Gamma Rays from Molecular Clouds and the Origin of Galactic Cosmic Rays. Stefano Gabici APC, Paris

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1 Gamma Rays from Molecular Clouds and the Origin of Galactic Cosmic Rays Stefano Gabici APC, Paris

2 The Origin of galactic Cosmic Rays Facts: the spectrum is (ALMOST) a single power law -> CR knee at few PeVs extremely isotropic, up to very high energies energy density -> ω CR = 1 ev/cm 3 δ = 2.7 CR knee δ = 3.0 extragalactic

3 The Origin of galactic Cosmic Rays Facts: the spectrum is (ALMOST) a single power law -> CR knee at few PeVs extremely isotropic, up to very high energies energy density -> ω CR = 1 ev/cm 3 Most popular explanation: acceleration in SuperNovaRemnants -> CR energy density if efficiency 10% diffusive shock acceleration -> roughly the required spectrum... propagation in the Galaxy -> isotropy δ = 2.7 CR knee δ = 3.0 extragalactic

4 Why is it so difficult? CR...magnetic field... CR source you We cannot do CR Astronomy. Need for indirect identification of CR sources.

5 Gamma-ray astronomy p + p p + p + π 0 π 0 γ + γ CR ISM E γ E CR /10 E peak = m π 0 2 same slope as CR spectrum FERMI IACT knee? 70 MeV 100 GeV 100 TeV

6 The E>100 MeV (FERMI)

7 We need to know: Which are the sources of CRs? which acceleration mechanism? -> injection spectrum total energy in CRs maximum energy of accelerated particles How do CRs propagate? magnetic field in the Galaxy spatial distribution of sources spatial distribution of CRs injected -> observed spectrum Which is the chemical composition of CRs?

8 We need to know: Which are the sources of CRs? which acceleration mechanism? -> injection spectrum total energy in CRs maximum energy of accelerated particles How do CRs propagate? magnetic field in the Galaxy spatial distribution of sources spatial distribution of CRs injected -> observed spectrum Which is the chemical composition of CRs?

9 Why molecular clouds? Molecular Clouds -> sites of star formation dense -> n ~ 100 cm -3 massive -> Mass up to 10 6 M

10 Why molecular clouds? Molecular Clouds -> sites of star formation dense -> n ~ 100 cm -3 massive -> Mass up to 10 6 M Cloud mass L γ σ c dv n CR n ISM = σ c n CR dv n CR n ISM M cl...because they are massive

11 Molecular Clouds are gamma-ray sources The galactic centre ridge as seen by HESS HESS collaboration, 2006

12 Molecular Clouds are gamma-ray sources The galactic centre ridge as seen by HESS good match between CS lines and TeV emission HESS collaboration, 2006

13 Molecular Clouds as CR barometers (Issa & Wolfendale, 1981 ; Aharonian, 1991) Zero-th order approximation: the CR spectrum everywhere in the Galaxy is identical to the spectrum we observe at Earth F γ = A ( Mcl d 2 ) known constant

14 Molecular Clouds as CR barometers (Issa & Wolfendale, 1981 ; Aharonian, 1991) Zero-th order approximation: the CR spectrum everywhere in the Galaxy is identical to the spectrum we observe at Earth F γ = A ( Mcl d 2 ) detectable with EGRET if: Akharonian, 1991 M 5 d 2 kpc > 10 only a few (Orion, Monoceros) -> Digel et al.2001 we need FERMI

15 Molecular Clouds as CR barometers F γ = A ( Mcl d 2 ) Conversely, if we know M cl and d (from CO measurements) we can derive A and estimate both the normalization and spectrum of CRs at the cloud -> Molecular Clouds are CR Barometers Two caveats: error in the determination of the mass (CO -> H2 conversion) effective penetration of CR into the cloud (if not see Gabici et al. 2007)

16 Detectability at TeV energies: the role of CTA Gamma-ray flux from the δ M 5 d 2 kpc T ev/cm 2 /s > ( ɛ CT A 0.1 ) ( ) θ 0.1 T ev/cm 2 /s flux from a passive cloud enhancement with respect to passive cloud mass and distance of the cloud Gabici, 2008

17 Detectability at TeV energies: the role of CTA Sensitivity of δ M 5 d 2 kpc T ev/cm 2 /s > ( ɛ CT A 0.1 ) ( ) θ 0.1 T ev/cm 2 /s HESS sensitivity divided by 10 how much CTA is better than HESS angular resolution Gabici, 2008

18 Detectability at TeV energies: the role of CTA Simplifying assumption: δ M 5 d 2 kpc T ev/cm 2 /s > ( ɛ CT A 0.1 ) ( ) θ 0.1 T ev/cm 2 /s all the clouds have the same density (~ 100 cm -3 ): θ 1 M 1/3 5 d kpc Gabici, 2008

19 Detectability at TeV energies: the role of CTA Detectability condition: d kpc < 2 δ M 2/3 5 HESS cannot detect passive clouds CTA will be able to detect local passive clouds (~ kpc distance scale) CTA (HESS) will probe the Cosmic Ray pressure in regions of the Galaxy where δ >> 1 (δ >> 10) Gabici, 2008

20 Tomography with gamma rays Casanova...SG...et al, 2009 NANTEN: CO (J=1-0) -> tracer of H 2 LAB HI Survey (Karberla et al 05) ASSUMPTION: CR spectrum is universal

21 Tomography with gamma rays Casanova...SG...et al, 2009 NANTEN: CO (J=1-0) -> tracer of H 2 LAB HI Survey (Karberla et al 05) ASSUMPTION: CR spectrum is universal 1 GeV ]!1 s!1 sr!1 TeV!2 Gamma!Ray Profile [ cm sum HI H Galactic Longitude [deg] 10 GeV ]!1 s!1 sr!1 TeV!2 Gamma!Ray Profile [ cm !3 sum HI H Galactic Longitude [deg] structured (clouds) smooth (gas) 100 GeV ]!1 s!1 sr!1 TeV!2 Gamma!Ray Profile [ cm !6 sum HI H2 1 TeV ]!1 s!1 sr!1 TeV!2 Gamma!Ray Profile [ cm ! sum HI H2 there s a peak here <- Gamma ray emission Galactic Longitude [deg] Galactic Longitude [deg]

22 Tomography with gamma rays Casanova...SG...et al, 1 GeV -> FERMI )!1 s!1 kpc!2 (photons cm!8 10 sum!9 10 HI H2 most of the emission comes from a relatively small region at D ~ 1-2 kpc!10 10! Distance (kpc) with FERMI data we will be able to use MCs to probe the CR spectrum in specific regions of the Galaxy

23 Montmerle s SNOBs adapted from Montmerle, 1979 ; Casse & Paul, 1980 Massive (OB) stars form in dense regions -> molecular cloud complexes OB stars evolve rapidly and eventually explode forming SNRs SNR shocks accelerate COSMIC RAYS CRs escape from their sources and diffuse away in the DENSE circumstellar material -> molecular cloud complex and produce there gamma rays! An association between cosmic ray sources and molecular cloud is expected

24 Conclusions Galactic Centre Ridge W28 IC443

The VHE future. A. Giuliani

The VHE future. A. Giuliani The VHE future A. Giuliani 1 The Cherenkov Telescope Array Low energy Medium energy High energy 4.5o FoV 7o FoV 10o FoV 2000 pixels 2000 pixels 2000 pixels ~ 0.1 ~ 0.18 ~ 0.2-0.3 The Cherenkov Telescope