Near-field cosmology: the first stars

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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


The First Galaxies Chapter 2 of my PhD Thesis 2009

Please come to my office (room 34 first floor @ Arcetri) or email me for any questions, comments, and feedback

Lectures available at


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



Subsequent generations

M > Msf (z) ?# YES# Z >Zcr=10−5±1 Z?#



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



Quenching of PopIII stars

PopIII stars (Z < Zcr) are very rare and they disappear at z ~ 2



Simulating the cosmic metal-enrichment


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


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


Freeman&Bland-Hawthorn 2002

Metal enrichment in the Milky Way



History of most iron-poor stars


Spectra of iron-poor stars

Keller+2014, Science


N* = 2756 r < 20 kpc


HE0107-5240 HE0557-4840

Metallicity Distribution Function

Galactic halo stars





N* = 2756 r < 20 kpc

HE1327-2326 HE0107-5240 HE0557-4840

Metallicity Distribution Function

Galactic halo stars



Log Z/Z Zcr =10 −5±1Z


Dwarf spheroidal galaxies

dSph galaxies satellites of the MW


kpc Galactic center

Total masses M < 109 M. Gas-free systems. Old and metal poor stars

Outer halo


Metallicity-Luminosity relation


Milky Way dwarf spheroidal satellites


Other new faint satellites



Simulating the Milky Way assembling


≈ 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


≈ 1,000,000,000 dark matter


mp= 4.100×103M


Aquarius simulation


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


MMW = 1012 M


z = 0


Comparison with N-body Binary scheme


ψ = ε







dt = −


+ dR

dt + dMinf

dtdMej dt



dt = −Z


ψ + dY

dt + Z




dt − Z





Z wZ w


Zvir Zvir

Physical prescriptions/free parameters

εw tinf


Model calibration


SFR ≈ 1.3 M/yr M* ≈ 6×1010 M

Mg/M* ≈ 0.1





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


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



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




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.


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


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




Simulations: different SF efficiencies


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


103 106 107 108


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



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




•  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!!




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