Fermi_ICRC2011.ppt Fermi Fermi- -LAT Study of LAT Study of Diffuse γ rays and CRs Diffuse γ -rays and CRs Diffuse Diffuse rays and CRs rays and CRs in the outer Galaxy in the outer Galaxy Aug. 11, 2011 @ ICRC2011 T Mizuno L Tibaldo and T. Mizuno, L. Tibaldo and I. Grenier On behalf of the Fermi-LAT On behalf of the Fermi-LAT collaboration T. Mizuno et al. 1
Fermi_ICRC2011.ppt Introduction Introduction • Objective – Cosmic-ray (CR) origin and propagation C i (CR) i i d ti (and ISM) • Method M th d – Diffuse γ -rays seen by Fermi-LAT Probe CRs in distant location E γ /Ep ~ 1/6 (e.g., Mori 97): CR CR spectrum up to > 100 GeV t t 100 G V – Detailed study of γ -rays in outer Galaxy T. Mizuno et al. 2
Fermi_ICRC2011.ppt The Outer Galaxy (LAT view) The Outer Galaxy (LAT view) Abdo+10, ApJ 710, 133 Ackermann+11, ApJ 726, 81 GC anticenter E γ > 200 MeV NASA/DOE/Fermi-LAT Collab. T. Mizuno et al. 3
Fermi_ICRC2011.ppt The Outer Galaxy (Top view) The Outer Galaxy (Top view) Galactic Center Sun II quad III quad NASA/JPL-Caltech/R. Hurt T. Mizuno et al. 4
Fermi_ICRC2011.ppt The Outer Galaxy (Top view) The Outer Galaxy (Top view) • No distance ambiguity in velocity separation of gas of gas – suitable to study CRs associated with arms and interarm regions arms and interarm regions Sun Local Arm II quad III quad NASA/JPL-Caltech/R. Hurt T. Mizuno et al. 5
Fermi_ICRC2011.ppt CR Spectra CR Spectra • We found no significant variation of emissivity spectra with Galactocentric distances spectra with Galactocentric distances • Agree well with the model for CRs directly measured at the Earth Γ ~2.7 for Ep<=100 GeV 0.1 1 10 GeV T. Mizuno et al. 6
Fermi_ICRC2011.ppt Optical Depth Correction for HI gas Optical Depth Correction for HI gas • Uncertainty is dominated by N(HI) determination, often overlooked in the past often overlooked in the past II quad q III quad • No large emissivity (CR density) gradient typical range of optical depth correction Sun T. Mizuno et al. 7
Fermi_ICRC2011.ppt CR Density Gradient CR Density Gradient • CR density profile flatter than expected from SNR/pulsar distribution and standard CR halo (4kpc) SNR/pulsar distribution and standard CR halo (4kpc) Emissivity (CR density) profile (CR density) profile SNR/pulsar distribution 4kpc (solid line) T. Mizuno et al. 8
Fermi_ICRC2011.ppt Implications Implications • CR density profile flatter than expected from SNR/pulsar distribution and standard CR halo (4kpc) SNR/pulsar distribution and standard CR halo (4kpc) • Implications: – Large CR halo (>10 kpc) Emissivity (CR density) profile (CR density) profile SNR/pulsar distribution & Different CR halo 20kpc 15kpc p 10kpc 4kpc (solid line) T. Mizuno et al. 9
Fermi_ICRC2011.ppt Implications Implications • CR density profile flatter than expected from SNR/pulsar distribution and standard CR halo (4kpc) 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. T. Mizuno et al. 10
Fermi_ICRC2011.ppt Implications Implications • CR density profile flatter than expected from SNR/pulsar distribution and standard CR halo (4kpc) SNR/pulsar distribution and standard CR halo (4kpc) • Implications: also possible: – Large CR halo (>10 kpc) - Non-uniform diffusion - Vast amount of missing gas – Flat CR source distribution (e.g., Evoli+08, Papadopoulos+02) T. Mizuno et al. 11
Fermi_ICRC2011.ppt Summary Summary • Diffuse γ -ray emission: Powerful probe of CRs in distant locations 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 Thank you for your Attention T. Mizuno et al. 12
Fermi_ICRC2011.ppt Backup Slides Backup Slides Backup Slides Backup Slides T. Mizuno et al. 13
Fermi_ICRC2011.ppt Data Analysis Data Analysis γ -rays “Standard” approach: • linear combination of ISM maps - no a-priori assumption of CR spectra - no a-priori assumption of CR spectra = = N(HI), 2 N(HI), 1 N(HI), 3 N(HI), 4 +q HI,2 X q HI,1 X +q HI,3 X +q HI,4 X interarm interarm Perseus arm Perseus arm Outer arm Outer arm Local arm Local arm W CO,1 W CO,2 W CO,3 +q CO,1 X +q CO 1 X +q CO 2 X +q CO,2 X +q CO 3 X +q CO,3 X E(B V) E(B-V) res +q EBV X +IC+Iso+PS T. Mizuno et al. 14
Fermi_ICRC2011.ppt Data Analysis Data Analysis γ -rays Improvement over the EGRET era: • Energy coverage, statistics • dust to trace gas unaccounted by HI and Wco • dust to trace gas unaccounted by HI and Wco • Careful evaluation of uncertainty due to N(HI) determination = = N(HI), 2 N(HI), 1 N(HI), 3 N(HI), 4 +q HI,2 X q HI,1 X +q HI,3 X +q HI,4 X interarm interarm Perseus arm Perseus arm Outer arm Outer arm Local arm Local arm W CO,1 W CO,2 W CO,3 +q CO,1 X +q CO 1 X +q CO 2 X +q CO,2 X +q CO 3 X +q CO,3 X E(B V) E(B-V) res +q EBV X +IC+Iso+PS T. Mizuno et al. 15
Fermi_ICRC2011.ppt Summary Spectra Summary Spectra • Dominated by γ -rays from HI gas Emissivity ratio to that of the Local arm T. Mizuno et al. 16
Fermi_ICRC2011.ppt Emissivity Ratio Emissivity Ratio • Consistent with “No spectral variation” Emissivity ratio to that of the Local arm T. Mizuno et al. 17
Fermi_ICRC2011.ppt Optical Depth Correction for HI gas Optical Depth Correction for HI gas • Uncertainty is dominated by N(HI) determination, often overlooked in the past often overlooked in the past II quad q III quad • No large emissivity (CR density) gradient • No large arm-interarm g contrast Local arm interarm Perseus arm Sun T. Mizuno et al. 18
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