- The EBL as Cosmic background
- Direct observations and limits
- Limits from EBL-TeV photons interaction
- Cosmological implications
Contents
Prandini Elisa 1 Extragalactic Background Light
Extragalactic Backgrounds
EBL [COB+CIB] ~ 5% of CMB
COB ~ CIB
Prandini Elisa 2 Extragalactic Background Light
Extragalactic Background Light
Dole et al., 2006, A&A, 451, 417
Last scattering surface
Stellar light
Stellar light absorbed and re-emitted by dust
Contributions integrated and redshifted through the history of the Universe!
EBL [COB+CIB] ~ 5% of CMB
COB ~ CIB
Prandini Elisa 3 Extragalactic Background Light
EBL contains informations about:
• Structure formation and evolution– Stars– Galaxies
• Cosmological parameters • Energetic budget of the Universe
– Dust component importance
EBL: the integrated history of the Universe after recombination
Prandini Elisa 6 Extragalactic Background Light
• Population III stars (M.L.Norman):
– Massive metal poor stars– All these stars died in an earlier stage
– From DM halos of M~105-6 M at z~30
Stars throughout the Universe
Prandini Elisa 7
From fluctuations to star formation
Prandini Elisa 8
• Super horizon fluctuations of the scalar field during inflation
--> density fluctuations
• Density fluctuations over the Jeans mass --> accretion
• Non collisional matter favoured after equivalence (no pressure force that inhibits the accretion)
• DM distributes in thin filaments and halos
• Barionic matter gravitationally attracted by DM
• Barionic matter undergoes stellar collapse (DM cannot undergo cooling)
• Population III stars (M.L.Norman):
– From DM halos of M~105-6 at Z~30 (from primordial fluctuations)
– Massive metal poor stars– All these stars died in an earlier
stage
• Population II stars:
– Metal poor stars quite old
– In the spherical component of galaxies
• Population I stars (Sun)
– Metal rich young stars
– In the disk of the galaxies
Stars throughout the Universe
Prandini Elisa 9
Fig . 1 .— Co sm ic c o m o v in g SFR in un its o f so la r m a sse s pe r ye a r pe r c ub ic m e g a pa rse c a s a fun c tio n o f re d sh ift.We a ssum e th a t c o o lin g in prim o rd ia l g a s is d ue to a to m ic h yd ro g e n o n ly a n d th at th e sta r fo rm a tio n e ffic ie n c y is ¼ 1 0 % . (a) La te re io n iza tio n (zre io n 7 ). So lid lin e : To ta l c o m o v in g SFR. Do tte d lin e s: Co n trib utio n to th e to ta l SFR fro m Po pula tio n I / II a n d Po p. III fo r th e c a se o f w e a k c h e m ic a l fe e d b a c k . Dash e d lin e s: Co n trib utio n to th e to ta l SFR fro m Po pula tio n I / II a n d Po pula tio n III fo r th e c a se o f stro n g c h e m ic a l fe e d b a c k . (b ) Ea rly re io n izatio n (zre io n 1 7 ). We a d o pt th e sam e c o n v e n tio n fo r th e lin e s a s in pa n e l (a ). In a ll c a se s, Po pula tio n III sta r fo rm a tio n is re stric te d to h ig h re d sh ifts b ut e xte n d s o v e r a s ig n ific a n t ra n g e , z 1 0 – 1 5 .
Bromm & Loeb 2006, ApJ 642, 382
Dust in galaxies• Obscures light• Re-emits at higher
wavelength• It is believed that amount of
dust in galaxies changes with the age
• Composition of dust is difficult to know
• Evolution of dust is even more difficult to know
EBL main characteristics
• Extragalactic radiation
• Isotropy on large scale
• No distinctive spectral signature
• Consistent fluctuations (discrete sources)
Prandini Elisa 11 Extragalactic Background Light
• find diffuse emission:– satellites– ground based
• source counts
• stacking method
• fluctuation analysis
EBL measurements
EBL density at z=0
Prandini Elisa 12 Extragalactic Background Light
Foregrounds and Backgrounds
There are several contribution to the large errors related to EBL measurements...
• Technical difficulties: absolute luminosity measurement (zero problem)
• Theoretical difficulties: strong foregrounds
– Light from stars in our galaxy– Zodiacal light– Diffuse light from IPD and ISD– λ>400 µm: CMB
observed Zodiacal light
Bright galactic sources
Interstellar medium
Faint galactic sources
COBE data (Hauser & Dwek, 2001)
Prandini Elisa 13
Foregrounds and Backgrounds
There are several contribution to the large errors related to EBL measurements
• Technical difficulties: absolute luminosity measurement (zero problem)
• Theoretical difficulties: strong foregrounds
– Light from stars in our galaxy– Zodiacal light– Diffuse light from IPD and ISD– λ>400 µm: CMB
Prandini Elisa 14 Extragalactic Background Light
4
• Direct measurements are difficult
• Lower limits from source counts and stacking
• Upper limits from fluctuation analyses and direct
UV-optical NIR MIR FIR
EBL status of the measurements
Prandini Elisa 15
From the actual limits on integrated EBL energy density can be argued that:
• The EBL energy density is a small fraction of CMB energy density: well below the critical density
• Dominant sources of EBL photons are dusty (2nd peak)
• Star formation rate in the past was higher than at present time
• The EBL energy comes mainly from hydrogen fusion in stars (and only a small fraction from AGN)
more implications: from the analysis of EBL evolution
Implications
Prandini Elisa 16
EBL evolutionInitial mass function, stellar popul spectra,
star formation rate, emissivity,
galaxy types
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Many models, lots of variables...limits from TeV Physics
Gamma-EBL interactions
• measure of ebl energy density!• Measure of cosmological parameters and distances!
Prandini Elisa 18 Extragalactic Background Light
Optical Depth and GRH
−+→ eeEBLHE γγThen the γ-ray flux is suppressed while travelling from the emission point to the detection point.
The e-fold reduction ( τ(E,z) = 1) is the Gamma Ray Horizon (GRH).
( )z,Ee τ−⋅Φ=Φ 0
-rays traversing cosmological distances are expected to be absorbed through their interactions with the EBL by:
Where the Opacity τ(E,z) is:
Prandini Elisa 19 Extragalactic Background Light
( ) ( )( )
∫∫∫∞
+
′⋅′
′=
2
22
12
2
00
,2
,
zExcm
z
q zndxdxzddtzdzE
q
γ γσεετ
Optical Depth
Prandini Elisa 20 Extragalactic Background Light
( ) ( )( )
∫∫∫∞
+
′⋅′
′=
2
22
12
2
00
,2
,
zExcm
z
q zndxdxzddtzdzE
q
γ γσεετ
EBL EBL densitydensity Cross Cross
section section
( )( )[ ] 2/12
0 )2(1)1(
1/1
λΩ+−+Ω+
+⋅=zzzzH
zcM
dzdt
cosmologycosmology:
Franceschini et al. 2008:
geometrygeometry
Extragalactic TeV emission
Fermi AGN sources simulation (E: 30MeV-300GeV)
EBL absorption effect!
Prandini Elisa 23 Extragalactic Background Light
• 24 sources: AGN• Redshift < 0.6
Observables
• Observed spectra after absorption at different redshift
• From some hp on emitted spectrum
UPPERLIMITS ON EBL
DENSITY
Prandini Elisa 24 Extragalactic Background Light
Observables
• Observed spectra after absorption at different redshift
• From some hp on emitted spectrum
UPPERLIMITS ON EBL
DENSITY
Prandini Elisa 25 Extragalactic Background Light
EBL allowed region from observations and TeV constraints
UV-optical NIR MIR FIR
N e w ly e x c lude d re g io n
Prandini Elisa 26 Extragalactic Background Light
• Upper limits using VHE spectra + assumptions about AGN physics
• Recent constraints are already very tight
• HESS II, MAGIC II and Fermi will remove these uncertainties
• References: – Aharonian et al, Nature 440– Mazin&Raue, AA 471– Aharonian et al., AA 475– Albert et al., Science 320
EBL allowed region from observations and TeV constraints
UV-optical NIR MIR FIR
N e w ly e x c lude d re g io n
Prandini Elisa 27 Extragalactic Background Light
• Upper limits using VHE spectra + assumptions about AGN physics
• Recent constraints are already very tight
• HESS II, MAGIC II and Fermi will remove these uncertainties
• References: – Aharonian et al, Nature 440– Mazin&Raue, AA 471– Aharonian et al., AA 475– Albert et al., Science 320
EBL allowed region from observations and TeV constraints
UV-optical NIR MIR FIR
N e w ly e x c lude d re g io n
S till a llo w e d E B L re g io n
• Upper limits using VHE spectra + assumptions about AGN physics
• Recent constraints are already very tight
• HESS II, MAGIC II and Fermi will remove these uncertainties
• References: – Aharonian et al, Nature 440– Mazin&Raue, AA 471– Aharonian et al., AA 475– Albert et al., Science 320
CLOSE TO SOURCE COUNTS!
Prandini Elisa 28 Extragalactic Background Light
Cosmological distancesDistance ladder: SNIa
If one knows Intrinsic AGN
spectrum EBL density
• determine distance to the sources using the EBL signature in the measured spectra• Can cover range fromz=0.004 to z ~ 2
Prandini Elisa 29 Extragalactic Background Light
Simulatedmeasurements
Blanch & Martinez 2004
Prandini Elisa 30 Extragalactic Background Light
GRH as a function of redshift: A POSSIBLE DISTANCE ESTIMATOR!
Independent from SNIa No standard candle required Up to redhift 2 (AGNs)
-- Parameters:
H0
Cosmological densities EBL density (z)
Cosmological distances
Conclusions
• EBL is the second cosmic background
• It contains informations about the history of the Universe after recombination (star and galaxies evolutions)
• Constraining EBL means constraining the actual models of star formation, evolution and dust contribution to the Universe
• Direct measurement are very difficult due to strong foregrounds
• GeV-TeV observation of extragalactic objects are becoming more and more important for EBL evolution determination
• The Universe seems more transparent to gamma rays than previously known
• Gamma observations can also be used for measuring cosmological distances and parameters (independently and complementary to supernovae Ia method)
Prandini Elisa 31 Extragalactic Background Light