Fermi-LAT Study of Diffuse g-rays and CRs in the outer Galaxy Aug. 11, 2011 @ ICRC2011 T. Mizuno, L. Tibaldo and I. Grenier On behalf of the Fermi-LAT collaboration Hello everybody,

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

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.

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

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

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 0.1 1 10 GeV

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

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)

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)

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.

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)

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

Backup Slides

= 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

= 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

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

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

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