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Dwarf Spheroidal Galaxies : From observations to models ... · Dwarf Spheroidal Galaxies : From...
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Dwarf Spheroidal Galaxies :From observations to models
and vice versa
Yves Revaz
The good reasons to study dSphs :
Test for the LCDM paradigm : small structures are predicted in abundance
Bower et al. 2003
Galaxy luminosity function
The good reasons to study dSphs :
The missing satellite problem
LCDM simulations predict the existence of a high number of dwarf galaxies orbiting around Andromeda and the MilkyWay... ...but only a modest population is observed.
Simon & Geha 2007
Via Lactea (Diemand et al. 2007)
The good reasons to study dSphs :
In the hierarchical LCDM paradigm, dSphs are the building blocks of larger objects
Dwarf galaxies
Spiral galaxies
Elliptical galaxies
Galaxy clusters
time
The good reasons to study dSphs :
Much more complex than usually thought
Tinguely 1970
Agenda
What is a dSph galaxy ?
How do we model these objects ?➢ The driving parameters➢ Intrinsic or extrinsic evolution ?
Can we predict the stellar mass for a given halo mass
Some conclusions
What is a dSph galaxy ?
Faint stellar system (Mv < -15)
Low velocity dispersion (~10 km/s)
Evidence for dark matter
No clear rotation (dominated by random motions)
No ongoing star formation
No gas (or at least not detected)
[Fe/H] follows luminosity
Mateo 2008
dSphs are interior to transition systems, themselves interior to dIrrs
clustered around spiral galaxies
core
tidal
core
tidal
diffuse objects
Fornax Carina
Piatek, 2007; 2003
they form a sequence
Adapted from Tolstoy, Hill & Tosi 2009
Vt=198 ±50 km/sVr=79 ±6 km/sPeri : 68 kpc (33, 83)Apo :122 kpc (97,133)Period : 2.2 Gyr
Sculptor
Piatek et al. 2006, 2003
Vt=85 ±39 km/sVr=22 ±24 km/sPeri : 20 kpc (3, 63)Apo :102 kpc (102,113)Period : 1.4 Gyr
Carina
todaytoday
...but are also very different
Sculptor Carina
Time Time
Sta
r Fo
rmati
on
Rate
Rizzi et al. 2004
Out
In
also Tolstoy et al. 2004; Battaglia et al. 2006
disparate, structures, diff. stellar pop as fct of r
What is at the origin of the diversity ?
● Is the environment the driving parameter ?➢ Chance encounters produce a variety of properties
● Can we think otherwise ?➢ How much of the variety is intrinsic ?➢ When and how is interaction required ?➢ Sequence = single framework of formation ?
Current limitations
➢ Too simplistic chemical evolution
➢ Arbitrary fixed SFH
➢ Small number of simulations
➢ Evolution stopped at high z
➢ Confront results only with global relations
Global relations are not all...
Objects with very similar :- Lv- [Fe/H]- [M/Lv]
...
... may have completely different stellar and chemical properties !
SFR, M* AGE [Fe/H] [Mg/Fe]
Observations : Where do we stand ?
● Accurate abundances measurement from individual stars for:➢ Fornax (Letarte et al. 2010)
➢ Sculptor (Hill et al. in prep.)
➢ Sextans (Shetrone et al., 2001, Aoki et al. ,2009, Jablonka et al., in prep.)
➢ Carina (Koch et al. , 2008, Lesmale et al. , 2011, Venn et al., in prep.)
DART TeamDART Team
Modelisation
Code & physical processes
The initial conditions
Tests and Robustness
GEAR : a self-consistent Tree/SPH code (Revaz & Jablonka 2012)
Skeleton : Gadget-2 (Springel 2005)
Gravity :
-> treecode (Barnes & Hut 86)
Hydrodynamics :
-> SPH : Smooth Particles Hydrodynamics (Lucy 77, Gingold & Monaghan 77)
NlogN instead of N2
Hydrodynamics values are obtained by convolution of neighbors particles with a kernel function
the resolution follows the mass
GEAR : a self-consistent Tree/SPH code
The baryon physics
(Katz 92)
Cooling function (metal dependent) :
- above 104K (Sutherland & Dopita 93)
- below 104K, H2, HD, OI, CII, SiII, FeII
(Maio et al. 07)
Star formation :
classical recipes : Schmidt law (Schmidt 59)
+ star formation density (0.1 a/cm3)
+ starformation temperature (3x104K)
Chemical evolution SSP scheme : (Poirier 03, PhD thesis)
- SNIa, SNII nucleosynthesis + feedback from SN explosions
- elements followed : Fe, Mg
GEAR : a self-consistent Tree/SPH code
Tim
e
Formation time of a stellar
particle (SSP)
mass fractionyieldsenergy
IMF : Krupa 01
Kodama & Arimoto 97
Injection into the system :
nearest neighbors
SNII :
yields of massive stars (Iwamoto et al. 99)
Energy : eSN
1051 ergs/SN (thermal)
SNIa :
model from Kobayashi et al. 2000
yields from Tsujimoto et al. 95, updated models
Energy : eSN
1051 ergs/SN (thermal)
stellar particles :➢ Fe, Mg, Z, age -> Lv (Vasdekis et al. 96)
gas particles : ➢ Fe, Mg, Z, density, temperature
all particles : ➢ positions, velocities, mass
Outputs of models and observables
Initial conditions
Initial conditions
- 2 Mpc/h3 box, dark matter only (WMAP V)
- 134'217'728 particles
=> resolution of 150 pc/h, 4.5 104 Msol/h
more than 150 dSph haloes withmasses between 108 > M > 109 M
sol
Z=6.53 3.72 2.46 1.72 1.46 0.25 0.0
Z=6.53 3.72 2.46 1.72 1.46 0.25 0.0
50% of the haloes have their density profiles already in place at z=6 (in physical coordinate)
98% have an NFW profile, wich central densities varying by a fractor ~3 only (for masses between 108 and 109 M
sol)
● Models of dSphs in a static Euclidean space, where the expansion of the universe is neglected, are justified. The physics of baryons that depends on the density in physical coordinates is correct.
● The densities of haloes with mass between 108 and 109 Msol exhibit a small dispersion, a factor 3 to 4, which could help understanding the variety in the observed properties of the dSphs
Core profile supported by the observations (Blais-Ouellette et al. 2001; de Blok & Bosma 2002; Swaters et al. 2003; Gentile et al. 2004, 2005; Spekkens et al. 2005; de Blok 2005; de Blok et al. 2008; Spano et al. 2008)
Initial conditions : isolated models
Energy conservation, Robustness & Convergence tests
Conservation better than 5% over more than 250 dynamical times !
9.5x108Msol
chemical properties : - metallicity distribution - abundances
density profile
Results
Parameters (see also Revaz et al. 2009)
More than 400 simulations,Exploring the effect of the parameters
Efficiency of supernova energy :
➢ If eSN
=100% (1051 ergs per SN) → no Fe/H enhancement (need to be below 10%)
➢ no strong winds : the gas is kept around the system
➢ 107 Msol of gas linked to the dSph (see also Marcolini et al. 06, Valcke et al. 08)
eSN
= 100%
eSN
= 1%
Mass = 4x108Msol
Mass or density ?
mass
dens
ity
density x 5 : Δ[Fe/H]= 1.0 dexmass x 9 : Δ[Fe/H]= 0.5 dex
•Cooling stronger for larger densities•Mass increases luminosity but has •negligible impact on the chemical •evolution
c*=0.05 eSN=0.03
Fornax
Sculptor
Sextans
Carina
7x108Msol
5x108Msol
3x108Msol
1x108Msol
Yes
● The dominant driving parameters are the mass and density (compatible with the cosmology)
● More massive and dense systems, forms stars continuously
➢ → high [Fe/H] → high Lv● Less massive and less dense sytems forms stars episodically
➢ → low [Fe/H] → low Lv
But we need outer physical processes
➢ to truncate the star formation
➢ to get rid of the remaining gas
Do we observe a sequence ?
Metallicity gradients
Sculptor (Tolstoy et al. 2004)
Sextans (Battaglia et al. 2011)
Fornax (Battaglia et al 2006)
Metallicity gradients
metalicity gradients ?
Metallicity gradients : the effect of gas motion
High resolution model : M=3.5 108Msol, 4x106 of particles
➢ hot gas heated by SNs accumulates at the center➢ due to strong Archimedes forces this gas is driven outside
➢ high metallic gas is ejected into the IGM (1.5 105 Msol)
High resolution model : M=9.5 108Msol, 4x106 of particles
Metallicity gradients : the effect of gas motion
How galaxies populates their dark matter halo ?
Abundance matching
•Millennium Simulation (MS; Springel et al. 2005)•High resolution MS (Boylan-Kolchin et al. 2009)•SDSS/DR7
Assume
•main subhaloes and satellite subhaloes have galaxies at their centres,
• the stellar masses of these galaxies are directly related to the maximum dark matter mass ever attained by the subhalo during its evolution. In practice, this mass is usually the mass at z= 0.
• one-to-one and monotonic relationship between Mhalo and M*
n(>Mhalo
) = n(>M*)
Guo et al. 2010
SDSS+Millenium Guo et al 2010
Sawala et al 2011
Extrapolation Sawala et al 2011
Abundance matching :
Sawala et al 2011
SDSS+Millenium Guo et al 2010
Who is right ?Who is wrong ?
Abundance matching
SDSS+Millenium Guo et al 2010
Faber-Jackson / Tully-Fisher relation
SDSS+Millenium Guo et al 2010
Faber-Jackson / Tully-Fisher relation
SDSS+Millenium Guo et al 2010
Faber-Jackson / Tully-Fisher relation
Can we find 10 km/s stellar systems in 1010M
sol halos ?
Large sample of halo populated by stars with random properties (N-body systems)
● Halo : generalized NFW (HonhSheng Zhao 1996, Treu et al. 2002) CUSP or COREM
h : [108,1011] M
sol
rs,h : [0.5-4] kpc
h : [0,1]
● Stars : generalized NFWM
h : [105,109] M
sol
rs,s : [0.5-4] kpc
s : [0,1]
Velocities computed from the Jeans Equations
Carina-like Fornax-like
Baryonic fractions at odd with plausible star formation histories
bf : [3x10-4, 3x10-3]b
f : [3x10-5, 3x10-4]
Maccio et al. 2006
Conclusions
● GEAR : a self-consistent Tree/SPH code for dSphs simulations
➢ robust + good convergence
● In LCDM, the density profiles of small haloes does not evolve much betweeen z=6 and z=0
→ Possible to study dSph in an isolated context
● To fit the metallicity of dSphs, the feedback efficiency cannot be large :
➢ no strong winds but a large amount of gas left
● Global relations are reproduced
● But also the variety in stellar populations and chemical abundances for : Fornax, Sculptor, Sextans & Carina
Conclusions
● Evolution driven by :
● Intrinsic processes : mass+density
● Extrinsic processes : get rid of the remaining gas + truncate the SFR
● The low stellar content predicted by LCDM for 1010Msol seems to be incompatible with our models
● Tidal stripping ?● Ram pressure stripping ?
Credit : Olivier Tiret
The futur of GEAR
Cosmological simulations
dIrr and spiral galaxies
The End