EGRET upper limits and stacking searches of gamma-ray observations of luminous and ultra-luminous infrared galaxies

We present a stacking analysis of EGRET $\gamma$-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, whereas no positive collective detection of $\gamma$-ray emission from these objects we determined both collective and individual upper limits. The upper most excess we find appears in the case of ULIRGs ordered by redshift, at a value of 1.8$\sigma$.