ISOPHOT'S SERENDIPITY SURVEY UNVEILS AN UNUSUAL ULTRALUMINOUS INFRARED GALAXY

摘要

We present multi-wavelength observations of the unusual ultraluminous infrared galaxy (ULIRG) ISOSS J 15080+7248. The object was identified using the 170μm ISOPHOT Serendipity Survey (ISOSS) in the search for cold and luminous extragalactic far-infrared sources. The dust emission of ISOSS J 15080+7248 is associated with a giant elliptical galaxy (M_V = -22.6 mag) at redshift z = 0.214. ~(12)CO(1-0) line observations reveal a molecular gas mass of M_(H_2) =2·10~(10) M_☉ which is unusually high for such an elliptical. Peculiar is moreover the absence of any optical line emission from this luminous ( L_(IR) = 2·10~(12) L_☉) galaxy with a star formation rate of 350 M_☉ yr~(-1), as inferred from the bolometric infrared luminosity. Our observations contrast with the known population of local ULIRGs, which are virtually all advanced mergers of gas rich spirals and characterized by optical emission line spectra and warm dust emission due to their strong star formation activity. The undisturbed elliptical morphology, the presence of a de Vaucouleur r~(1/4) brightness profile and a low gas-to-dust ratio and dust temperature let us suggest that ISOSS J 15080+7248 is either a giant elliptical merging a gas rich spiral or a relaxed merger between two spirals on the transition into a massive elliptical system. Such sources might have been overlooked in surveys based on the IRAS data, which were biased towards the warm dust emission at the peak of star formation activity.