Diffuse Galactic Emission
in the Fermi-LAT Era
Troy A. Porter
Santa Cruz Institute forParticle Physics
On behalf of the Fermi Large
Area Telescope collaboration

November 2nd, 20091

Why study the Diffuse Emission?
The Milky Way and its Structure

As a Foreground→ Origin and propagation of cosmic rays
Nature and distribution of sources
The propagation mode itself ↔ relationshipto magnetic turbulence in the ISM
Relative proportions of primary species
Production of secondary species
etc.

→ The diffuse emission is the foregroundagainst which sources are detected
Point sources : limitation on sensitivity
Extended sources : disentanglement→ Interstellar Medium
Distribution of HI, H2, HII gas
Nature of XCO relation in Galaxy
Distribution and intensity of interstellar
radiation field ↔ formation of H2→ Indirect dark matter detection
Predicted gamma-ray/cosmic-ray signals
rely on accurate subtraction of standard
astrophysical sources→ Foreground for isotropic diffuse background
Whatever its natureetc.

November 2nd, 20094

Why study the Diffuse Emission?
The Milky Way and its Structure

As a Foreground→ Origin and propagation of cosmic rays
Nature and distribution of sources
The propagation mode itself ↔ relationshipto magnetic turbulence in the ISM
Relative proportions of primary species
Production of secondary species
etc.

→ The diffuse emission is the foregroundagainst which sources are detected
Point sources : limitation on sensitivity
Extended sources : disentanglement→ Interstellar Medium
Distribution of HI, H2, HII gas
Nature of XCO relation in Galaxy
Distribution and intensity of interstellar
radiation field ↔ formation of H2→ Indirect dark matter detection
Predicted gamma-ray/cosmic-ray signals
rely on accurate subtraction of standard
astrophysical sources→ Foreground for isotropic diffuse background
Whatever its natureetc.

November 2nd, 20095

0pHalo~0.1-0.01
cm-3Ga~1 s, so00 ur
cm ces
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40kp
c

41

2kp
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Cosmic rays injected into
ISM propagate formillions of years
before escape tointergalactic space
Particle interactions withinterstellar gas andradiation fields
produce gamma rays
and other secondariesc

Cosmic Rays and Diffuse Emission

November 2nd, 20096

The Particle Beam: Cosmic Rays
Primary cosmic rays fromSNR, pulsars, …
Secondary cosmic rays (e±,pbar, ..) frominteractions with ISM
Propagation from sources
via `interactions' withmagnetic turbulence in
the ISM
Details of propagationinterpreted withincontext of model →comparison withmeasured cosmic-ray
spectraNovember 2nd, 2009

SNR RX J1713-3946
42 sigma (2003+2004 data)

HESS
PSF

CR distribution from diffuse
gammas (Strong & Mattox 1996)
SNR distribution (Case &Bhattacharya 1998)
Pulsar distribution(Lorimer 2004)

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The Targets #1: Interstellar gas
Neutral interstellar medium –most of the interstellar gas
mass
Obtain information via 21-cm
H I & 2.6-mm CO (second
most abundant molecule in
ISM - surrogate for H2)
Transitions excited even forinterstellar conditions
Allow determinations ofcolumn densities → Doppler
shifts of lines interpreted as
distance measure
Clemens (1985)
HII low density → obtained
from modelling pulsar
dispersion measurements
Helium ~10% by number`Metals' (i.e., Z > 2) contributevery small fraction comparedwith H and HeHICOW. KeelSun
November 2nd, 20098

The Targets #2: Interstellar Radiation Field
Interstellar radiation field = low
energy photon populations inGalaxy from stellar emission
and dust reprocessing ofstarlight
Only observed locally so use
modelling for spectral energy
and angular distributions
throughout Galaxy
Inner Galaxy ISRF energy
density > x100 local
The scale height above theGalactic plane is large (~10kpc) → pervasive contribution
by IC over the sky

Stellar + infrared

Z=0, R=0 kpc
4 kpc
8 kpc
12 kpc
16 kpcopticalNovember 2nd, 2009

IRCMB9

Starting Point
We use a code called `GALPROP' to study
the relation between cosmic-ray
production and diffuse emission in theGalaxy
Starting point for our studies: the cosmicray spectra consistent with local
observations (cosmic-ray nuclei, Fermi
LAT electrons) → reference model
Model skymaps compared with data using
maximum likelihood
Data we use are same as for the isotropic
gamma-ray background analysis →improved background rejection withrespect to the standard `diffuse' classevents

November 2nd, 2009

Model Skymaps
π0-decay

Inverse Compton

Bremsstrahlung

See Poster P4-138

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Summary #1
The `a-priori' model works fairly well alreadyGamma rays are showing missing details
Gas → cosmic rays `see' all phases of ISM andusual tracers do not give everything → gamma
rays probe of the ISM
Providing evidence for an extended cosmic-ray
halo → `isotropic' background
Minor modifications improve the agreement
LAT measurements of the DGE allow an increased
sophistication
The targets (gas, ISRF) are obtained using
observations at other wavelengths and modelling
Cosmic ray sources and transport → improveunderstanding with knowledge of diffuse emission
Exploring this within the context of a physical model iscrucial for understanding what is missing
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Summary #2
Understanding what is missing is key
Unresolved source populations
EGRET had them, so will the LATThis modelling is next to be included into ourdiffuse emission studies (see poster P4-139)
Targets → ongoing studies (see poster P4-137)
Information from specific regions for gammas andcosmic rays (see DGE-related poster summary)
There will be many claims of `excesses'
Caution: need to demonstrate understanding of thebeam (CRs), targets (gas, ISRF), and unresolved
source populations
This is best done using a physical modelNovember 2nd, 200918

Summary of DGE-related Posters
GALPROP modelling of the Galaxy (P4-138)
Contributions of source populations to the Galactic diffuse emission (P4139)
HI spin temperature with Fermi-LAT (P4-137)
High Energy Gamma-ray Emission Around the North Polar Spur
Diffuse Gamma-ray Observations of the Orion Molecular Clouds
Fermi-LAT study of the cosmic-ray gradient in the outer Galaxy: Fermi-LAT
view of the 3rd quadrant (P4-120)
Fermi measurements of the diffuse gamma-ray emission beyond the solar
circle: Cassiopeia, Cepheus and the Perseus arm (P4-136)
Particle Background Effects on Efficiency and Residual BackgroundContamination of the LAT Diffuse Class Photon Sample
Extending the Galactic cosmic-ray positron + electron spectrum measured
by the Fermi-LAT
Searches for Cosmic-ray Electron Anisotropies in the Fermi-LAT Data
The High Energy Cosmic Ray Electron Spectrum measured with the Fermi
Space Telescope: some possible interpretations

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