Evolutionary paths to and from the red sequence: star formation and H i properties of transition galaxies at z ∼ 0

We investigate the properties of galaxies between the blue and the red sequence (i.e. the transition region, 4.5 < NUV - H < 6 mag) by combining ultraviolet (UV) and near-infrared imaging to 21-cm H i line observations for a volume-limited sample of nearby galaxies. We confirm the existence of a tight relation between colour and H i fraction across all the range of colours, although outside the blue cloud this trend becomes gradually weaker. Transition galaxies are divided into two different families, according to their atomic hydrogen content. 'H i-deficient' galaxies are the majority of transition galaxies in our sample. They are found in high-density environments and all their properties are consistent with a quenching of the star formation via gas stripping. However, while the migration from the blue cloud is relatively quick (i.e. less than or similar to 1 Gyr), a longer amount of time (a few Gyr at least) seems required to completely suppress the star formation and reach the red sequence. At all masses, migrating 'H i-deficient' galaxies are mainly discs, implying that the mechanism responsible for today's migration in clusters cannot have played a significant role in the creation of the red sequence at high redshift. Conversely, 'H i-normal' transition galaxies are a more heterogeneous population. A significant fraction of these objects show star formation in ring-like structures and evidence for accretion/minor-merging events suggesting that at least part of the H i reservoir has an external origin. The detailed evolution of such systems is still unclear, but our analysis suggests that, in at least two cases, galaxies might have migrated back from the red sequence after accretion events. Interestingly, the H i available may be sufficient to sustain star formation at the current rate for several billion years. Our study clearly shows the variety of evolutionary paths leading to the transition region and suggests that the transition galaxies may not be always associated with systems quickly migrating from the blue to the red sequence.