Observations of the Hubble Deep Field with the Infrared Space Observatory .3. Source counts and P(D) analysis

We present source counts at 6.7 and 15 mu m from our maps of the Hubble Deep Field (HDF) region, reaching 38.6 mu Jy at 6.7 mu m and 255 mu Jy at 15 mu m. These are the first ever extragalactic number counts to be presented at 6.7 mu m, and are three decades fainter than IRAS at 12 mu m. Both source counts and a P(D) analysis suggest that we have reached the Infrared Space Observatory (ISO) confusion limit at 15 mu m: this will have important implications for future space missions. These data provide an excellent reference point for other ongoing ISO surveys. A no-evolution model at 15 mu m is ruled out at > 3 sigma, while two models which fit the steep IRAS 60-mu m counts are acceptable. This provides important confirmation of the strong evolution seen in IRAS surveys. One of these models can then be ruled out from the 6.7-mu M data.