Probing intergalactic radiation fields during cosmic reionization through gamma-ray absorption

0.17 MB·English

by  Susumu Inoue

Mon.Not.R.Astron.Soc.000,1–??(2009) Printed25January2010 (MNLATEXstylefilev1.4)
Probing intergalactic radiation fields during cosmic
⋆
reionization through gamma-ray absorption
†
Susumu Inoue1 , Ruben Salvaterra2, Tirthankar Roy Choudhury3,
Andrea Ferrara4, Benedetta Ciardi5 and Raffaella Schneider6
0
1 1Department of Physics, KyotoUniversity, Oiwake-cho, Kitashirakawa, Sakyo-ku, Kyoto606-8502, Japan
0 2INAF, Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate, Italy
3Harish-Chandra Research Institute, Chhatnag Road, Jhunsi, Allahabad 211 019, India
2
4Scuola Normale Superiore, Piazza dei Cavalieri 7, 56126 Pisa, Italy

n 5Max-Planck-Institut fu¨rAstrophysik, K.-Schwarzschild-Str. 1, 85748 Garching, Germany
a 6Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, 50125 Firenze, Italy
J

5
2
Accepted ?.Received?;inoriginalform?

]
O
ABSTRACT
C
We discuss expectations for the absorption of high-energy gamma-rays by γγ pair
h. productionwithintergalacticradiationfields(IRFs)atveryhighredshifts(z ∼5−20),
p andtheprospectsthereofforprobingthecosmicreionizationera.FortheevolvingIRF,
- a semi-analytical model incorporating both Population II and Population III stars
o
is employed, which is consistent with a wide variety of existing high-z observations
tr including QSO spectral measurements, WMAP Thomson depth constraints, near-IR
s
source count limits, etc. We find that the UV IRF below the Lyman edge energy
a
[ with intensities in the range of a few times 10−19 erg cm−2s−1Hz−1sr−1 can cause
appreciable attenuation above ∼12 GeV at z ∼ 5, down to ∼ 6 − 8 GeV at z ∼>
2 8−10. This may be observable in the spectra of blazars or gamma-ray bursts by the
v FermiGamma-raySpaceTelescope,ornextgenerationfacilitiessuchastheCherenkov
5
TelescopeArray,AdvancedGamma-rayImagingSystemor5@5,providinginvaluable
9
insight into early star formation and cosmic reionization.
4
2
Key words: galaxies: high-redshift – intergalactic medium – cosmology: theory –
.
6 gamma-rays:bursts – galaxies: active
0
9
0
:
v 1 INTRODUCTION with clues to such processes in the early universe, cosmic
i
X reionizationalsoprofoundlyaffectstheensuingformationof
Some time after the epoch of cosmic recombination at red-
starsandgalaxies, soelucidatingthiseraisoneofthemost
r shift z ∼ 1100, the bulk of the intergalactic gas in the
a pressing issues in cosmology today (see Barkana & Loeb
universe must have been somehow reionized by z ∼ 6, as
2001; Ciardi & Ferrara 2005; Fan et al. 2006, Choudhury
indicated observationally from the spectra of high-z QSOs
2009 for reviews).
and the polarization of the cosmic microwave background
(CMB).However,thesources,historyandnatureofthiscos-
mic reionization process are still largely unknown, as most
In the majority of scenarios for reionization of hydro-
of this redshift range has yet to be explored through di-
gen in the intergalactic medium (IGM), the main protago-
rectobservations.Becausethefirststarsandgalaxiesinthe
nists are UV photons with energies above the Lyman edge
universe must have formed during this period, the primary
(ǫ ≥ ǫLE = 13.6 eV). Although those with lower energies
suspect is photoionization by UV radiation from such ob-
do not contribute to photoionization, they are also crucial
jects, potentially involving metal-free, Population (Pop) III
sincei)theygiveindicationsastothestrengthandnatureof
stars. Alternative possibilities include mini-quasars, super-
the ionizing radiation, ii) those in the Lyman-Werner band
novaremnantsanddarkmatterdecay.Besidesprovidingus
(ǫ=11.2−13.6 eV) can photodissociate H2 molecules and
suppress early star formation (e.g. Ciardi & Ferrara 2005),
and iii) Lyα photons (ǫ = 10.2 eV) can strongly affect the
⋆ Numerical data of the model results will be available at HIspin temperatureandtheassociated cosmological 21cm
http://www-tap.scphys.kyoto-u.ac.jp/˜inoue/hizabs/ signatures (e.g. Furlanetto et al. 2006). Thus, having some
† E-mail:[email protected] observationalmeanstoprobetheevolutionofUVintergalac-
(cid:13)c 2009RAS
2 S. Inoue, R. Salvaterra, T. R. Choudhury, A. Ferrara, B. Ciardi & R. Schneider
tic radiation fields (IRFs) ‡ in the cosmic reionization era as predicted by these models, the γγ opacity is evaluated
would be of paramount importance, complementing exist- for theredshift range z =5−20. We also briefly assess the
ing observations that probe the neutral or ionized gaseous detectability of the resultant absorption features in high-z
components of the IGM. However, direct detection of this sources such as blazars or gamma-ray bursts (GRBs) with
diffuseemissionfromveryhighz isextremelydifficultifnot currentandfuturegamma-rayfacilities,andtheconsequent
impossible. § implications.
Anindirectbutpowerfulmeansofprobingdiffuseradia-
tionfieldsisthroughphoton-photon(γγ)absorptionofhigh-
energygamma-rays(e.g.Gould&Schreder1967; Steckeret 2 INTERGALACTIC RADIATION FIELD
al. 1992). Gamma-rays with energy E emitted from extra- MODEL
galactic sources will be absorbed during intergalactic prop-
agation by interacting with photons of the diffuse radia- Thesalientfeaturesofoursemi-analyticalmodelsareasfol-
tion field with energy ǫ to produce electron-positron pairs lows (see Choudhury & Ferrara 2005; 2006; 2007; Choud-
(γ+γ →e++e−), as long as there is sufficient opacity for hury et al. 2008; Choudhury 2009 for more details): (1)
energies satisfying the threshold condition Eǫ(1−cosθ) ≥ Adopting a lognormal distribution of IGM inhomogeneities
2m2c4, where θ is the incidence angle of the two photons. (Miralda-Escud´eetal.2000),theionizationandthermalhis-
e
Theobservedspectraofthegamma-raysourcesshouldthen tories of the neutral, HII and HeII phases of the IGM are
exbihit corresponding attenuation features, from which one tracked simulaneously and self-consistently. (2) The forma-
can effectivelyinferorlimit thepropertiesof thediffusera- tion and evolution of dark matter halos are described by a
diation.ThismethodhasbeenutilizedinrecentTeVobser- Press-Schechter-based approach. (3) Three types of radia-
vationsof blazars byground-basedCherenkov telescopes to tionsourcesareconsidered:a)metal-free PopIIIstarswith
set important constraints on the extragalactic background a Salpeter initial mass function (IMF) in the mass range
light in the near infrared to optical bands at relatively low 1−100M⊙, with spectra according to Schaerer (2002) and
z (Aharonian et al. 2006; Albert et al. 2008). including nebular and Lyα emission lines (see Salvaterra &
As first discussed by Oh (2001; see also Rhoads 2001), Ferrara2003); b)low-metallicity (Z =0.02Z⊙)Pop IIstars
UV IRFs with sufficient intensities to cause IGM reioniza- with spectraaccording toBruzual &Charlot (2003), other-
tion are also likely to induce significant γγ absorption in wise being the same as Pop III; and c) QSOs with power-
gamma-ray sources at z > 6 at observed energies in the lawspectraandemissivitybasedontheobservedluminosity
∼
range of a few to tens of GeV. However, these estimates i) function at z < 6, considering only those above the break
were made before WMAP observations indicating an early luminosity (Choudhuryet al. 2008). (4) Pop II and Pop III
start of reionization and were limited to z ≤ 10, and ii) stars each form from gas in virialized halos with efficiencies
did not include the possibility of metal-free Pop III stars, ε∗,II and ε∗,III, respectively, and the corresponding escape
which may have been active during the first epochs of star fractions of ionizing photons from the host halos are pa-
formation and are more prodigious UV emitters compared rameterized by fesc,II and fesc,III. Included self-consistently
to normal stars ¶. The recent launch of the Fermi satellite are the consequent effects of radiative feedback that sup-
k with the Large Area Telescope (LAT) operating in the presses star formation in sufficiently small halos, as well as
∼0.1−100 GeV domain motivates us to reevaluate the γγ a “genetic” merger-tree-based treatment of chemical feed-
absorptionopacityatveryhighz,incorporatingmorerecent backthatinducesthetransitionfromPopIIItoPopIIstar
observational and theoretical developments concerning the formation (Schneideret al. 2006).
cosmic reionization era. The free parameters of the model are ε∗,II, ε∗,III, ηesc,
For this purpose, we employ updated versions of the which fixes both fesc,II and fesc,III, and λ0, related to the
semi-analytical models of Choudhury & Ferrara (2005; meanfreepathofionizingphotonsduetoHIinhigh-density
⋆⋆
2006),whichself-consistentlydescribeinhomogeneousreion- regions. Theseareascertainedsoastosimultaneouslyre-
ization of theIGM, accounting for both Pop II and Pop III produce a large set of high-z observational data: i) redshift
starsandtheirradiativeandchemicalfeedbackeffects.With evolution of Lyman-limit absorption systems; ii) effective
only a few free parameters, they are able to fit a wide va- opticaldepthsoftheIGMforLyαandLyβfromQSOspec-
riety of high-z observational data. Using the evolving IRFs tra; iii) electron scattering optical depth τe from WMAP
††
3rd yearresults (Spergel et al. 2007); iv) temperature of
the mean IGM; v) cosmic star formation history; and vi)
‡ Althoughoftenreferredtoas“extragalacticbackgroundlight” limits on J-band source cou‡‡nts from NICMOS HUDF. In
thefiducial,best-fitmodel, Hreionization beginsrapidly
forlowerz,hereweavoidtheterm“background”,sinceIRFscan
at z ∼ 15, initially driven by Pop III stars, and is 90 %
be highly inhomogeneous in the reionization era, especially for
ǫ≥ǫLE,eventhough itturns outtobemoreorlessuniformfor completebyz∼8.Thereafteritissloweddownbyfeedback
thespectral regimerelevanttoγγ absorption;seebelow.
§ Earlier indications of a large contribution from Pop III stars
tothelocalnear-IRbackground arenowdisfavoredfromJ-band ⋆⋆ The adopted cosmological parameters are h = 0.73, Ωm =
sourcecountlimits(Salvaterra&Ferrara2006). 0.24, ΩΛ = 0.76, Ωbh2 = 0.022, σ8 = 0.74, ns = 0.95 and
¶ Note that γγ absorption measurements in low-z blazars have dns/dlnk=0(Spergel etal.2007).
set strong constraints against a large contribution from Pop III †† The fiducial model gives τe = 0.07, consistent with the 5th
starstothelocalnear-IRbackground(Aharonianetal.2006;see yearresultsaswell(Dunkleyetal.2009).
alsoRaueetal.2009). ‡‡ Therelevantparametersareε∗,II=0.1,ε∗,III=0.02,fesc,II=
k http://fermi.gsfc.nasa.gov 0.0578andfesc,III=0.54.
(cid:13)c 2009RAS,MNRAS000,1–??
Gamma-ray absorption during cosmic reionization 3
λ [A]
102
104 103
10-18 6 5 4 3 2 76 5 4 3 2 7 101 z=4
6
z=4
-1r]10-19 68 100 180
s
-1z 10 E) 10-1 12
H (
-2-1m s 10-20 12 14 τlocal 10-2 1164
c10-21 10-3
g 16
r 18
[e 10-4
ε) 10-22 18
( 20
J 10-5
2 4 6 8 2 4 6 8 2 4 6 8
20 101 102 103 104
10-23 E [GeV]
2 3 4 5 67 2 3 4 5 67 2 rest
1 10
ε [eV]
Figure2.Localγγopticaldepthτlocal(E)vs.rest-framegamma-
rayenergyErest atredshiftsz aslabelledforthefiducialmodel.
Figure 1. Volume-averaged intensity of the intergalactic radia- ThecontributionfromtheCMBisnotshown.
tion field J(ǫ) vs. energy ǫ (or wavelength λ) at redshifts z as
labelledforthefiducialmodel.
3 GAMMA-RAY ABSORPTION OPACITY
We first estimate the “local γγ optical depth” at each
z by the optical depth across a Hubble radius l (z) =
H
c/H0(Ωm(1+z)3+ΩΛ)1/2,
effectsandtakenoverbyPopIIstarsatz∼7,finallyreach-
ingcompletionbyz∼6(seeFig.2ofChoudhury2009).The ∞
cosmic star formation rate is always dominated by Pop II τlocal(z,E)=lH(z)Z dǫ n(ǫ,z)
stars and is at the level of 0.05−0.08 M⊙ yr−1Mpc−3 for ǫth
z∼6−8(Fig.2(b)ofChoudhury2009),inlinewiththatde- 1 1
× dµ(1−µ)σ (E,ǫ,µ), (1)
ducedfromobservedGRBrates(e.g.Salvaterraetal.2008, 2Z γγ
−1
Kistler et al. 2009).
Shown in Fig.1 is the volume-averaged intensity of the where n(ǫ,z) is the IRFphoton numberdensity per energy
IRFascalculatedfromthismodel,whichdeclinesmonoton- interval, µ = cosθ, ǫth = 2m2ec4/E(1−µ) is the thresh-
ically with z following the evolution of the star formation old energy, and σ (E,ǫ,µ) is the γγ pair production cross
γγ
rate (SFR). We caution that at z ∼> 6 before intergalactic section. For given E, σγγ rises sharply from ǫ = ǫth, peaks
HIIregionshavecompletelyoverlapped,theIRFisexpected at ǫ = 2ǫth, and then falls off as ǫ−1. Thus τlocal roughly
to be inhomogeneous and fluctuating along different lines mirrors the IRF spectrum at each z, although its detailed
of sight, particularly strongly for ǫ ≥ ǫLE. However, it is featuresaresmeared out.DisplayedinFig.2 intermsofthe
also evidentthat ionizing photonsarestrongly absorbed by rest-frame gamma-ray energy Erest, we see that the opac-
the neutral IGM and the mean IRF spectrum cuts off very ity may be significant out to z ∼ 10 for Erest ∼ 102−104
sharplyaboveǫLE,sothatthisportionhasnegligibleeffects GeV. §§ Note the steep drop in τlocal at Erest < ELE ∼
ontheγγopacity(Madau&Phinney1996,Oh2001,Rhoads (mec2)2/ǫLE ≃ 18 GeV, corresponding to the sharp cutoff
2001). On the other hand, UV radiation with ǫ<ǫLE have intheIRFspectrumabovetheLymanedge.Aspointedout
muchlongermeanfreepathsintheIGM,andthenotionofa byOh(2001;seealsoRhoads2001),thisiscrucialinthatit
nearlyuniformandisotropicbackgroundmaystillbeappro- allowsappreciablecontributionstothetotalγγopacityfrom
priateforthisregime,whichisalsothemostrelevantforγγ higher z even when the IRF intensity is relatively weaker,
absorption.Ofparticularnoteisthebandǫ=10.2−13.6eV and which should be uncontaminated from absorption at
wherethespectrumdipssomewhatduetoblanketingbythe lowerz.However,wealso seethatduetothedecliningIRF
Lyman series lines, but which should nevertheless be very intensitytogetherwiththereducedpathlength,theopacity
important for the γγ opacity. from z>10 is likely to bequitesmall.
∼
Being optimized for the cosmic reionization era, the ThiscanbeseenmoreexplicitlyinFig.3whereweshow
main shortcoming of the present model is that it does not theintegratedγγopticaldepthsfordifferentsourceredshifts
account for Pop I stars or dust that can become important
atlowerz.OurIRFcalculationsareavailableonlyforz ≥4,
and may be somewhat less reliable near z ∼ 4 as the com-
parison with observations has not been as thorough as for §§ ThecontributionfromtheCMB(e.g.Steckeretal.2006)also
z > 5. A more complete model describing the evolution of becomes important at Erest ∼> 2 TeV, but is irrelevant for our
theIRFat all redshifts awaits future studies. resultsbelowandnotplottedinFig.2.
(cid:13)c 2009RAS,MNRAS000,1–??
4 S. Inoue, R. Salvaterra, T. R. Choudhury, A. Ferrara, B. Ciardi & R. Schneider
¶¶
10 compared with those of an alternative model that does
7 not include Pop III stars, in which reionization is driven
6 fiducial only by Pop II stars and occurs relatively late at z ∼ 6
5 K04
4 (similar to the late reionization model of Gallerani et al.
3 2008). The fact that Pop II stars are less efficient sources
2 of ionizing photons compared to Pop III stars mandates a
larger SFR, more intense IRF for ǫ< ǫLE and hence larger
(E) 1 γγ opacity.However,sincetheSFRatz∼<6isobservation-
τ ally constrained, notable differences appear only at z > 8,
67 6 5 5 4 3 whichshouldbechallengingtodistinguishinpractice.T∼hus
5 2
4 8 gamma-ray absorption may not be a sensitive probe of the
3 reionization historyitself.Wehavealsoinvestigatedvarious
10
2 1 other models, e.g. those with more realistic prescriptions
20
for radiative feedback that fit the current high-z observa-
0.1 tions nearly equally well, and found that they generally do
5 10 15 20 25 30 not lead to large differences. Conversely, being constrained
E [GeV]
byexistingdata,ourpredictionsmaybeconsidered reason-
ably robust, at least within the framework of our model.
Figure3.Integratedγγopticaldepthτ(E)vs.observedgamma-
Nevertheless, we caution that relaxing some of the present
ray energy E for source redshifts z as labelled, for the fiducial
assumptions,e.g.regardingthestellarIMFortheQSOcon-
model(solid)andK04’shighstellarUVmodel(dashed).
tribution,may yetallow awiderrangeofpossibilities. Note
that although some other recent models (e.g. Razzaque et
al.2009,Gilmoreetal.2009)predictsomewhatlessabsorp-
z, tionatz∼5−6,theyarenotdirectlycomparablewithours
z dl ∞ as their focus is on the z < 6 universe (e.g. Gilmore et al.
τ(z,E)= dz′ dǫ n(ǫ,z′) 2009 do not attempt to fit the Lyα effective optical depths
Z dz′ Z
zmin ǫth at z∼>5.5 as we do).
1
1
× dµ(1−µ)σ(E(1+z′),ǫ,µ), (2)
2Z
−1
4 DISCUSSION
wheredl/dz′ =l (z′)/(1+z′)andEistheobservedgamma-
H
rayenergy at z=0.Asmentionedabove,thelower limit of The fact that γγ absorption is sensitive to photons with
z-integrationthatcanbetakeninourmodeliszmin=4;for energiesbelowtheLymanlimit ratherthantheionizingra-
additionalabsorptionfromtherangez=0−4,wecanonly diation (§2)actually pointstoauniqueprobeofthecosmic
consultothermodelsatthemoment(e.g.Kneiskeetal.2004, reionizationepochthatcomplementsmeasurementsofQSO
hereafter K04; Stecker et al. 2006; Razzaque et al. 2009; Gunn-Peterson troughs or CMB polarization anisotropies,
Gilmoreetal.2009).OverlayedhereforcomparisonisK04’s whichprobetheneutralandionizedcomponentsoftheIGM,
“high stellar UV model”, which gives theirbest description respectively.Ontheonehand,observationally deducingthe
of QSO proximity effect measurements at z∼2−4. global UV emissivity and hence the cosmic star formation
AswecouldinferfromFig.2,ourmodelpredictsappre- rate from thelatter two is problematic dueto uncertainties
ciable opacity at observed energies E <12 GeV for sources intheinhomogeneityoftheIGM(clumpingfactor) andthe
∼
at z > 5, with notable differences out to z ∼ 8. However, escape fraction of ionizing photons from the host galaxies
∼
therelative effectsof furtherabsorption from z>8 may be (Madau et al. 1999, Wyithe et al. 2009). On the other, the
∼
practically indiscernible. Nevertheless, the spectral attenu- directcensusofthehigh-zUVluminositydensityfromdeep,
ation feature itself should be observable in high-z gamma- near-IRsurveysareaffectedbytheuncertainintegratedcon-
ray sources by current or future gamma-ray facilities, and tributionoffaintgalaxiesbelowthetelescopedetectionlimit
possibly distinguishable in the range z ∼ 5−8 for suffi- (e.g.Bouwensetal.2007).Observingγγabsorptioninhigh-
cientlybrightobjects(§4).Owingtothedropinγγ opacity z sources may allow more robust measurements of the evo-
at Erest < 18 GeV (Fig.2), the differences in absorption in lutionofthecosmicUVemissivity,andincombinationwith
thisz rangeare caused in-situbytheevolution ofUVIRFs other data, possibly the determination of the escape frac-
justbelowtheLymanedgeenergy,includingthecrucialLyα tion and/or the IGM clumping factor as well. These infer-
andLyman-Wernerbands.Wealsorecallthatinthismodel, ences are general and independent of any particular model
Pop IIIstarscontinuedtobesignificant contributorstothe for reionization, but will be investigated in more quantita-
UVIRFdowntoz∼7,wheretheyarecomparablewithPop tivedetail in thenear future. Likewise, the implications for
II stars for ionizing photons. Measurements of these effects constraining Lyα or H2-dissociating radiation from γγ ab-
wouldthusprovideanimportantcheckofcurrentmodelsof sorption will be discussed in future work.
cosmic reionization in its latter stages, as well as a unique We now briefly address whether the effects discussed
andinvaluableprobeofevolvingUVIRFsinthesub-Lyman
edge regime duringtheera of early star formation (§4).
InFigs.4and5,respectively,weplotthespectralatten- ¶¶ Therelevantparametersareε∗,II=0.1andfesc,II=0.0928.
uationfactorexp[−τ(E)]andtheobservedenergyE(τ =1) The model gives τe = 0.06, marginally consistent with the 5th
wheretheopticaldepthisunity.Herethefiducialresultsare yearWMAPconstraints.
(cid:13)c 2009RAS,MNRAS000,1–??
Gamma-ray absorption during cosmic reionization 5
for which deep, pointed observations may indeed reveal the
1.0
IRFabsorption features described above.
0.8
20 1 GRBs are also promising as they are known to occur
10
0.6
8 at z > 6 (Kawai et al. 2005; Greiner et al. 2009), at least
6 2 up to z ∼ 8.2 (Tanvir et al. 2009; Salvaterra et al. 2009),
E)) 0.4 5 54 3 and perhaps out to the very first epochs of star formation
(
τ
- in the universe (e.g. Bromm & Loeb 2006; Salvaterra et
(
p
x al. 2008). Although the spectral properties of GRBs in the
e
0.2 GeV domain are still rather uncertain, previous detections
fiducial
by CGRO/EGRET (Hurley et al. 1994) and the recent de-
K04
tection of GRB 080916C at z=4.35 by Fermi (Abdoet al.
1.0 2009a) kk demonstrate that at least some GRBs have lu-
0.8 minous emission extending to >10 GeV, which can also be
0.6 20 expected theoretically (e.g. Zhang & Meszaros 2001; Asano
10
8 et al. 2009). A burst similar to GRB 080916C may still be
E)) 0.4 6 5 detectable at several GeV by Fermi/LAT at z ∼< 7, and
τ( even out to higher z if the spectrum was somewhat harder.
-
p( Even better for this purpose would be proposed ground-
x
e based telescopes with much larger effective area and multi-
0.2
GeV energy threshold, such as the Cherenkov Telescope
no Pop III ∗∗∗
Array (CTA) , Advanced Gamma-ray Imaging System
∗∗∗
(AGIS) or the 5@5 array (Aharonian et al. 2001). To-
5 10 15 20 25 30 getherwithmeasurementsofLyαdampingwings(McQuinn
E [GeV] et al. 2009) and possibly radio dispersion (Ioka 2003; In-
oue 2004), future, broadband observations of very high-z
Figure 4. Spectral attenuation factor exp[−τ(E)] vs. observed GRBs should open new windows onto the cosmic reioniza-
gamma-ray energy E for source redshifts z as labelled. Upper tion epoch.
panel: fiducial model (solid) and K04’s high stellar UV model Even if IRF-induced spectral features are detected, a
(dashed). Lowerpanel:modelwithoutPopIIIstars. generic problem for γγ absorption studies is distinguishing
them from spectral cutoffs intrinsic to the source. In this
regard, spectral variability should offer an important clue.
BothblazarsandGRBsarehighlyvariablegamma-rayemit-
20
18 ters, and in general, changes in physical conditions of the
16 fiducial sourcethatcausevariationsinfluxshouldalsobeaccompa-
14 no Pop III nied by variations of the intrinsic cutoff energy, whether it
] K04
V is due to injection of freshly accelerated particles, changes
e12
G inthemagneticfields,internalradiationfields,bulkflowve-
1) [10 locity, etc. In contrast, cutoffs of IRF origin should be sta-
=
τ( 8 ble in time and independent of the variability state of each
E object. Acquisition of time-resolved spectra should thus al-

lowthedeconvolutionofthetwoeffects.Anotherindication
6
shouldcomefromstatisticalstudiesofasufficientsampleof
measurements. IRF-related cutoffs should occur at similar
4 6 8 10 12 14 16 18 20
energies for sources at similar z, and also exhibit a system-
z
atic evolution toward lower energies for higher z, whereas
thereis no strong reason to expect such trends for intrinsic
Figure 5. Observed gamma-ray energy E where τ = 1 vs. red-
cutoffs.Boththeabovestrategiesmotivatetheconstruction
shiftz,forthefiducialmodel(solid),modelwithoutPopIIIstars
offuture,high-sensitivitymulti-GeVfacilities suchasCTA,
(dotted) andK04’shighstellarUVmodel(dashed).
AGISand5@5,whichshouldbepowerfultoolstoprobethe
evolutionofUVIRFsinthecosmicreionizationerathrough
γγ absorption in very high-z sources.
above are observable in real sources with current or future
gamma-ray instruments. For blazars, the most prominent
andnumerousextragalacticsourcesofGeVgamma-rays,the
highest redshift confirmed so far is z ∼ 3 (Hartman et al.
1999; Abdo et al. 2009b). However, objects similar to the
mostpowerfulknownblazarssuchas3C454.3withapparent
luminositiesL∼1049erg s−1 shouldbedetectablebyFermi kk For z = 4.35 and E = 13.2 GeV, our fiducial model gives
outtoz ∼8−10iftheyexistatsuchredshifts(e.g.Romani
τγγ ≃ 0.4, consistent with the actual detection of a photon at
etal.2004).Accordingtothelatestblazarevolutionmodels thisenergyfromGRB080916C.
(Inoue & Totani 2009), it may be plausible for Fermi to ∗∗∗ http://www.cta-observatory.org
detect some blazars above z ∼ 6 during its survey period, ∗∗∗ http://www.agis-observatory.org
(cid:13)c 2009RAS,MNRAS000,1–??
6 S. Inoue, R. Salvaterra, T. R. Choudhury, A. Ferrara, B. Ciardi & R. Schneider
ACKNOWLEDGMENTS SalvaterraR.etal.,2009,Nature,461,1258
SchaererD.,2002,A&A,382,28
We thank F. Aharonian, P. Coppi, Y. Inoue, N. Kawai, F.
SchneiderR.,SalvaterraR.,FerraraA.,CiardiB.,2006,MNRAS,
Miniati, N. Omodei, J. Rhoads, M. Teshima and T. Totani 369,825
for valuable discussions, T. Kneiske for making her models SpergelD.N.etal.,2007,ApJS,170,377
available, and the anonymous referee for very helpful and SteckerF.W.,deJager,O.C.,Salamon,M.H.,1992,ApJ,390,L49
constructive comments. S. I. is supported by Grants-in-Aid Stecker F.W.,MalkanM.A.,ScullyS.T.,2006,ApJ,648,774
for ScientificResearch Nos. 19047004 and 19540283 and for TanvirN.R.etal.,2009,Nature,461,1254
theGlobalCOEProgram”TheNextGenerationofPhysics, Wyithe J.S.B.,Hopkins A.M.,Kistler M.D.,Yu¨ksel H., Beacom
J.F.,2009,MNRAS,submitted(arXiv:0908.0193)
Spun from Universality and Emergence” from the Ministry
ZhangB.,M´esz´arosP.,2001, ApJ,559,110
of E.C.S.S.T. (MEXT) of Japan.
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