The main sequence of star-forming galaxies-I. the local relation and its bending

By using a set of different star formation rate (SFR) indicators, including Wide-field Infrared Survey Explorer (WISE) mid-infrared and H a emission, we study the slope of the main sequence (MS) of local star-forming galaxies at stellar masses larger than 1010 M. The slope of the relation strongly depends on the SFR indicator used. In all cases, the local MS shows a bending at high stellar masses with respect to the slope obtained in the low-mass regime. While the distribution of galaxies in the upper envelope of the MS is consistent with a lognormal distribution, the lower envelope shows an excess of galaxies, which increases as a function of the stellar mass but varies as a function of the SFR indicator used. The scatter of the best lognormal distribution increases with stellar mass from ~0.3 dex at 1010 M to ~0.45 at 1011 M. The MS high-mass end is dominated by central galaxies of group-sized haloes with a red bulge and a disc redder than the lower mass counterparts. We argue that the MS bending in this region is due to two processes: (i) the formation of a bulge component as a consequence of the increased merger activity in groups, and (ii) the cold gas starvation induced by the hot halo environment, which cuts off the gas inflow on to the disc. Similarly, the increase of the MS scatter at high stellar masses would be explained by the larger spread of star formation histories of central group and cluster galaxies with respect to lower mass systems.