Near-field cosmology: the first stars
Useful books/notes/reviews for this lecture:
The First Galaxies Bromm & Yoshida ARA&A 2011, vol. 49, p. 373
Galaxy Formation and Evolution – Chap. 8/9 - Cambridge 2010, Mo, van den Bosch & White First Light in the Universe Saas-Fe lectures 2008 by Loeb, Ferrara & Ellis
http://www.sns.it/en/didattica/scienze/menunews/personale/docenti/ferraraandrea/materiale/lectures/
download/SaasFeeLectures.pdf/
The First Galaxies Chapter 2 of my PhD Thesis 2009
http://www.astro.rug.nl/~salvadori/PhdThesis
Please come to my office (room 34 first floor @ Arcetri) or email me salvadori@astro.rug.nl for any questions, comments, and feedback
Lectures available at
http://www.astro.rug.nl/~salvadori/Stefania_Salvadori/Lectures_2015.html
First stars & galaxies: a simple sketch
M ~ 106M @ z ~ 25 Tvir < 104K H2-cooling
tcool << tff H2-cooling
Tc ~ 200K, nc ~104cm−3 Mclump ≈ MJ ≈ 700 M
Maccr ≈ Tc3/2
m* > 10 M
τlifc ≈ few Myr Feedback processes:
LW photons H2 dissociation Ionizing photons HII regions
Metal production/dispersion driven by SN explosions
Low binding energy:
gas/metals ejection
The minimum halo mass able to form stars increases Msf(z)
The metallicity Z of the ISM and IGM
increases
Subsequent generations
M > Msf (z) ?# YES# Z >Zcr=10−5±1 Z?#
YES# NO#
NO#
dark halo no stars
Different evolution, photon production, metal enrichment,
SN energy
Mclump ≤ M
m*=(0.1-100) M m* > 10 M
Mclump≈ 700M
Observing the high-z Universe
Observing the high-z Universe
The evolution of the star formation rate density
Madau & Dickinson 2014
Observing the high-z Universe
Probes of metal-enrichment: gas metallicity in different environments
Madau & Dickinson 2014
Galaxies (DLAs)
Inter Galactic Medium Galaxy
Clusters
Quenching of PopIII stars
PopIII stars (Z < Zcr) are very rare and they disappear at z ~ 2
Tornatore+2008
Simulating the cosmic metal-enrichment
Pallottini+2014
PopIII stars (Z < Zcr) are very rare and they disappear at z ~ 2
Observing the tip of an Iceberg
At the moment, we are only able to observe the brightest, highly star-forming galaxies at z > 6. It is extremely difficult to directly observe the “first galaxies” that host PopIII stars simply because
they are rare, faint, and distant!
Galactic Archaeology is a valid alternative probably even more powerful to constrain the properties of first stars and galaxies!
The next generation of space (James Webb Space Telescope, expected lunch 2018), and ground based telescopes (Extremely Large Telescope, ~ 2030) will possibly observe these galaxies
New exciting era to understand early galaxy formation!
Stellar lifetimes
z = 25 Age = 0.13 Gyr
τ = 13.5 Gyr
z = 6 Age ~ 1 Gyr τ = 12.7 Gyr
z = 2 Age ~ 3.3 Gyr
τ = 10.3 Gyr
Surviving stars
Initial Mass Function
ϕ(m*) ~ m*−1+x exp(−mcut/m*) x = −1.35
mcut = 0.35 M
Looking for metal-poor stars
If the formation of “normal” low mass popII stars is triggered by the presence of metals and dust exceeding
Zcr =10 −5±1Z#
then the most metal-poor stars, Z ~ Zcr , that survive until today may represent the oldest stellar relics of the early Universe.
Where can we observe the most metal-poor stars?
thick disk
Mbulge ≈ 2. 1010 M
Mgas ≈ 1010 M
Mdisc ≈ 6. 1010 M
Mhalo ≈ 3. 109 M Stellar halo
thin disk
30 kpc
8 kpc
Sun
The structure of the Milky Way
4 kpc
thick disk open clusters
bulge thick disk globulars
young halo globulars
old halo globulars thin disk
thick disk
halo
Freeman&Bland-Hawthorn 2002
Metal enrichment in the Milky Way
[Fe/H]=log10(NFe/NH)-log10(NFe/NH)
History of most iron-poor stars
Spectra of iron-poor stars
Keller+2014, Science
N* = 2756 r < 20 kpc
HE1327-2326
HE0107-5240 HE0557-4840
Metallicity Distribution Function
Galactic halo stars
Beers&Christlieb2005
Caffau+11
SM03103-6708
N* = 2756 r < 20 kpc
HE1327-2326 HE0107-5240 HE0557-4840
Metallicity Distribution Function
Galactic halo stars
Beers&Christlieb2005
Caffau+11
Log Z/Z Zcr =10 −5±1Z
Dwarf spheroidal galaxies
dSph galaxies satellites of the MW
kpc
kpc Galactic center
Total masses M < 109 M. Gas-free systems. Old and metal poor stars
Outer halo
Metallicity-Luminosity relation
Kirby+08
Milky Way dwarf spheroidal satellites
Kirby+2008
Other new faint satellites
discovered
Simulating the Milky Way assembling
Diemand+2007/2008
≈ 1,000,000,000 dark matter
particles mp= 4.100×103M
Simulating the Milky Way assembling
NOTE: in the movie you see the merging of dark matter haloes.
Merging of galaxies is something we do really observe, and not only a prediction of the hierarchical LCDM model.
In the following you can see a sequence of images from the Hubble Space Telescope (HST) that underline this.
Via Lactea simulation
Diemand+2007/2008
≈ 1,000,000,000 dark matter
particles
mp= 4.100×103M
Aquarius simulation
Springel+2008
Increasing resolution
4,252,607,000 mp = 1.712×103 M 148,285,000
mp = 4.911×104 M 2,316,893
mp = 3.143×106 M
Monte Carlo approach
MW
MMW = 1012 M
Time
z = 0
Redshift
Comparison with N-body Binary scheme
€
ψ = ε
*M
gt
ff€
dMg
dt = −
ψ
+ dRdt + dMinf
dt − dMej dt
€
dM
Zdt = −Z
ISMψ + dY
dt + Z
virdM
infdt − Z
wdM
ejdt
Z wZ w
ZISM
Zvir Zvir
Physical prescriptions/free parameters
εw tinf
Model calibration
Evoli&Ferrara2011
SFR ≈ 1.3 M/yr M* ≈ 6×1010 M
Mg/M* ≈ 0.1
68%
99%
99%
95%
Simplified case: only stars/gas no infall
The free parameters
General rule for semi-analytical models:
the higher is the number of equations (physics) involved the higher is the number of free parameters
the higher is the number of observational constraints needed
Example: if we also want to follow the evolution of metals along the build-up of the Milky Way we have to reproduce
the final metallicity of the gas/stars (~ Z)
along with the observed Z-range of Galactic halo stars
Constraining high-z properties
Once fixed the main free parameters (SF/wind efficiency) we can investigate (and then constrain?)
the properties of the first stars/galaxies
and/or the efficiency of feedback processes acting at high-redshifts
• What is the value of the critical metallicity?
• Can we constrain the mass range of the first stars?
• What is the efficiency of star formation in H2-cooling haloes?
• Are H2-cooling mini-haloes a “suicide” population?
• What is the mass of the first galaxies?
Questions we can try to address:
Signatures of first stars
The most iron-poor star: [Fe/H] < −7
Keller+14
Very massive first stars ~ 200 M
First stars ~ 60 M
Other fossil stars at [Fe/H] < −4
Iwamoto+2003, Science
First stars ~ 25 M
Second generation stars ?
1. If the total metallicity reflects that of the ISM from which these stars form ZISM > 10 −3Z >Zcr. Faint primordial
supernovae with relatively high mass M ~ 25 M
2. If the iron abundance reflects the metallicity of the ISM from which they form ZISM ≈ 10 −5Z ≈ Zcr .
CNO have to be accreted from a companion star
Caveat: for these stars [Fe/H] is not a good metallicity indicator!
Even if [Fe/H] < −4.8 the total metallicity is Z > 10 −3Z
No binary companions observed!
Imprints of chemical feedback
Varying the critical metallicity
The predicted Metallicity Distribution Function of Galactic halo stars strongly depends on the assumed critical metallicity and IMF of primordial stars. The existence of the most
metal-poor star implies Zcr < 10 −4 Z
Salvadori+07
Imprints of chemical feedback
Varying the critical metallicity
The predicted Metallicity Distribution Function of Galactic halo stars strongly depends on the assumed critical metallicity and IMF of primordial stars. The existence of the most
metal-poor star implies Zcr < 10 −4 Z
Salvadori+07
Z/Z
Just a “common” metal-poor star
The chemical abundance patter of the most pristine star ever, with Z ~ 10−5 is consistent with that of stars with − 4 ≤ [Fe/H] ≤ − 3 no characteristic features produced by massive first stars !
Where are second-generation stars enriched by very massive (>140 M) primordial supernovae?
Cayrel+04 Mean [X/Mg] value for 35 stars with [Mg/H] < − 3
Second generation stars
Second generation stars are extremely rare. The expected number of second generation stars in the currently limited Galactic halo sample is < 1-2.
Salvadori+07
Zcr = 10 – 4 Z Zcr = 10 – 6 Z Zcr = 0
2nd generation vs all generations
A very rare star!
1 out of 500 stars at “high” iron abundance [Fe/H] ~ −2.5
Signatures of feedback processes
Number of DM haloes
Madau+08
Via Lactea simulation
The missing satellites problem
If all the haloes are able to form stars with a fixed efficiency
The number of predicted luminous satellites exceeds by several orders of magnitude the one observed.
The higher is the resolution of the simulation
the higher is the expected number of luminous satellites at z = 0
Radiative feedback processes are expected to gradually
reduce the SF in minihaloes and increase the minimum mass of haloes that are able to form stars. This is really a problem?
The SF efficiency of minihaloes
105 104
103
Ltot/L
Observations
Simulations: different SF efficiencies
Madau+08
The SF efficiency of mini -haloes has to decrease at decreasing mass in order to reproduce the observed
luminosity function of dwarf satellites
Imprints of radiative feedback
Munoz+09 105
104
103 106 107 108
Ltot/L
The number of luminous satellite galaxies predicted at z = 0 strongly
depends on the evolution of Msf(z)
The faintest dwarf galaxy
Ultra-faint dwarf galaxies
Kirby+08
What are ultra-faint dwarfs?
H2-cooling minihaloes!
Salvadori+2015 Salvadori & Ferrara 2009
See also Bovill & Ricotti 2009/11/12, Munoz+09, Salvadori & Ferrara 2012
Star-formation histories of ultra-faint dwarfs
Brown+2014
Conclusions
• Current observations support the idea that first stars were more massive than today stars ( M >10 M ) and rapidly disappeared
• Ultra-faint dwarf galaxies might be the living fossils of the first H2 -cooling minihaloes we are likely observing the first galaxies.
Big surveys combined with theoretical predictions will allow to constrain the mass function of the first stars!
Powerful systems to understand the role of radiative feedback processes and chemical enrichment in the early Universe!
What we learnt ?
• Semi-analytical models are “cosmological bridges” that connect the physical processes acting at high-z with the Local observations.
• They are powerful tools to investigate the feedback imprints left in the Local Universe and the properties of the first stars/galaxies.
• If you want to build up a good semi-analytical model you have to compare your results with most of the available observations
• There are still many puzzling questions about the first cosmic objects that can be solved using these methods and the new observations!!
