Study of the origin and propagation of cosmic-rays in a starburst galaxy by observations of TeV gamma-rays

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We have tried to study the spatial distribution of TeV cosmic rays in the galaxy NGC 253 by observing TeV gamma-ray emission with Imaging Atmospheric Cerenkov Telescope. NGC 253 is located relatively near at 2.5 Mpc and is well known by its large radio halo extended to 〜10 kpc and is also known by its very high frequency of supernova occurrence. We observed the galaxy with 10 m telescope of CANGAROO and succeeded in detection of TeV gamma-rays with a statistical significance of 11 sigma. This is the first detection of TeV gamma-rays from normal galaxies other than our galaxy. The emission region is extended to distances much larger than the optical image of the galaxy. This emission region, the gamma-ray halo, is of the largest scale so far detected. The most probable emission mechanism is the inverse Compton scattering of the Cosmic Microwave Background Radiation by very high energy cosmic ray electrons. Hadronic model is unlikely due to insufficient amount of the interstellar material. We have confirmed by a numerical simulation that electrons can propagate out to the observed emission region without cooling by server synchrotron losses if the diffusion coefficient is larger by a factor of 100 than the Bohm diffusion limit.