EGRET Upper Limits and Stacking Searches of Gamma-Ray Observations of Luminous and Ultraluminous Infrared Galaxies

We present a stacking analysis of EGRET γ-ray observations at the positions of luminous and ultraluminous infrared galaxies. The latter were selected from the recently presented HCN survey, which is thought to contain the most active star-forming regions of the universe. Different sorting criteria are used, and since there is no positive collective detection of γ-ray emission from these objects, we determined both collective and individual upper limits. The uppermost excess we find appears in the case of ULIRGs ordered by redshift, at a value of 1.8 σ.