc

Fornax dSph

G-dwarfproblem7

Strong Radial Metallicity Gradient

The MDF for the inner (outer) region has a peak at [Fe/H]〜-1.4([Fe/H]〜-1.9). We find this is just due to the metallicity gradientin the simulated system.

Velocity Dispersion Profile[Fe/H]>-1.7[Fe/H]<-1.7

Within the radius of about 0.6 kpc, the metal poor population
have larger velocity dispersion than the metal rich one.

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Caveats
Our simulation demonstrates that a system formed at a high redshift
can reproduce the two stellar populations whose chemical anddynamical properties are distinctive.
However,• In the observational data, there are no stars at [Fe/H]<-2.8,
while the simulated galaxy has a significant fraction of stars withsuch low metallicity (G-dwarf problem).
・ The velocity dispersion of our simulated galaxy is too
small compared with the observed values.
・ The V-band magnitude of the simulated galaxy (Mv=-7.23)
is also small compared with the Sculptor dSph (Mv=-10.7).

Role of SNeIa & SNeII
Tolstoy (2005) astro-ph, 0506481

Star formation stopped at <1Gyr, well before
SNeIa started to contribute significantly.
Scatter is very large, indicating a serious
problem of the current chemical evolution
model in the particle based simulation.

9Role of Intermediate Mass Stars

０．３Gyr

４−８Mo

The enriched gas is blown out at a high redshift around z=17,
due to a strong feedback by SNeII and relatively shallow potentialof subgalactic clumps. As a result, the chemical enrichment by themassive stars becomes less important and the enrichment fromintermediate mass stars (4-8Mo) becomes important.

Sculpter dSph Simulation
In the simulation dwarf spheroidals formed
via hierarchical clustering, but stars formed fromcold gas and stars at the galaxy center tend to form
from metal-enriched infall gas, which builds up
the metallicity gradient.
Infalling gas has larger rotational velocity
and small velocity dispersion.

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