H.E.S.S. observations of the globular clusters NGC6388 and M15 and search for a dark matter signal E. Moulin, C. Medina, J.-F. Glicenstein and A. Viana IRFU, CEA-Saclay for the H.E.S.S. Collaboration A.Abramovski et al (HESS), ApJ 735, 12 (2011) 32nd International Cosmic Ray Conference, Beijing

Indirect dark matter search strategy WIMP annihilation flux into rays observed in solid angle : fAP: dark halo model particle model Astrophysical factor:  dense targets dN/dE given by a particle model: in our case neutralinos, assume cc W+W- signal boost in some models (W+W-) by internal brehmstrahlung FSR IB HESS DM search towards NGC6388 & M15 ICRC 2011, Beijing 2

Motivation ― annihilation rate of DM  (DM density)2 ― expect DM density compression near black holes and dense cores of globular clusters (GC) ―result on M15 GC published by Whipple collaboration Woods et al, ApJ 678, 594 (2008) ― 1.2 h observation time ― 400 GeV threshold ― M1595%CL < 0.38 Crab Units UMi Draco M15 (adiabatic compression) MSSM + WMAP constraints Better constraints from Whipple with M15 than with dwarf galaxies HESS DM search towards NGC6388 & M15 ICRC 2011, Beijing

M15 and NGC6388 M15 Globular cluster, located at 10 kpc from Sun Estimated total mass: 105 Msol Undergoing core collapse, with a very small r = 0.04 pc core low metallicity (« primordial ») NGC6388 Globular cluster, located at 11.5 kpc from Sun Estimated total mass: 106 Msol Core radius r=0.4 pc Possible 5 103 Msol IMBH in the center HESS DM search towards NGC6388 & M15 ICRC 2011, Beijing

H.E.S.S. Observations and data analysis Good quality data : 15.2 hours (M15), 27.2 hours (NGC6388) No signal at the target position of NGC6388 and M15  95% upper limits on the numbers of gamma-ray from targets - NGC6388: N95%CL = 21.6 - M15: N95%CL = 11.5 HESS DM search towards NGC6388 & M15 ICRC 2011, Beijing

Dark matter modeling of M15 initial DM halo with NFW profile (Mvir = 107 Msol, c=50) adiabatic compression of dark matter following baryon collapse in core (as in Woods et al 2008). heating of DM in core by baryons on timescale Th=104 years Final DM density in core density (Msol/pc3) Woods et al 2007 rheat radius (pc) HESS DM search towards NGC6388 & M15 ICRC 2011, Beijing

Heating of dark matter by stars (1) Kinematical heating of DM by stars (Merritt ‘04, Merritt ‘07) → depletion of DM in the central region of globular clusters - heating time given by :  Case of M4 (Bertone & Fairbarn PRD 2008): - DM compression - but Tr << TGC ~1010 yr old The authors assumed a maximum density of 50 Msunpc-3 density up to the radius where the timescale for heating of DM becomes larger than the age of the Universe For M15, Tr ~ 104 yr → maximum density of 25 Msunpc-3 HESS DM search towards NGC6388 & M15 ICRC 2011, Beijing

Dark matter exclusion limits for M15 Excluded (95% CL) HESS limit Same (Woods et al) DM halo modelling HESS DM search towards NGC6388 & M15 ICRC 2011, Beijing

Heating of dark matter by stars (2) DM profile steepening by the adiabatic growth of a central black hole (Gondolo & Silk ’99) → spike in the DM profile If presence of a central black hole → DM final density  r-3/2 inside the black hole influence radius rh after a few Tr (Merritt ‘07) For NGC6388, Tr << 1010 yr - DM ~ 90 Msunpc-3 for rh < r < rheat - for r ≤ rh : - for r > rheat , DM density unaffected Merritt et al, 2007 PRD 75, 043517 HESS DM search towards NGC6388 & M15 ICRC 2011, Beijing

Dark matter halo profiles for NGC6388 Initial NFW profile: Mvir = 107 Msun DM contraction by baryon infall Adiabatic growth of the BH for r ≤ rh But Heating of DM by stars - for rh ≤ r ≤ rheat ~ 5pc : max. DM density ~ 90 Msun pc-3 -for r <rh : Observed stellar density → HESS PSF rheat rh HESS DM search towards NGC6388 & M15 ICRC 2011, Beijing

Dark matter exclusion limits for NGC6388 Excluded (95% CL) HESS limit HESS DM search towards NGC6388 & M15 ICRC 2011, Beijing

Conclusion No gamma-ray signal at the target position of NGC6388 and M15 Dark matter halo modelling : - DM adiabatic contraction by baryons - DM profile steepening by adiabatic contraction by a central BH - DM depletion by kinematical scattering of stars on DM Dark matter constraints on NGC6388 and M15: - NGC6388: <v> at the level of a few 10-25 cm3s-1 for the final DM profile - M15: <v> of a few 10-24 cm3s-1 for the final DM profile only slightly better than Whipple limit HESS DM search towards NGC6388 & M15 ICRC 2011, Beijing