The First Galaxies … a theoretical prospective …

Useful books/notes: Mo, van den Bosch & White: Galaxy Formation & Evolution – Chapters 8/9

Andrea Ferrara’s Saas-Fe lectures: http://www.sns.it/en/scienze/menunews/docentiscienze/ferraraandrea/lectures/

Introduction of my Phd Thesis: http://www.astro.rug.nl/~salvadori/thesis.pdf

Dust, metals and photons from the first stars

The Universe is metal-free, neutral and homogeneous

Gradual growth of density

perturbations

Today observable Universe

z ~1100

z ~ 30

z ~ 6

z = 0 ~ 13.7 billion

Redshift Synthesis of light nuclei

The Big Bang

Protons and electrons recombine

The reionization is complete

The first stars form within the first

virializing haloes

Galaxy formation proceeds

hierarchically

LAEs < 7

QSOs < 8.3 Salvaterra+09 Tanvir+09

Ono+10

The Cosmic History

•  Well defined and simple initial conditions: no metals, no dust, no magnetic field, no turbulence •  Simple physical processes involved •  Perfect pedagogical objects to understand the physics of galaxy formation and evolution •  Strong impact onto the subsequent structure formation history through feedback processes

We need to understand the properties of the first galaxies and of the first stars in order to have a complete picture of the overall structure formation history

So why the first galaxies?

1.  What is the typical mass of the first galaxies ? 2.  What is the typical mass of the first stars ? 3.  How feedback processes depend on the properties of

the first cosmic objects ? 4.  How feedback processes influence the properties of the

subsequent generations of stars and galaxies ?

Questions

The first virializing haloes

Mo & White 2002

Sigma density perturbations

Abundance of DM haloes

99% 95%

68%

Haloes ! 3-" are extremely rare

Physical properties

Dark matter halo with total mass M that virialize at redshift z and has a gas mass content Mg = #b/#m M

where :

Barkana&Loeb2001

How massive were the first galaxies?

This is a necessary but not sufficient condition for the collapse.

We need to cool down the gas very rapidly

The Jeans mass after recombination

Fundamental timescales

Cooling : Free-fall :

Hubble :

tc > tH no cooling tH >tc > tff quasi-static cooling tc < tff rapid cooling ! free fall collapse

The cooling function

Barkana&Loeb2001

H2

H

nH = 0.045 cm-3

nH2 = 0.1% nH

Primordial composition gas bremsstrahlung

Lya-cooling Roto-vibrational transition

H2-cooling mini-haloes

Molecular hydrogen formation

main formation channel:

main destruction channel:

The presence of electrons is crucial in order to produce H2 After recombination ye $ 3 %10-4 from which we get fH2 $ 2%10-6

This is too low to allow the collapse

H2 enhancement during virialization

Tegmark+97

z = 100 z = 50 z = 25

When is the cooling efficient?

Tegmark+97 TCMB > Tvir

tcool > tH

tcool < tff

How massive were the first galaxies?

Tegmark+97 TCMB > Tvir

tcool > tH

tcool < tff

3!

The first galaxies are expected to be associated with H2-cooling minihaloes,

that have total masses M ! 106M⊙ virial temperatures Tvir ~ 2000K,

and that assembled at redshift z ! 25–30. These objects correspond to 3" density fluctuations

of the density field.

How massive were the first galaxies?

How massive were the first stars?

The primordial gas cool down and reaches a preferential state that only depends on the H2 microphysics Tc ~ 200K nc ~ 104 cm-3

The corresponding Jeans mass is MJ ~ 700 M"

The energy deposited by gravitational contraction cannot balance the radiative losses: T decreases with increasing &.

The cloud cools and then fragments.

RF " #J " cs tff " T 1/2tff

Proto-stellar gas cloud with tcool << tff

How massive were the first stars?

In absence of turbulence/magnetic field no further fragmentation is expected and most of the gas accrete in few million of years.

Since feedback processes become important only once m* " 20 M⊙

the first stars are expected to be very massive m* " (30-300) M⊙

The accretion rate onto the proto-stellar gas cloud is very efficient because of the high temperature of the gas

dM/dt " cs3/G " T3/2

Evolution

The lack of metals do not allow the CNO cycle to start The gravitational collapse is counterbalanced only by the p-p chain

'p-p $ XH &T4

'CNO $ XH XC &T17

Metal-free stars are hotter and have harder spectra: they emit more ionizing radiation (hv >13.6 eV)

per unit stellar mass of metal-enriched stars.

As a result of low nuclear energy generation the stars reach high core temperatures Tc $ 108 K and start the 3-( reactions.

Spectrum

Tumlinson&Shull2000

Final fate

Heger&Woosley2002

Predicted mass range

m* " (30-300) M⊙

Pair Instability SN m* " (140-260) M⊙

No

rem

nant

s: c

ompl

ete

dist

rupt

ion

Zero Metallicity Non rotating stars

Products of the first stars

If the first stars are very massive pair instability supernovae (PISN) represent the major contributors to the metal (dust)

enrichment and the energy deposition in the early Universe

EPISN ~ 2.7 %1052erg

YZ ~ 0.45 YFe ~ 0.022

Mdust ~ 0.15-0.3 mPISN

ESNII ~ 1.2 %1051erg YZ ~ 0.01

YFe = 5 %10)4

Mdust ~ 0.003-0.03 m*

Average PISN : m* =200M⊙ Integrated contribution : m*<100M⊙

Odd-even effect : distinctive signature of primordial stars

Predicted chemical abundances

Heger&Woosley2002

SNII only

SNII +PISN

Feedback processes

Back reaction of a process on itself or on the causes that have produced it. The system may become self-regulated

@ z ~ 25 M ~ 106 M" m* >30 M"

The efficiency of feedback processes depends on the properties of the first stars and of the first galaxies and affects the subsequent generations of stars and galaxies.

1? Are the first galaxies able to form stars after the first SF event? 2? What are the properties of the second generations of stars? 3? What are the properties of the second generations of galaxies?

Feedback processes

Radiative feedback: ionization/dissociation of hydrogen atoms and molecules

Mechanical feedback: mechanical energy injection of massive stars in form of winds or SN explosions

Chemical feedback: metal/dust enrichment of the ISM/IGM driving the transition from massive to normal stars

Radiative feedback on minihaloes

Photo-dissociation: Lyman-Werner photons (11.2-13.6 eV) can dissociate H2 molecules. Two steps (Salomon) process

Photo-evaporation: the gas can be heated by UV radiation above Tvir and ejected out of the gravitational potential.

Complications: Lyman-Werner flux? Self-shielding? Positive feedback in recombined ionized regions or behind SN shocks?

Implications for minihaloes

The star-formation efficiency is strongly reduced in minihaloes of decreasing mass/temperature $ T3

vir

Because of the gradual build-up of a Lyman-Wernar background the star formation is quenched in progressively more massive objects with M < Mth

Machecek+2001

Reionization

time

redshift

Isolated HII regions

Overlapping stage

At zrei the reionization is complete

zrei $ 6 from QSOs absorption spectra

zrei $ 11 from CMB Thompson optical depth

Courtesy of A. Maselli

Implications for galaxy formation

The heating associated with photoionization raises the IGM temperature thus increasing the Jeans mass in progressively ionized cosmic regions.

The “filtering mass” below which the gas infall is progressively quenched depends on the entire reionization history.

When reionization is complete only the more massive haloes with vc ! 20-30km/s (Tvir > 104K) are able to form stars

Minimum halo mass for SF reionization

Increasing LW background

Minimum absolute

mass

Chemical feedback

The presence of metals and dust drives the transition from a “primordial” star formation mode (massive stars) to the normal one we observe today in the local Universe.

"  Z < 10-4 Z! "  m*=[30-300] M! "  IMF?

"  Z = [10–4–1] Z! "  m*=[0.1-100]M! "  IMF : Salpeter

Local observations

Z ! Zcr Z > Zcr

Theoretical studies

PopIII stars PopII/I stars

Zcr=10 –5±1Z!

Chemical feedback

Schneider+2006

Chemical feedback

fdep " Mdust/MZ

Courtesy of R.Schneider

Mechanical feedback

The energy deposition associated with the first SN explosions may induce partial or total gas removal from the galaxy itself

The ejection efficiency depends on the halo binding energy (Eb) and on the kinetic energy released during the explosion (Ekin)

Eb = GM2/2rvir $ 2.3%1053 erg (M/108M")5/3[(1 + z)/10] Esn = *w NSN <ESN >

The star formation is reduced in low mass objects The IGM out of which haloes form is gradually metal enriched

Mechanical feedback M = 108M"#

@ z $ 9

multiple SN explosions

Mori, Madau, Ferrara 2002

48 kpc 12 kpc 3 kpc

~ 5 Myr

~ 8 Myr

~ 20 Myr

~ 35 Myr

~ 175 Myr

Mechanical feedback M = 1011M"

Stars log[&*cm3]

Gas log[&gcm3]

Metals [O/H]

Mori&Umemura2006

Implications

Radiative feedback influences the mass of the subsequent “generation of galaxies” by increasing the minimum halo mass able to form stars

Chemical feedback influences the mass of the second generation stars by driving the transition from massive PopIII stars to normal PopII/I stars.

Mechanical feedback influences both the mass of subsequent generation of galaxies and stars by removing the gas in low mass haloes and by enriching the IGM out of which more massive haloes virialize.