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									      First Stars:
The Cosmological Context



      Benedetta Ciardi
           MPA
               Timeline in cosmic history
 Years since              Big Bang: the Universe is filled with hot plasma
the Big Bang
                          The gas cools and becomes neutral: recombination
 ~350000
 (z~1000)
                                   The Dark Ages

~100 million              The first structures begin to form:
 (z~20-30)                reionization starts




 ~1 billion               Reionization is complete
  (z~6)




 ~13 billion
   (z=0)                  Today’s structures
               Timeline in cosmic history
 Years since                     Cosmic Microwave Background
                          Big Bang: the Universe is filled with hot plasma
the Big Bang
                          The gas cools and becomes neutral: recombination
 ~350000
 (z~1000)
                                   The Dark Ages

~100 million              The first structures begin to form:
 (z~20-30)                reionization starts
                                          FIR/Radio?




 ~1 billion               Reionization is complete
  (z~6)



                                          UV/Optical/IR



 ~13 billion
   (z=0)                  Today’s structures
Cosmological model




            M. Tegmark Webpage
Cosmological model




            M. Tegmark Webpage
Cosmological model




            M. Tegmark Webpage


                                 (Bennett et al 2003)
                  The formation sites of the first stars
                  DM halos virial temperature




                                            (Barkana & Loeb 2001)
                              3-σ
Temperature [K]




                             2-σ                                    The gas needs to cool to be
                                                                    available for star formation
                       1-σ




                               Redshift
                     The formation sites of the first stars
                     DM halos virial temperature                                                  Cooling function (no metals!)




                                               (Barkana & Loeb 2001)
                                  3-σ
Temperature [K]




                                                                               Λ/n² [erg cm³/s]
                                 2-σ                                                    The gas needs to cool to be
                                                                                                             Atomic cooling

                                                                                        available for star formation
                           1-σ


                                                                                                  HD cooling
                                                                                                                     H2 cooling




                                   Redshift                                                               Temperature [K]


                  Tvir > 10 4 K                                        Atomic hydrogen cooling

                  Tvir < 10 4 K                                        Molecular hydrogen cooling
                     The formation sites of the first stars
                     DM halos virial temperature                                                       Cooling function (no metals!)




                                                    (Barkana & Loeb 2001)
                                  3-σ
Temperature [K]




                                                                                    Λ/n² [erg cm³/s]
                                 2-σ                                                         The gas needs to cool to be
                                                                                                                  Atomic cooling

                                                                                             available for star formation
                           1-σ          H2 is a key species in the early universe
                                                                                                       HD cooling
                                                                                                                          H2 cooling




                                   Redshift                                                                    Temperature [K]


                  Tvir > 10 4 K                                             Atomic hydrogen cooling

                  Tvir < 10 4 K                                             Molecular hydrogen cooling
                      Feedback effects

Radiative feedback

Mechanical feedback

Chemical feedback

               The first generation of stars affects
               the subsequent star formation process
                      Feedback effects
                              H2 photodissociation
Radiative feedback
                           (L-W photons, 11.2-13.6 eV)

Mechanical feedback

Chemical feedback
                      Feedback effects
                                 H2 photodissociation
Radiative feedback
                              (L-W photons, 11.2-13.6 eV)

Mechanical feedback
                                     Photoheating
Chemical feedback

               The first generation of stars affects
               the subsequent star formation process




                                                            (Gnedin 2000)
                      Feedback effects
                                 H2 photodissociation
Radiative feedback
                              (L-W photons, 11.2-13.6 eV)

Mechanical feedback
                                     Photoheating
Chemical feedback

               The first generation of stars affects
               the subsequent star formation process
                                   Photoevaporation




                                                                            (Shapiro et al. 2003)
                                                            (Gnedin 2000)
                      Feedback effects
                                       Blowout/blowaway
Radiative feedback
                                                          Blow-out
                           Blow-away
Mechanical feedback




                                                                     (Mac Low & Ferrara 99)
Chemical feedback
                      Feedback effects

Radiative feedback
                                                                       Change in fragmentation mode due to Z
Mechanical feedback                                                                                  (Bromm et al, Schneider et al)




Chemical feedback




                        Fraction of metals depleted onto dust grains
                                                                                                                                            ~ 100 Msun


                                                                                                                                            ~ 1 Msun


                                                                                                                                            ~ 0.1 Msun




                                                                                                                        (Schneider et al)
                                                                               Metallicity / Solar Metallicity
                           Feedback effects

 Radiative feedback

 Mechanical feedback

 Chemical feedback

 IGM reionization

 IGM metal enrichment




See Ciardi & Ferrara 2005 for a review
             IGM reionization
Spectra of high-z QSOs




                         (Fan)
             IGM reionization
Spectra of high-z QSOs
                                 Ly-alpha optical depth




                                                      (White et al. 2003)
                         (Fan)
             IGM reionization
Spectra of high-z QSOs
                                            Ly-alpha optical depth




        The IGM is highly ionized at z<6.
      The abundance of neutral hydrogen
      increases with increasing redshift.




                                                                 (White et al. 2003)
                         (Fan)
                                       IGM reionization

                            H0 redshift evolution




                                                    UV radiation from the
                                                    first stars drives the
(BC, Stoehr & White 2003)




                                                       IGM reionization
              IGM metal enrichment


Metal Abundance at z=3                    - Metals are observed down to very
                                            low densities in the IGM




                         (Pettini 2000)
                                            far away from their production sites
                                          - Metals are relatively cold
                                            they had time to cool down




                                          Existence of an early population
                                           of stars that polluted the IGM
            First stars: what do they look like?

First generation of stars formed out of metal free gas:

-   fragmentation scale
-   accretion efficiency
-   emission properties
-   mass loss efficiency

are different from those of metal enriched stars



                    Which is the mass/IMF of the first stars?
            First stars: what do they look like?

First generation of stars formed out of metal free gas:
                                       SNII        PISN
-   fragmentation scale
-   accretion efficiency
-   emission properties
-   mass loss efficiency

are different from those of metal enriched stars



                    Which loss mass/IMF free first stars?
               Is the massis thefrom metalof thestars negligible?




                                                             (Heger & Woosley 2002)
          First stars: what do they look like?

First generation of stars formed out of metal free gas:
                                       SNII       PISN
- fragmentation scale
- accretion efficiency
- emission properties
- mass loss efficiency
                The IMF of the first stars is crucial to determine
are different from those of metal enriched stars
                  the properties of the star formation process,
                   the IGM reionization and metal enrichment.

                  Which loss mass/IMF free first stars?
             Is the massis thefrom metalof thestars negligible?




                                                                (Heger & Woosley 2002)
          The mass of the first stars

  (Bromm et al; Abel et al;
    Nakamura&Umemura)




                                    (Omukai&Nishi;
                                    Ripamonti et al)




                                       (Omukai&Palla;
                                   Tan&McKee; Bromm et al)


 (Nakamura&Umemura;
Omuka; Uehara&Inutsuka)




                                        Ciardi & Ferrara 2005
          The mass of the first stars

  (Bromm et al; Abel et al;
    Nakamura&Umemura)




                                              (Omukai&Nishi;
                                              Ripamonti et al)




 How does accretion work for metal free stars?
                                                 (Omukai&Palla;
                                             Tan&McKee; Bromm et al)


 (Nakamura&Umemura;
Omuka; Uehara&Inutsuka)




                                                 Ciardi & Ferrara 2005
   Reionization of the IGM

                  CMB power-spectrum




                                                 (Hu webpage)
Thomson scattering optical depth:   τ (z) = ∫ ... n e (z') dz'
   Reionization of the IGM

        WMAP TE Cross-Power Spectrum
              CMB power-spectrum




                                                 (Hu webpage)
                                                       (Kogut et al. 2003)


Thomson scattering optical depth:   τ (z) = ∫ ... n e (z') dz'
      Reionization of the IGM

           WMAP TE Cross-Power Spectrum
                 CMB power-spectrum




                                                    (Hu webpage)
More electrons than previously thought have been
   produced during the reionization process
contribution from very massive metal-free stars?




                                                          (Kogut et al. 2003)


   Thomson scattering optical depth:   τ (z) = ∫ ... n e (z') dz'
          Reionization of the IGM
Mildly top-heavy IMF: M < 70 M⊕




                                                          τ e = 0.16 ± 0.04
                                                                         68% CL (Kogut et al. 2003)




                         (BC, Ferrara & White 2003)
                                                       We don’t need very massive
                                                      metal-free stars to reproduce
                                                           the WMAP results
                                                             (see also Schneider et al 2005)
                                   NIRBackground
- Excess emission at 1.2-4 μm
          (e.g. Wright 2001; Matsumoto et al 2004; Santos et al 2003; Salvaterra & Ferrara 2003)


- Fluctuations of the background in the same range
          (e.g. Kashlinsky et al 2002, 2005; Magliocchetti et al 2003)


            cannot to be accounted for by normal galaxies




                             Primordial, very massive stars
                                   NIRBackground
- Excess emission at 1.2-4 μm
          (e.g. Wright 2001; Matsumoto et al 2004; Santos et al 2003; Salvaterra & Ferrara 2003)


- Fluctuations of the background in the same range
          (e.g. Kashlinsky et al 2002, 2005; Magliocchetti et al 2003)            (Salvaterra & Ferrara 2003)


            cannot to be accounted for by normal galaxies
                                                        PopIII stars+gals.




                                                                                  Unknown excess




                             Primordial, very massive stars
                                                                    Galaxies
                                       NIRBackground
   - Excess emission at 1.2-4 μm
              (e.g. Wright 2001; Matsumoto et al 2004; Santos et al 2003; Salvaterra & Ferrara 2003)


   - Fluctuations of the background in the same range
              (e.g. Kashlinsky et al 2002, 2005; Magliocchetti et al 2003)


                cannot to be accounted for by normal galaxies




                                 Primordial, very massive stars



BUT:   i) too demanding in terms of primordial star formation efficiency (Madau & Silk ‘05)
       ii) they would have been observed in HST-UDF (Salvaterra & Ferrara ‘05)
                                       NIRBackground
   - Excess emission at 1.2-4 μm
              (e.g. Wright 2001; Matsumoto et al 2004; Santos et al 2003; Salvaterra & Ferrara 2003)


   - Fluctuations of the background in the same range
              (e.g. Kashlinsky et al 2002, 2005; Magliocchetti et al 2003)


                cannot to be accounted for by normal galaxies
               Do we really need very massive primordial stars?
              Is there any strong evidence for their existence?



                                 Primordial, very massive stars



BUT:   i) too demanding in terms of primordial star formation efficiency (Madau & Silk ‘05)
       ii) they would have been observed in HST-UDF (Salvaterra & Ferrara ‘05)
                                       NIRBackground
   - Excess emission at 1.2-4 μm
              (e.g. Wright 2001; Matsumoto et al 2004; Santos et al 2003; Salvaterra & Ferrara 2003)


   - Fluctuations of the background in the same range
              (e.g. Kashlinsky et al 2002, 2005; Magliocchetti et al 2003)
               Do we really need very massive primordial stars?
                cannot to be accounted for by normal galaxies
               Is there any strong evidence for their existence?
               Do we really need very massive primordial stars?
               Is the mass strong evidence for stars negligible?
              Is there any loss from metal freetheir existence?
                How does accretion work for metal free stars?
                          Which is the IMF of primordial stars?
                                 Primordial, very massive stars



BUT:   i) too demanding in terms of primordial star formation efficiency (Madau & Silk ‘05)
       ii) they would have been observed in HST-UDF (Salvaterra & Ferrara ‘05)

								
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