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Deciphering the gamma-ray background: stafrorming galaxies, AGN, and

the search for Dark Matter in the GeV Band.

Vasiliki PavlidouEinstein Fellow

Shin’ichiro Ando (Caltech)Brandon Hensley (Caltech)Luis Reyes (U. Chicago)Jennifer Siegal-Gaskins (Ohio State) Tonia Venters (Goddard)

The gamma-ray sky

4 days of Fermi LATCredit: LAT collaboration

What is making the GeV isotropic diffuse

background?

• Guaranteed sources: active galaxies, starforming galaxies

• Hypothesized source classes: starburst galaxies, galaxy clusters, dark matter cusps

What physics can we learn about galaxies?

• How galaxies make gamma rays:– Gas makes stars– Stars blow up and make supernova remnants– Supernova remnants accelerate cosmic rays– Cosmic rays collide with gas, make pions, – Pions decay into gamma rays

• How much diffuse gamma-ray emission due to all galaxies, everywhere, ever? Physics input to this calculation: – Cosmic star formation history (how much star formation, gas)– Cosmic-ray -- gas interactions– Cosmic-ray acceleration, confinement, escape Learn about B-field

at high-z!

Q u ic k T im e ™ a n d a d e c o m p re s s o ra re n e e d e d to s e e th is p ic tu re .

Q u ic k T im e ™ a n d a d e c o m p re s s o ra re n e e d e d to s e e th is p ic tu re .

VP & Fields 02, Ando & VP 09

What physics can we learn about AGN?• How AGN make gamma rays:

– Gamma-ray loud AGN (blazar) has relativistic jet aimed at you– Some disturbance (?) accelerates electrons (?) to relativistic

energies– Relativistic electrons Compton-upscatter soft photons

(synchrotron? accretion disk? rescattered from broad line region?)to GeV energies

• How much diffuse gamma-ray emission due to all AGN, everywhere, ever? Physics input to this calculation: – Luminosity function– Energy spectrum– Duty cycle– Extragalactic UV, optical, IR backgrounds!

Credit: J. Buckley 1998 (Science),illustration: K. Sutliff

More input parameters, more complicated problem, less understood physics, fewer well-constrained inputsBut also: more observables, more potential for discovery!

Venters, Reyes & VP 09VP & Venters 08

Why look for dark matter in -rays?

(Kolb 98)

Bang!

Time passes, structure forms, density cusps develop, annihilation rate should be higher @ cusps!

Where do we look for DM with -rays?

Where should we be looking?Individual sources:– Galactic Center (but: messy)– Nearby low-gas dwarf galaxies (but: faint)– Nearby MW substructure clumps (but: where?)

Unresolved emission (isotropic diffuse): – Contribution from MW halo substructure, extragalactic sources– But: astrophysical foregrounds! DM signal could be

subdominant…If only we could somehow enhance a subdominant DM signal…

Siegal-Gaskins 08

Siegal-Gaskins & Pavlidou 2009, PhysRevLett.102.241301, arXiv:0901.3776

1GeV 10 GeV

Can we enhance a subdominant DM signal?

Yes, we can!Take into account information about angular anisotropies– MW subhalos: few, nearby, lots of power at small scales– Blazars: many, faraway, very little power at small scales– There should be a transition in angular power

at small scales as we move from low E (no DM) to high E (some DM)

Siegal-Gaskins & Pavlidou 2009 PhysRevLett.102.241301, arXiv:0901.3776

How well can we do?

Hensley, Siegal-Gaskins & Pavlidouin preparation

How much can we learn?

We could measure the annihilation spectrum!

Hensley, Siegal-Gaskins & Pavlidouin preparation

Conclusions

1. A wealth of information on high-energy processes is encoded in the isotropic diffuse background in GeV energies starforming galaxies, blazars, dark matter(+ galaxy clusters, starburst galaxies, …)

2. A dark matter signal from the Milky Way substructure could be hiding in the isotropic diffuse background:More substructure away from Galactic center, where clumps are not tidally disrupted collective unresolved clump emission appears isotropic

3. The DM signal can be robustly separated from other astrophysical contributions by combining spectral and anisotropy information

4. DM particle mass, annihilation spectrum can be recovered

5. Fermi is performing spectacularlyThe future is bright, stay tuned!