Search for TeV gamma-ray emission from nearby starburst galaxies.

摘要

Observations of four starburst galaxies (SBGs), which are IC342, M81, M82, and NGC3079, have been conducted with the Whipple 10-m gamma-ray telescope from January 2001 to March 2004. A search was made for TeV gamma-ray radiation from cosmic ray (CR) interactions with the local ambient gas in the galaxies. SBGs are galaxies that have regions of intensive star formation often associated with high density interstellar medium (ISM) and a supernova (SN) rate 10 to 100 times that of the Milky Way. These regions are expected to have a high CR production and interaction rate, which may result in a large flux of gamma rays with energies between 100 MeV and 100 TeV. It has long been thought that CRs are accelerated in the strong shocks formed in supernova remnants (SNRs), producing large fluxes of gamma rays. However, the gamma-ray production mechanism in the detected SNRs has not been unambiguously identified with secondary neutral pion decay resulting from CR interactions, or with Inverse Compton (IC) scattering from ultra-relativistic electrons. SBGs provide another interesting site to test this SNR CR acceleration hypothesis. Selected SBGs are located just outside the Local Group, at a distance of a few Mpc, sufficiently close that attenuation by a distance factor will be small and the very high energy (VHE) gamma-ray flux may still be detectable. As part of this work, the fluxes of VHE gamma rays from a number of SBGs were estimated by extrapolation from the known CR spectrum and SN rate of the Milky Way. A new analysis method was developed allowing for the unknown spectral properties of the sources by utilizing a multidimensional maximal likelihood method (MLM). The results of applying this method and the standard Whipple analysis method to the observations are presented. No flux is detected, and an upper limit on the emission rate from each source is calculated. These limits constrain only the most extreme predictions. Predictions for repeated observations with a more sensitive, next-generation ground-based gamma-ray instrument, such as VERITAS, are made.