Extragalactic Background Light - · PDF fileExtragalactic Background Light Dole et al., 2006, A&A, 451, 417 Last scattering surface Stellar light Stellar light absorbed and re-emitted

Extragalactic Background Light

Elisa Prandinidottorato 23° ciclo

30 Ottobre 2008

- 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


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


Extragalactic Backgrounds: observations


Our History


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


• 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)


• find diffuse emission:– satellites– ground based

• source counts

• stacking method

• fluctuation analysis

EBL measurements

EBL density at z=0


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


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


Many models, lots of variables...limits from TeV Physics

Gamma-EBL interactions

• measure of ebl energy density!• Measure of cosmological parameters and distances!


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:


( ) ( )( )

∫∫∫∞

+

′⋅′

′=

2

22

12

2

00

,2

,

zExcm

z

q zndxdxzddtzdzE

q

γ γσεετ

Optical Depth


( ) ( )( )

∫∫∫∞

+

′⋅′

′=

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

The optical depth evolution

Franceschini et al. 2008

The Gamma ray Horizon

Franceschini et al. 2008


Extragalactic TeV emission

Fermi AGN sources simulation (E: 30MeV-300GeV)

EBL absorption effect!


• 24 sources: AGN• Redshift < 0.6

Observables

• Observed spectra after absorption at different redshift

• From some hp on emitted spectrum

UPPERLIMITS ON EBL

DENSITY


Observables

• Observed spectra after absorption at different redshift

• From some hp on emitted spectrum

UPPERLIMITS ON EBL

DENSITY


EBL allowed region from observations and TeV constraints


N e w ly e x c lude d 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












S till a llo w e d E B L re g io n





CLOSE TO SOURCE COUNTS!


Cosmological distancesDistance ladder: SNIa


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


Simulatedmeasurements

Blanch & Martinez 2004


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)


Documents

Extragalactic Background Light - · PDF fileExtragalactic Background Light Dole et al., 2006, A&A, 451, 417 Last scattering surface Stellar light Stellar light absorbed and re-emitted