First Stars, QSO’s and Reionization in our Universe

1. First Stars, QSO’s and Reionization in our Universe By Simon Strasser & Jon Vermedahl 22 April 2003

2. Outline • Ionization • Halo formation • First Stars • Quasi-Stellar Objects • Observational aspects of the first stars, QSO’s, and CMB. • Conclusions • Future Work

3. Reionization(End of the Dark Ages) • Period of time when the universe changes from atomic/molecular to ionized gas. • Benefit = Universe is transparent to light. • Mass fraction of baryons to ionize universe = ~10-5 • Hydrogen: Reionized at z ~10. • Helium: Reionized at z ~3. • Reionization heats up the universe and slows down star formation.

4. Reionization • Oldest stars formed in halos at z ~ 20 - 30. - corresponds to 3-s peaks in density fields. • Reionization occurs when the photons emitted in one recombination time equal the number of H-atoms.

5. Formation of Halos • Large-scale structure starts with collapse of low-mass halos, and grow via mergers. • Because of reionization, galaxies could only form in halos with velocity dispersion greater than 30 km/s. • Consequently, most of the gas is in diffuse form until z < 2. • Confirmed in numerical simulations.

7. Physical Characteristics of First Stars • No metals. - ≥15M may build up some C nuclei prior to onset of MS burning. • Proton-Proton burning very inefficient. • Exhibit high effective temperatures and smaller radii. • Zero-Age MS on HR Diagram of a Pop III star and Pop I star. - diamonds reflect changes in metallicity.

8. Physical Characteristics of First Stars (Pop III) • Lifetimes of zero-metallicity are similar in length to metal-enriched stars (Pop I). • Stars cool and become more luminous, similar to Pop I stars. • Ionizing radiation has ~50% more ionizing radiation in Pop III than Pop I. • Pop III stars can be identified via their spectra Teff = 36,000 K (Pop II) Teff = 63,000 K (Pop III)

9. Unknowns of the first stars • Will the first stars be massive? -Jeans mass of clumps in halos ~103M • Will many stars be formed by rapid fragmentation? • What differences in the star formation process will be caused by the absence of heavy elements and magnetic fields? • If the first stars had the same IMF (Salpeter) as today, with a lower cutoff at about 0.1 M , one might expect stars smaller than 0.8 M to display arbitrarily low metallicity.

10. Where are the first stars? • Between z = 20 and z = 9, only ~400 million years pass. • Current ages of oldest stars accurate to ~109 years! • Prediction that oldest stars will be metal poor will be difficult to test. • Strong bias of objects on small scale – oldest stars should be more abundant in central parts of galaxies and clusters. • Bulge of Milky Way should contain largest abundance of Pop III stars (assumes bulge formed first).

11. Quasi-Stellar Objects • Farthest objects detected in our universe. • First detected via radio telescopes in 60’s. • Optical counter-parts appear as bright, starlike nuclei surrounded by faint halo. • ~105 more luminous than typical galaxies. • Z = 5.8

12. Unified theory of AGN • QSOs are essentially Active Galactic Nuclei. • Host galaxies have a mass of ≥1012M • Luminosity is powered by a super massive black hole (~ 108M)

13. Evolution of QSOs? • There were more brighter quasars at higher redshift.

14. Evolution of QSOs • Number density (comoving) of QSOs has remained ~constant (z < 2). • 1000 times as many QSOs with MB≥ -25.9 at z = 2 than at z = 0. • Indicates an evolutionary effect of QSOs! • Above z = 2, events can not be explained by a constant number of QSOs with changing luminosity. - Comoving number density decreases by a factor of ~5 between z = 2 to z = 4. - Verdict is still out!