Molecular content of a Type Ia supernova host galaxy at z=0.6

We study the properties and the molecular content of the host of a Type Ia supernova (SN1997ey). This z = 0.575 host is the brightest submillimetre source of the sample of Type Ia supernova hosts observed at 450 and 850 mu m by Farrah et al. Observations were performed at the Institut de Radioastronomie Millimetrique 30 m telescope (IRAM 30 m) to search for CO(2-1) and CO(3-2) lines in good weather conditions but no signal was detected. The star formation rate cannot exceed 50 M circle dot yr(-1). These negative results are confronted with an optical analysis of a Keck spectrum and other data archives. We reach the conclusion that this galaxy is a late-type system (0.7L(*)(B)), with a small residual star formation activity (0.2 M-circle dot yr(-1)) detected in the optical. No source of heating (active galactic nucleus or starburst) is found to explain the submillimetre-continuum flux and the non-CO detection excludes the presence of a large amount of cold gas. We thus suggest that either the star formation activity is hidden in the nucleus (with A(V) similar to 4) or this galaxy is passive or anemic, and this flux might be associated with a background galaxy.