1. GRB 090423: high-z GRBs as a new tool to study the reionization epoch
by Ruben Salvaterra
Universita’ dell’Insubria - Como

2. most distant object ever observed !
GRB : TNG spectrum
Tanvir et al. 2009
Telescopio
Nazionale
Galileo
~14hrs after trigger
obs=1216 A (1+z) z=
most distant object ever observed !

3. GRB : a typical long GRB
Amati relation
Typical long GRB with a canonical afterglow light-curve, likely the result of the collapse of a PopII star similar to low-z bursts

4. GRB 090423: a rare event? + evolution
Basic assumptions: 1) GRBs follow the cosmic SFR
2) GRB LF is constant
+ evolution
model free parameters are obtained by fitting the BATSE logN-logP
BATSE
Swift

5. GRB : need of evolution
No evolution models fail to reproduce the number of high-z (e.g. z>2.5) GRBs and to account for the detection of GRB : luminosity or density evolution is needed
GRB
lum evo.
dens evo.
no evo.

6. High-z GRBs: expected numbers
density evo.
The high-z population:
GRB z=6.3
GRB z=6.7
GRB z=8.2
z>6
1-7% of all Swift GRBs at z>6 assuming Plim=0.4 ph/s/cm2
Next generation of satellites (e.g. EXIST, XENIA, SVOM, JANUS) will rapidly increase the high-z sample. In particular EXIST will be able to detect tens of GRBs/yr at z>6 allowing a systematic study of the high-z GRB population

7. High-z GRBs: host galaxies
Cosmological simulaton with GADGET:
L=10 Mpc/h, Np=2563, Mp= 2x106Msun
with chemical feedback  PopIII-PopII trans.
radiative feedback
mechanical feedback (SNyy and SNII)
HST/WFC3 dropout LF
simulated JWST H-band sky
The simulation nicely reproduces the recent HST/WFC3 data without the need of any fine-tuning

8. High-z GRBs: host galaxies
high SSFR
bursty
observed/low-z
simulated/high-z
low-Z
environments

9. Mass-metallicity relation at high-z
050904
Berger et al. 2007
z=6
z=8
090423
Tanvir’s talk

10. High-z galaxies: reionization
Only a small fraction (<20% at z>6) of the ionizing photons come from bright galaxies that we resolve in current deep field surveys.
GRBs can be the signpost of those faint galaxies that provide the bulk of UV photons that reionized the Universe
JWST will resolve the bulk of the sources of ionizing photons up to z=7.3

11. Constraining the reionization history
zreion~ 7
zreion~ 6
GRB
(Kawai et al. 2006)
zreion~ 7
zreion~ 6
The dimension of the largest dark gap in the afterglow spectrum between Lya and Lyb can help to constrain the reionization history
60%
45%
30%
15%
40<Wmax<80 A
5 d
3 d
1 d
0.3 d
0.2 d
0.1 d
GRB Wmax=63 A
80<Wmax<120 A

12. Conclusions
The detection of GRB has shown that GRBs do form (and are detectable!!!) at very high-z.
High-z GRBs represent a fundamental tool for the study of the reionization epoch, e.g.:
signposts of faint galaxies that contributed mostly to the reionization process
constrain the M-Z relation at very high-z
constrain the reionization history ….