1. Fermi-LAT Study of Diffuse g-rays and CRs in the outer Galaxy
Aug. 11, ICRC2011
T. Mizuno, L. Tibaldo and I. Grenier
On behalf of the Fermi-LAT collaboration
Hello everybody,

2. Introduction Objective Cosmic-ray (CR) origin and propagation Method
Diffuse g-rays seen by Fermi-LAT
Detailed study of g-rays in outer Galaxy
(and ISM)
Probe CRs in distant location
Eg/Ep ~ 1/6 (e.g., Mori 97):
CR spectrum up to > 100 GeV

3. The Outer Galaxy (LAT view)
Abdo+10, ApJ 710, 133
Ackermann+11, ApJ 726, 81 
anticenter GC
Eg > 200 MeV
NASA/DOE/Fermi-LAT Collab.

4. The Outer Galaxy (Top view)
Galactic
Center
Sun
II quad
III quad
NASA/JPL-Caltech/R. Hurt

5. The Outer Galaxy (Top view)
No distance ambiguity in velocity separation of gas
suitable to study CRs associated with arms and interarm regions
Sun
Local Arm
Outer arm
Perseus Arm
II quad
III quad
NASA/JPL-Caltech/R. Hurt

6. CR Spectra
We found no significant variation of emissivity spectra with Galactocentric distances
Agree well with the model for CRs directly measured at the Earth
G~2.7 for Ep<=100 GeV
GeV

7. Optical Depth Correction for HI gas
Uncertainty is dominated by N(HI) determination, often overlooked in the past
II quad
III quad
No large emissivity (CR density) gradient
typical range of
optical depth correction
Sun

8. SNR/pulsar distribution
CR Density Gradient
CR density profile flatter than expected from SNR/pulsar distribution and standard CR halo (4kpc)
Emissivity
(CR density) profile
SNR/pulsar distribution
4kpc (solid line)

9. SNR/pulsar distribution
Implications
CR density profile flatter than expected from SNR/pulsar distribution and standard CR halo (4kpc)
Implications:
Large CR halo (>10 kpc)
Emissivity
(CR density) profile
SNR/pulsar distribution
&
Different CR halo
20kpc
15kpc
10kpc
4kpc (solid line)

10. Implications
CR density profile flatter than expected from SNR/pulsar distribution and standard CR halo (4kpc)
Implications:
Large CR halo (>10 kpc)
Flat CR source distribution
Standard CR halo (4kpc)
&
modified source distr.

11. Implications
CR density profile flatter than expected from SNR/pulsar distribution and standard CR halo (4kpc)
Implications:
Large CR halo (>10 kpc)
Flat CR source distribution
also possible:
Non-uniform diffusion
Vast amount of missing gas
(e.g., Evoli+08, Papadopoulos+02)

12. Thank you for your Attention
Summary
Diffuse g-ray emission: Powerful probe of CRs in distant locations
Extensive study in the outer Galaxy
N(HI) is the dominant source of uncertainty
Flat CR density distribution, implying
Larger CR halo
More CR sources
Non-uniform diffusion/missing gas
Thank you for your Attention

13. Backup Slides

14. = Data Analysis “Standard” approach: linear combination of ISM maps
g-rays
“Standard” approach:
linear combination of ISM maps
- no a-priori assumption of CR spectra =
N(HI),1
N(HI),2
N(HI),3
N(HI),4
qHI,1X
+qHI,2X
+qHI,3X
+qHI,4X
Local arm
interarm
Perseus arm
Outer arm
WCO,1
WCO,2
WCO,3
+qCO,1X
+qCO,2X
+qCO,3X
E(B-V)res
+qEBVX
+IC+Iso+PS

15. = Data Analysis Improvement over the EGRET era:
g-rays
Improvement over the EGRET era:
Energy coverage, statistics
dust to trace gas unaccounted by HI and Wco
Careful evaluation of uncertainty due to N(HI) determination =
N(HI),1
N(HI),2
N(HI),3
N(HI),4
qHI,1X
+qHI,2X
+qHI,3X
+qHI,4X
Local arm
interarm
Perseus arm
Outer arm
WCO,1
WCO,2
WCO,3
+qCO,1X
+qCO,2X
+qCO,3X
E(B-V)res
+qEBVX
+IC+Iso+PS

16. Summary Spectra Dominated by g-rays from HI gas
Emissivity ratio to that of the Local arm

17. Emissivity Ratio Consistent with “No spectral variation”
Emissivity ratio to that of the Local arm

18. Optical Depth Correction for HI gas
Uncertainty is dominated by N(HI) determination, often overlooked in the past
II quad
III quad
No large emissivity (CR density) gradient
No large arm-interarm contrast
Local arm
interarm
Perseus arm
Sun