Transcript Observational Data

Sagittarius Dwarf Galaxy
(HST, NASA, ESA)
In current favorite Λ-dominated CDM cosmology, small objects
form first, and larger systems are built up by the assembly of
smaller systems.
Therefore,
dwarf spheroidal
galaxies might beinthe
Two Distinct
Populations
first generation of galaxies that survived from
canibalization by larger systems.
Dwarf Spheroidal Galaxies
Hence, dSphs might contain a record of the epoch
of the end of the dark age.
Nobuo Arimoto (NAOJ, Tokyo)
The Full Fledged
Dwarf Irregular Galaxy Leo A
Vansevicius, V., Arimoto, N., Hasegawa, T., Ikuta, C., Jablonka, P.,
Narbutis,
The Local
GroupD., Ohta, K., Stonkute, R., Tamura, N., Vansevicius, V.,
Yamada, Y. (2004) ApJ 617, L119
Grebel (2000)
Leo A Dwarf Irregular Galaxy
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Distance, kpc
800
Distance modulus, (m-M)0 24.5
Absolute magnitude, MV
-11.7
Integrated color, (B-V)
0.32
Holmberg’s size
7’.0 x 4’.6; b/a = 0.64
Isophote 25 mag/arcsec2 5’.1 x 3’1; b/a = 0.61
Size of HI envelope 14’ x 8’.2; b/a = 0.60
Detection of RR Lyr 8
CO
non-detection
X-ray source
non detection
Leo A Dwarf Irregular
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most isolated in the Local Group
very gas rich HI mass ～ 1.1x107 MO
very low stellar mass ～ 3·106 MO
the lowest metallicity ～ 0.0004
Stellar ages
10 Myr - 10 Gyr
Young & Lo (1996)
Leo A Suprime-Cam Photometry
• Suprime-Cam observation: (20-21 Nov 2001)
B (5x600 s), V (5x360 s), I (5x240 s)
• Seeing < 0”.8
• 6 central CCDs (~20’ x 26’) employed
• Standard reductions - NekoSoft
• Crowded-field stellar photometry – DAOPHOT
• Transformed to the HST system F439W, F555W,
F814W (LG photometric data archive)
• Transformation accuracy: ~0.01 (V&I); ~0.02 (B)
• Number of measured (BVI) objects: ~22000
Subaru/Suprime-Cam
a<3’
4’<a<5’
HST (Tolstoy et al. 1998)
RGB Stars in LeoA
a<12’
BVI available: N ~ 22000
Coordinate accuracy in BVI:
Dr < 0”.3; <Dr> ~ 0”.06
Photometric accuracy:
sI < 0.06; sV < 0.08
Magnitude range: 20.4<I<23
QBVI=-0.4<[(B-V)
-(V-I)]<+0.1
c2 < 1.5; |sharpness| < 0.4
--------------------N = 1394
RGB stars
<sI> ~ 0.010; <n> = 4.7
<sV> ~ 0.015; <n> = 4.7
<sB> ~ 0.019; <n> = 4.6
a<12’, N=12604 3’<a<5’, N=2462 5’.5<a<7’.5, N=974
C1-V01
RGB star radial density profile
• 1) crowded central part: a = 0’.0 – 2’.0, completeness from
80 to 90 % at I = 23m;
• 2) old exponential disk extending well beyond the previously
estimated size of the galaxy:
a = 2’.0 – 5’.5, S-L 1’.03 ± 0’.03;
• 3) discovered old “halo”:
a = 5’.5 – 7’.5, S-L 1’.84 ± 0’.09;
• 4) cut-off of RGB star distribution: a = 7’.5 – 8’.0;
• 5) sky background zone, a = 8’.0 – 12’.0;
• 6) young (< 1 Gyr) disk population
(I < 24 & (V-I) < 0.25):
a < 5’, S-L 0’.56 ± 0’.06;
• 7) exponential HI distribution:
a < 7’.0, S-L 1’.40 ± 0’.10.
Conclusions
The young and old Leo A disks together with the
discovered old halo and sharp stellar edge closely
resemble structure as well as stellar and gaseous
content found in the large full-fledged disk
galaxies.
This suggests complex build-up histories of the very
low mass galaxies like Leo A, which are supposed
to form directly from the primordial (～1s) CDM
density fluctuations in the early universe, and
challenges contemporary understanding of galaxy
formation and evolution.
Two Distinct Ancient Populations
in 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.Abeｌ
CM diagram for the WFI coverage of Scl.
Spatial Distribution of BHB and RHB Stars
in the Sculptor dSph
Radial Metallicity Gradient
Kinematical Properties of Scl dSph Stars
Two Distinct Ancient Populations
in 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-poor stars have σ= 11 ± 1
km/s.
What Mechanism Can Create Two Ancient stellar
Compositions in a Small dSph Galaxy?
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The formation of these dSph galaxies began with an initial burst of star
formation, resulting in a stellar population with a mean [Fe/H]<-2.
Subsequent supernovae explosions would have been sufficient to cause
gas and metal loss such that star formation was inhibited until the
remaining gas could sink deeper into the center and begin star
formation again (Mori et al. 2002).
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Another possible cause is external influences, such as minor mergers,
or accretion of additional gas.
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Events surrounding the epoch of reionization strongly influenced the
evolution of these small galaxies and resulted in stripping of
photoevaporation of the outer layers of gas in the dSph galaxy, meaning
that the subsequent more metal-enhanced star formation occurred
only in the central regions.
The DART survey of the Fornax Dwarf
Spheroidal Galaxy using VLT/FLAMES
G.Battaglia, E.Tolstoy, A.Helmi, M.J.Irwin, B.Letarte, P.Jablonka, V.Hill,
P.Francois, K.A.Venn, M.D.Shetrone, F.Primas, A.Kaufer, T.Szeifert, T.Abel,
N.Arimoto, K.Sadakane (2006)
Fornax dSph Galaxy
AGB
MS
RHB
RG
B
RC
BHB
Old : Intermediate: Young
Populations
Radial Distribution of Stellar Populations in
Fornax dSph
1)
2)
3)
4)
5)
The
The
The
The
The
main sequence and blue-loop stars disappear in the outer regions,
average colour of BL stars becomes bluer at large radii,
RC (2-8Gyr) in less extended in luminosity,
BHB becomes clearly visible at the outer most region,
shape of RGB changes.
BHB
RHB
RC
前景星
Blue RGB
BRGB: low [Fe/H]
Old 13 GYr
Red RGB
RRGB: high [Fe/H]
Intermediate 3 Gyr
Old stellar populations (BRGB, RHB, RHB, RR; ～10Gyr) show
almost identical distribution and are more extended than
the intermediate age populations (RCs; 2-8Gyr).
Intermediate age stellar populations (RRGB, RC, AGB ; 2-8Gyr)
show almost identical distribution and are more concentrated than
the old populations (RHBs; 10Gyr). These stars all formed from the
same distribution of gas.
Radial Stellar Density Distribution
There is no clear evidence for flattening the density profile at outer
radii, suggesting no old halo populations in the Fornax dSph galaxy.
Stellar Velocity Dispersion
For r<0.4, the metal poor population exhibits a larger velocity
dispersion than the metal rich one, while in the outer regions
the velocity dispersions are similar.
Peculiar dSph Galaxy?
The velocity dispersion of metal poor stars at r<0.4 is far from
being Gaussian, it is flat or even double peaked.
Double peaked velocity histogram for metal-poor stars in the Fornax dSph.
Spatial Distribution, Metallicity & Kinematics
of the Fornax dSph Stellar Population
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As in Sculptor dSph, two main stellar components are present in
Fornax; an old one, metal poor and extended, and a younger one,
more metal rich and more centrally concentrated; the MR stars in
Fornax have a colder dispersion than the MP ones.
The dominant stellar population (RCs) in Fornax are metal rich and
much younger than in Sculptor.
Fornax contains young stars (MS and blue-loop) at the inner most
region (r<0.4).
The MP stars at r<0.4 deg have peculiar kinematics: the velocity
dispersion is flat or even double peak.
In Fornax for r<0.4 deg there is a high metallicity tail to the mean
([Fe/H]>-0.6) which is not present in Sculptor.
Accretion of A Metal Poor Gas-Rich Dwarf Galaxy
on to the Fornax dSph
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We can regard the Fornax as a Sculptor-like dSph, made of two
stellar populations, plus an overlaid population perturbed by an event
that causes the asymmetric distribution of young stars and high
metallicity stars.
Somewhat peculiar behaviour of stellar populations in Fornax could
be explained by the accretion of a gas-rich dwarf galaxy.
None of the other satellites of the Milky Way show an extended star
formation history and young dominant stellar population, except
perhaps Leo I. We notice that this kind of star formation history is
similar to the one of dIrr galaxies.
Origin of Two Distinct Populations in Dwarf
Spheroidal Galaxies
Hierarchical Growth (DM=287,491, gas=233,280)
Kawata, Arimoto, Cen & Gibson (2005)
Galactic Chemodynamics Code (GCD+)
Kawata & Gibson (2003) MNRAS 340, 908
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Three dimensional tree N-body/smoothed particle hydrodynamics
(SPH) code which incorporates
Self-gravity,
Hydrodynamics,
Radiative cooling,
Star formation,
Supernovae feedback,
The original GCD+ code has been updated to implement
Metal
enrichment bychemical
SNeII and
SNe Ia,of hydrogen and helium
non-equilibrium
reaction
Mass-loss from
intermediate
mass
species
(H, H+, He,
He+,stars,
He++, H2, H2+, H-)
andhistory
their cooling
Chemical enrichment
of gas processes.
and stars.
Although
Evolution
some of
minor
the mergers
distribution
areofinvolved,
the darkthe
matter
system
(top),
is forming
gas density
nd), and accretion,
through the(2smooth
K-band observed
somewhat
frame
likeluminosity
monolithic(bottom).
collapse.
of theproduced
gas density
(top),have
MoreEvolution
than 80% of
of the
the distribution
heavy elements
in stars
the gas temperature
(2nd),the
thesystem
iron abundance
escaped from
till z=5.9. of gas (3rd).
No Star Formation at z<5.9 due to re-ionization and/or galactic wind.
Although some minor
mergers are involved, the
system is forming through
the smooth accretion.
mrg
mrg
SNe feedback has a strong
effect on the gas dynamics, and
continuously 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
G-dwarf
problem
Fornax dSph
Radial Metallicity Distribution
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 gradient
in 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.
G-dwarf Problem (Caveats)
Our simulation demonstrates that a system formed at a high redshift
can reproduce the two stellar populations whose chemical and
dynamical properties are distinctive.
However,
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In the observational data, there are no stars at
[Fe/H]<-2.8, while the simulated galaxy has a significant fraction of
stars with such 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).
Solutions to the G-dwarf Problem
(Infall, MESF, PIE)
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It is likely that the disk of the Milky Way has been formed by continuous
accretion of gas from the reservoir, such as the Galactic halo and the IGM,
but our simulation already takes into account cosmological gas infall.
Our simulation does not take into account the effect of ionizing radiation
fields. If there is a background radiation field, some fraction of H 2 will be
photo-dissociated. Our simulation may overestimate the H 2 cooling and the
accretion rate.
MESF model is unlikely either, because our simulation takes into account
radiative cooling rate depending on the metallicity of the gas.
Pop.III PIE scenario looks most attractive. At high redshift (z～20) Pop.III
stars formed at the center of the building block and SNe explosions blew
up the gas and metals in the blocks, which helps to enrich the IGM.
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.
Role of Intermediate Mass Stars
The enriched gas is blown out at a high redshift around z=17,
due to a strong feedback by SNeII and relatively shallow potential
of subgalactic clumps. As a result, the chemical enrichment by the
massive stars becomes less important and the enrichment from
intermediate mass stars (4-8Mo) becomes important.
Sculpter dSph Simulation
In the simulation dwarf spheroidals formed
via hierarchical clustering, but stars formed from
cold 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.