Are passive red spirals truly passive? The current star formation activity of optically red disc galaxies

We used GALEX ultraviolet and WISE 22 mu m observations to investigate the current star formation activity of the optically red spirals recently identified as part of the Galaxy Zoo project. These galaxies were accurately selected from the Sloan Digital Sky Survey as pure discs with low or no current star formation activity, representing one of the best optically selected samples of candidate passive spirals. However, we show that these galaxies are not only still forming stars at a significant rate (greater than or similar to 1 M-circle dot yr(-1)) but, more importantly, their star formation activity is not different from that of normal star-forming discs of the same stellar mass (M-* greater than or similar to 10(10.2) M-circle dot). Indeed, these systems lie on the UV-optical blue sequence, even without any corrections for internal dust attenuation, and they follow the same specific star formation rate vs. stellar mass relation of star-forming galaxies. Our findings clearly show that at high stellar masses, optical colours do not allow to distinguish between actively star-forming and truly quiescent systems.