Transcript First Stars, Quasars and Reionization Observations

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