Two Distinct Populations inDwarf Spheroidal Galaxies

Nobuo Arimoto (NAOJ, Tokyo)

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 be the first generation of galaxies that survived from

canibalization by larger systems.

Hence, dSphs might contain a record of the epochof the end of the dark age.

The Full Fledged Dwarf Irregular Galaxy Leo A

Vansevicius, V., Arimoto, N., Hasegawa, T., Ikuta, C., Jablonka, P., Narbutis, D., Ohta, K., Stonkute, R., Tamura, N., Vansevicius, V., Ya

mada, Y. (2004) ApJ 617, L119 Grebel (2000)The Local Group

Leo A Dwarf Irregular Galaxy• 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

• 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, F

814W (LG photometric data archive)• Transformation accuracy: ~0.01 (V&I); ~0.02 (B)• Number of measured (BVI) objects: ~22000

Subaru/Suprime-Cam

HST (Tolstoy et al. 1998)a<3’ 4’<a<5’

BVI available: N ~ 22000Coordinate accuracy in BVI:Dr < 0”.3; <Dr> ~ 0”.06Photometric accuracy:

sI < 0.06; sV < 0.08Magnitude range: 20.4<I<23

QBVI=-0.4<[(B-V) -(V-I)]<+0.1c2 < 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’

RGB Stars in LeoA

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.

ConclusionsThe 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.Abe ｌ

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?

• 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).

• Another possible cause is external influences, such as minor mergers, or accretion of additional gas.

• 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

Old : Intermediate: Young Populations

MS

BHB

RHB RC

RGB

AGB

Radial Distribution of Stellar Populations in Fornax dSph

1) The main sequence and blue-loop stars disappear in the outer regions,2) The average colour of BL stars becomes bluer at large radii,3) The RC (2-8Gyr) in less extended in luminosity,4) The BHB becomes clearly visible at the outer most region,5) The shape of RGB changes.

BHB

RHB

RC

前景星

Blue RGB

Red RGB

BRGB: low [Fe/H] Old 13 GYr

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 outerradii, 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.

The velocity dispersion of metal poor stars at r<0.4 is far from being Gaussian, it is flat or even double peaked.

Peculiar dSph Galaxy?

Double peaked velocity histogram for metal-poor stars in the Fornax dSph.

Spatial Distribution, Metallicity & Kinematicsof the Fornax dSph Stellar Population

• 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

• 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

• 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.

The original GCD+ code has been updated to implement non-equilibrium chemical reaction of hydrogen and helium

species (H, H+, He, He+, He++, H2, H2+, H-)

and their cooling processes.

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

Although some minor mergers are involved, the system is forming through the smooth accretion, somewhat like monolithic collapse.

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

More than 80% of the heavy elements produced in stars have escaped from the system till z=5.9.

Although some minormergers are involved,

thesystem is forming

throughthe smooth accretion.

No Star Formation at z<5.9 due to re-ionization and/or galactic wind.

mrg

mrg

SNe feedback has a strongeffect on the gas dynamics, and continuously blows out the gas

from the system. Continuous gasaccretion, however, leads to further

star formation but with low rate.

Metallicity Distribution

G-dwarfproblem

Sculptor dSph

Fornax dSph

r<0.25 kpc r>0.25 kpc

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 populationhave larger velocity dispersion than the metal rich one.

Our simulation demonstrates that a system formed at a high redshift can reproduce the two stellar populations whose chemical and dynamical properties are distinctive.

G-dwarf Problem (Caveats)

• 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).

However,

Solutions to the G-dwarf Problem(Infall, MESF, PIE)

• 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 H2 will be photo-dissociated. Our simulation may overestimate the H2 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 seriousproblem of the current chemical evolutionmodel 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 fromintermediate 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 upthe metallicity gradient.

Infalling gas has larger rotational velocity and small velocity dispersion.