Searching for Dark Matter in the Galactic Center with Fermi LAT: Challenges Simona Murgia University of California, Irvine Debates on the Nature of Dark Matter Sackler 2014 19-22 May 2014
arxiv:0908.0195 Gamma rays from DM Annihilation
The Fermi Sky Fermi LAT data 4 years, E > 1 GeV
Understanding the Gamma-ray Sky = + + data sources galactic diffuse isotropic + dark matter??
Galactic Gamma-Ray Interstellar Emission The diffuse gamma-ray emission from the Milky Way is produced by cosmic rays interacting with the interstellar gas and radiation field and carries important information on the acceleration, distribution, and propagation of cosmic rays. = + + data sources galactic diffuse isotropic Inverse Compton Bremsstrahlung π 0 -decay x-ray, gamma-ray x-ray, gamma-ray proton
Galactic Gamma-ray Interstellar Emission Cosmic ray origin, propagation, and properties of the interstellar medium can be constrained by comparing the data to predictions. Generate models (in agreement with CR data) varying CR source distribution, CR halo size, gas distribution (GALPROP, http://galprop.stanford.edu) and compare with Fermi LAT data Assume locally measured CR spectra and intensities are representative of the Galaxy
Gamma Rays from Local HI Cosmic ray origin, propagation, and properties of the interstellar medium can be constrained by comparing the data to predictions. Generate models (in agreement with CR data) varying CR source distribution, CR halo size, gas distribution (GALPROP, http://galprop.stanford.edu) and compare with Fermi LAT data Assume locally measured CR spectra and intensities are representative of the Galaxy Gamma-ray emissivity of hydrogen gas from within ~ 1kpc from the Sun is in agreement with predictions based on directly measured CR spectra arxiv:0908.1171
All Sky Cosmic ray origin, propagation, and properties of the interstellar medium can be constrained by comparing the data to predictions. Generate models (in agreement with CR data) varying CR source distribution, CR halo size, gas distribution (GALPROP, http://galprop.stanford.edu) and compare with Fermi LAT data Fermi LAT data 21 months, 200 MeV-100 GeV (data - prediction)/prediction) for example model arxiv:1202.4039 On a large scale the agreement between data and prediction overall is good, however some extended excesses and deficits stand out. Example model: CR source distribution: SNRs CR confinement region: 20 kpc radius, 4 kpc height
arxiv:1202.4039 Inner Galaxy Cosmic ray origin, propagation, and properties of the interstellar medium can be constrained by comparing the data to predictions. Generate models (in agreement with CR data) varying CR source distribution, CR halo size, gas distribution (GALPROP, http://galprop.stanford.edu) and compare with Fermi LAT data Fermi LAT data 21 months, 200 MeV-100 GeV DGE π 0 -decay bremsstrahlung IC sources Total isotropic Inner galaxy
Galactic Center Region Steep DM profiles predicted by CDM Large DM annihilation/decay signal from GC! Good understanding of the conventional astrophysical background is crucial to extract a potential DM signal from this complex region of the sky: source confusion: many energetic sources near to or in the line of sight of the GC diffuse emission modeling: large uncertainties due to the overlap of structures along the line of sight, difficult to model
CR Source Distribution Steep DM profiles predicted by CDM Large DM annihilation/decay signal from GC! Good understanding of the conventional astrophysical background is crucial to extract a potential DM signal from this complex region of the sky: source confusion: many energetic sources near to or in the line of sight of the GC diffuse emission modeling: large uncertainties due to the overlap of structures along the line of sight, difficult to model Cosmic ray source density Fractional residual difference between models generated with two different CR source distributions arxiv:1202.4039 Pulsars (Lorimer 2006) Pulsars (Yusifov&Kukuk 2004) OB stars SNRs
Diffuse TeV Emission The gamma rays of the Galactic center ridge at TeV energies observed by HESS is hard (spectral index~2.3), suggesting spectrum of CRs is harder than measured locally HESS Aharonian et al 2006 Aharonian et al 2006
Interstellar Medium Steep DM profiles predicted by CDM Large DM annihilation/decay signal from GC! Good understanding of the conventional astrophysical background is crucial to extract a potential DM signal from this complex region of the sky: source confusion: many energetic sources near to or in the line of sight of the GC diffuse emission modeling: large uncertainties due to the overlap of structures along the line of sight, difficult to model Average gas density Integrated line intensity of CO (traces H2) z= 0 pc 100 pc 200 pc
Interstellar Radiation Field Steep DM profiles predicted by CDM Large DM annihilation/decay signal from GC! Good understanding of the conventional astrophysical background is crucial to extract a potential DM signal from this complex region of the sky: source confusion: many energetic sources near to or in the line of sight of the GC diffuse emission modeling: large uncertainties due to the overlap of structures along the line of sight, difficult to model Spatial variation of the radiation field in the Galaxy
Unresolved Sources Steep DM profiles predicted by CDM Large DM annihilation/decay signal from GC! Good understanding of the conventional astrophysical background is crucial to extract a potential DM signal from this complex region of the sky: source confusion: many energetic sources near to or in the line of sight of the GC diffuse emission modeling: large uncertainties due to the overlap of structures along the line of sight, difficult to model Also, unresolved source component likely Gregoire et al, arxiv:1305.1584 Predicted millisecond pulsar population undetectable by the LAT Models based on Story et al 2007, Faucher-Giguere & Loeb 2010
Fermi Bubbles Gamma-ray bubbles (Su et al 2010, ApJ 724, 1044): very extended (~ 50 o from plane) hard spectrum (~E -2, 1-100 GeV) sharp edges possible counterparts in other wavelengths (ROSAT, WMAP, and Planck) Fermi LAT data, E>10 GeV Outflow from the center of the Milky Way: jets from the supermassive black hole? starburst? Su et al 2010, ApJ 724, 1044 Fermi LAT Collaboration, ICRC 2013 PRELIMINARY
Dark Matter Distribution The dark matter annihilation (or decay) signal strongly depends on the dark matter distribution. Cuspier profiles can provide large boost factors Bertone et al., arxiv:0811.3744 NFW profile Navarro, Frenk, and White 1997 (r) = 0 r 0 r (1 + r 0 /a 0 ) 2 (1 + r/a 0 ) 2 0 = 0.3 GeV/cm 3 a 0 = 20 kpc, r 0 = 8.5 kpc 17 Via Lactea II (Diemand et al 2008) predicts a cuspier profile, ρ(r) r -1.2 Aquarius (Springel et al 2008) predicts a shallower than r -1 innermost profile
Fermi s View of the Inner Galaxy (15 o x15 o region) Fermi LAT preliminary results with 32 months of data, E>1 GeV (P7CLEAN_V6, FRONT): DATA DATA-MODEL (diffuse) W28 2FGL J1745.6-2858 LAT PSR J1732-3131 LAT PSR J1809-2332 Galactic diffuse emission model: all sky GALPROP model tuned to the inner galaxy Bright excesses after subtracting diffuse emission model are consistent with known sources.
Fermi s View of the Inner Galaxy (15 o x15 o region) Fermi LAT preliminary results with 32 months of data, E>1 GeV (P7CLEAN_V6, FRONT): DATA DATA-MODEL (diffuse+sources) Diffuse emission and point sources account for most of the emission observed in the region.
Dark Matter Constraints Fermi LAT Collaboration, arxiv:1308.3515 Prom pt Prom pt + ICS 10-22 10-23 10-24 3σ Einasto Burkert 10-27 10-28 5 10-22 20 10-23 cc Æ m + m - Hooper and Linden, arxiv: 1110.0006 NFW 10 10-25 Burkert 10-26 10-27 NFWc 10 Prom pt + ICS Optimized region Einasto for NFW -24 NFW 10-25 10-26 Prom pt cc Æ b b <sv > @cm 3 êsd <sv > @cm 3 êsd Use LAT data from the inner Galaxy to constrain DM by requiring that the DM contribution doesn t exceed observed data Constraints are not competitive unless background is (at least partially) subtracted or for contracted DM profiles m DM @GeVD 50 100 200 500 1000 2000 10-28 5 NFWc 10 20 m DM @GeVD 50 100 200 500 1000 2000
Dark Matter Constraints Use LAT data from the inner Galaxy to constrain DM by requiring that the DM contribution doesn t exceed observed data Constraints are not competitive unless background is (at least partially) subtracted or for contracted DM profiles Fermi LAT Collaboration, arxiv:1310.0828 h vi (cm 3 s 1 ) 10 22 10 23 10 24 10 25 10 26 Observed Limit Median Expected 68% Containment 95% Containment e + e uū Hooper and Linden, arxiv: 1110.0006 10 22 h vi (cm 3 s 1 ) 10 23 10 24 10 25 10 26 µ + µ b b 10 22 h vi (cm 3 s 1 ) 10 23 10 24 10 25 10 26 + W + W 10 1 10 2 10 3 Mass (GeV/c 2 ) 10 1 10 2 10 3 Mass (GeV/c 2 )
Sgr A* Image (width~0.5 o ) combines a near-infrared view from the Hubble Space Telescope (yellow), an infrared view from the Spitzer Space Telescope (red) and an X-ray view from the Chandra X-ray Observatory blue and violet) into one multi-wavelength picture.
Conclusions Our knowledge of the conventional astrophysical background is uncertain. This is currently the big limitation for the search of dark matter in the Galactic center with gamma rays. In addition, better understanding of the dark matter density distribution in the Galactic center is essential in interpreting observations. Difficult analysis, but the rewards are big! Thank you!