2002A&A...381L...1E

We report the detection of the extremely red object (ERO), HR10 (I-K∼6.5, z=1.44), at 4.9 and 6.1µm (rest-frame) with ISOCAM, the mid-infrared (MIR) camera onboard the Infrared Space Observatory (ISO). HR10 is the first ERO spectroscopically identified to be associated with an ultra-luminous IR galaxy (ULIG) detected in the radio, MIR and sub-millimeter. The rest-frame spectral energy distribution (SED) of HR10 is amazingly similar to the one of Arp 220, scaled by a factor 3.8±1.3. The corresponding 8-1000µm luminosity (∼7x10¹²h₇₀^–2L_☉) translates into a star formation rate of about 1200 h₇₀^–2M_☉/yr if HR10 is mostly powered by star formation. We address the key issue of the origin of the powerful luminosity of HR10, i.e. starburst versus active galactic nucleus (AGN), by using the similarity with its closeby clone, Arp 220.