STScI, Feb. 27, 2004
Jan 21, 2016
STScI, Feb. 27, 2004
Exploring Early Structure Formation Exploring Early Structure Formation with a Very Large Space Telecopewith a Very Large Space Telecope
Avi Loeb Avi Loeb Harvard UniversityHarvard University
Cosmic-Archeology : the First Stars and The Reionization on the Universe
The more distant a source is, the more time it takes for its light to reach us. Hence the light must have been emitted when the universe was younger. By looking at distant sources we can trace the history of the universe.
distanceEarth
First Year Data from WMAP
Polarization/temperature correlation implies an electron-scattering optical depth after cosmological recombination at z=1088 of:
ü= 0:17æ0:04
Implying that the universe was reionized at z = 17æ5Only 200 million years after the big bang
Kogut et al. 2003
Cosmic Hydrogen was significantly Neutral at z~6.3Cosmic Hydrogen was significantly Neutral at z~6.3
Wyithe & Loeb, Nature, 2004; astro-ph/0401188
Size of HII region depends on neutral fraction of IGM prior to quasar activity and quasar age
Ionization(Stromgren) sphere of quasar
line of sight
R(t)
Likelihood for Neutral Fraction at z~6.3Likelihood for Neutral Fraction at z~6.3
quasar lifetime= 4â 107f lt 0:1ïà á
yearsM 5=3
1 +M 5=32
(M 1+M 2)5=3
xHI
SDSS J1148+5251
But on small scales the universe is clumpy
Mean Density
Early times
Intermediate times
Late times
Ionization History of Hydrogen
REDSHIFT
TIME
10006
Million years
Billion years
Emergence of the First Star Clusters
molecular hydrogen
Yoshida et al. 2003
Astronomy of the Early Universe
• When did the first stars and quasars form?• When was the universe re-ionized?• How quickly was the intergalactic medium enriched
with metals from the first stars?
Most pressing questions:
Metal-Free Stars were massive: Simulations
Bromm, Coppi, & Larson 2000
~1 kpc ~ 2500 A.U.
Abel, Bryan, & Norman 2000
H2 à cooling
Spectrum of Pop-III Stars
---> unusually strong recombination lines (H_alpha)
Bromm, Kudritzki, & Loeb 2001
Formation of Massive Black Holes in the First Galaxies
Add Bromm
Low-spin systems: Eisenstein & Loeb 1995
Numerical simulations: Bromm & Loeb 2002
R < 1pc
M 1 ø 2:2â 106M ì
M 2 ø 3:1â 106M ì
õ = 0:05
H2 suppressed
Supermassive StarsSupermassive Stars
Teff ø 105K
L = L E / M
For a spherical (non-rotating) star:general-relativistic instability at
(Rc2GM)crit = 0:6295( M
M ì )1=2
Angular momentum mass shedding along equator
Collapse to a black hole is inevitable for M > 300M ì
Why a Very Large Space Telescope?Why a Very Large Space Telescope?
• Probe rest-frame UV emission at z=6-10. NIR capability is essential:
• Identify the galaxies that dominate the ionizing background during reionization
• Use UV spectra to infer the dominant stellar population during reionization (and gauge the role of massive, metal-poor, Pop-III stars)
• Find faint quasars and reveal the emergence of quasar seeds
õë = 0:85ömâ [(1+ z)=7]
Number of ionizing photons (>13.6eV) per baryon incorporated into stars:
Massive, metal free stars
Salpeter mass function
Bromm, Kudritzki, & Loeb 2001, ApJ, 552, 464
Teff ø 105KL = L E / M
M > 300M ì
M > 100M ì
ø 100;000
ø 40;000
Metal free
Z = 0:01Zì
ø 7;000
ø 3;500
Gain by up to a factor of ~30!Gain by up to a factor of ~30!
Cooling Rate of Primordial Gas
n=0.045 cm^-3
Atomic cooling
H_2 cooling
Virial Temperature of Halos
1-sigma 2-sigma
3-sigma
Atomic cooling
H_2 cooling
Reviews on the physics of reionization: *Barkana & Loeb 2001, Physics Reports 349, 125*Loeb & Barkana 2001, Ann.Rev.Ast.&Ap, 39, 19
1-sigma
2-sigma
3-sigma
Atomic cooling
H_2 cooling
2-sigma
Collapse Redshift of Halos
Why should VLST have High Resolution?Why should VLST have High Resolution?
Virial Radius of Halos
1-sigma 2-sigma 3-sigma
Why should VLST have a Wide Field?Why should VLST have a Wide Field?
Unusually Large Fluctuations in the Statistics of Galaxy Formation at High Redshifts
Barkana & Loeb 2003 (astro-ph/0310338)
Assumption : î L = 0
L
Bias in numerical simulations:
Exploring the Time Domain with VLST:Exploring the Time Domain with VLST: The High-Redshift Universe Can be Best The High-Redshift Universe Can be Best
Probed Through Transient EventsProbed Through Transient Events
• Gamma-Ray Burst afterglows [days x (1+z)]
• Supernovae [weeks x (1+z)]
• Quasars [Myr x (1+z)]
Binding Energy of Dark Matter Halos
1-sigma 2-sigma
3-sigma
Supernova
Self-Regulated Star Formation
Virial temperature < 10,000K
destruction by starlight (10.2<E<13.6 eV)H2
Evaporation of gas cloud by photo-ionization heating to >10,000K
As observed for local dwarf galaxies
in SDSS (Kaufmann et al. 2002)
(M ? < 3â 1010M ì )
Virial temperature > 10,000K
M ? / E SN ø 23M gasû2
f ? = M gas
M ? / û2 / M 2=3halo
(Dekel & Silk)
Bromm, Yoshida, & Hernquist 2003
SPH Simulation of a Hypernova Explosion
1kpc
E SN = 1053ergs;E vir = 1051ergs
Pair-Instability Supernova
Minimum Carbon and Oxygen Abundance Required for the Formation of Low-Mass Stars
Lines: Cooling rate by C II or O I allows fragmentation in metal-poor gas clumps
Filled points: halo dwarf stars
Open squares: giant stars
Bromm & Loeb 2003, Nature, in press
n ø 104 cmà 3
T ø 100à 200 KM J ø 100M ì
Self-regulation of Supermassive Black Hole Growth
quasar
! 108M ì
M bh = 1:5 200km=sû
ð ñ5
Ltdyn ø 23M gasû2
dynamical time of galactic disk maxf Lg = L E / M bh
halo velocity dispersion
After translating û ! û? this relation matches the observed correlation in nearby galaxies (Tremaine et al. 2002; Ferarrese & Merritt 2002)
M à ûã
Silk &Rees 1998; Wyithe & Loeb 2003
Quasar Luminosity Function
Wyithe & Loeb 2002
Simple physical model:
*Each galaxy merger leads to a bright quasar phase during which the black hole grows to a mass and shines at the Eddington limit. The duration of this bright phase is proportional to the (smaller than unity) mass ratio in the merger.
*Merger rate: based on the extended Press-Schechter model in a LCDM cosmology.
duty cycle ~10 Myr
M ï / v5c
The Earliest Quasar Detected: z=6.43
Fan et al. 2002
Gamma-Ray Bursts: Probing one Star at a TimeGamma-Ray Bursts: Probing one Star at a Time Progenitors
NS NS
Collapse of a Massive Star (possibly leading to a supernova)
÷÷ö ! e+eà
Coalescence of NS/NS or NS/BH Binaries
4ù1051ergs = 2â ù(ò=5î )2
1054ergsIn both cases the original progenitors are massive stars (which should exist at high redshifts)
Detectability of Afterglow Emission Near the Lya WavelengthPhotometric redshift identification: based on the Lya trough
z=15
z=5z=11 z=7z=9
JWST sensitivity
Barkana & Loeb 2003
astro-ph/0305470
Weak contamination by Lya emission line
Gamma-Ray Burst Afterglows Quasars
GRB
Infall onto Massive Host Galaxy
Ionized Gas
M ionized ø 4â 104M ì â 1051ergsE
ð ñ
flux
wavelength
Lyman-alpha Damping Wing
1216A â (1+ z)
Quasars get fainter with increasing redshift because galaxies are less massive and the luminosity distance increases at higher redshifts!
Advantages of GRB Afterglows Relative to Quasars Advantages of GRB Afterglows Relative to Quasars (see details in astro-ph/0307231)(see details in astro-ph/0307231)
• Luminous stellar sources high redshifts– outshine galaxies and quasars, as they become less luminous at increasingly higher
redshifts– -ray trigger allows all-sky monitoring
• Featureless broad-band spectra extending into the rest-frame UV– absorption features can be used to probe the ionization and metallicity states of the IGM
• Observed flux at a fixed observed age does not fade significantly with increasing GRB redshift (cosmological time stretching counteracts increase in
luminosity distance) • Weak Feedback on Surrounding IGM - Short-lived event: negligible ionizing effect of a GRB on the IGM
-Low mass host galaxies: weak IGM infall & Lya line emission
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