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First Stars, Quasars and Reionization Observations Presented by: Simon Strasser & Jon Vermedahl Observational Probes of Reionization Epoch • Population III stars – [Fe/H], ionizing photons • Quasars – Ly forest ionization • CMB – Late integrated Sachs-Wolfe effect • What do observations tell us about these? Population III Stars • Stellar populations: – Pop I: High metal content ([Fe/H]), mainly found in disks of spiral galaxies – Pop II: [Fe/H] poor stars, spiral galaxy halos and elliptical galaxies • But, we have not detected any stars with zero metallicity to date… Pop III stars The Observed [Fe/H] Distribution Milky Way Galaxy: • No clear drop in outer Galaxy • Lower limit in [Fe/H] of about -3 … no evidence for low mass Pop III stars … • To form massive stars we need a high DM halo velocity dispersion – expect a strong metallicity to dispersion correlation as massive stars enrich the surrounding medium the most – Stars with lowest [Fe/H] NOT expected to form at highest redshifts but low dispersion halos at z < 9. – Our stellar age models are not good enough to see this [Fe/H] to age dependence yet… – Would provide a strong test for galaxy formation by hierarchical clustering • Why have we not seen Pop III stars? – Only high mass stars formed? • Current initial mass functions (IMF) yield a large number of low mass stars for every high mass star… Quasars • Some of the earliest objects formed • Important probe of the reionization epoch – Hubble Deep Field • No evidence for expected number of faint unresolved sources Quasar 3C175, z = 0.77 Gunn-Peterson Trough • Gunn-Peterson Effect Neutral Hydrogen (HI) clouds absorb radiation shortward of Ly • The strength of the absorption can tell us about the amount of HI present • Becker et al (2001) find strong evidence for a large increase in HI optical depth at z ~ 6 Cosmic Microwave Background • Late integrated Sachs-Wolfe Effect – Redistribution of CMB power into quadrupole moment due to photons interactions with free electrons (Thomson Scattering) – Correlation of polarization and temperature – Provides a handle on redshift and efficieny of reionization • Wilkinson Microwave Anisotropy Probe (WMAP) gives some very interesting results WMAP Polarization cross-power spectrum: • We can see a very clear signature of reionization in the lowest multipoles • Consistent with an early epoch of reionizaton • The peak can be modelled with a Thomson optical depth, = 0.17 • This is much higher than what we can account for with a single reionization event WMAP Results continued • Complicates the interpretation of GunnPeterson trough around z = 6 • Modelling indicates that we have to invoke either – top-heavy IMF or – Low metal enrichment by Pop III stars, so we can prolong the reionization era Conclusions • What do we know? – Ionization epoch lasted from z ~ 20 to 3 – Pop III stars and Quasars both contributed z 20 - 30 >6 3 Events Reionization epoch begins (first stars form) • Gunn-Peterson trough becomes very strong • Detection of earliest (?) quasars He II ionization Many Open Questions… • Relative contribution of ionizing photons from Pop III stars / Quasars? • How did the multi-step process unfold exactly? • What is the IMF for the first stars? • How can the Ly forest results be interpreted? Future Work • Planck, WMAP, NGST – In recent years a much clearer picture has emerged on the observational side – Much more to come from these missions – especially NGST Probing the Edge of the Dark Ages • WMAP/COBE: Limit of Observations at z ~ 1200 • NGST: will reach z > 10 • SIRTF, z ~ 6 • SKA, ISM to z ~ 6 • Current instruments: − Hubble Space telescope, shy of reionization epoch. − VLA, z ~ 3 − Sloan and 2df, z ~ 6 NGST Expected IR Source Counts from SIRTF and NGST