We investigate the nature of star-forming galaxies with reduced specific star formation rate ( sSFR ) and high stellar masses , those ‘ ‘ green valley ’ ’ objects that seemingly cause a reported bending , or flattening , of the star-forming main sequence .
The fact that such objects host large bulges recently led some to suggest that the internal formation of bulges was a late event that induced the sSFRs of massive galaxies to drop in a slow downfall , and thus the main sequence to bend .
We have studied in detail a sample of 10 galaxies at 0.45 < z < 1 with secure SFR from Herschel , deep Keck optical spectroscopy , and HST imaging from CANDELS allowing us to perform multi-wavelength bulge to disc decomposition , and to derive star formation histories for the separated bulge and disc components .
We find that the bulges hosted in these systems below main sequence are virtually all maximally old , with ages approaching the age of the Universe at the time of observation , while discs are young ( \langle T _ { 50 } \rangle \sim 1.5 Gyr ) .
We conclude that , at least based on our sample , the bending of the main sequence is , for a major part , due to rejuvenation , and we disfavour mechanisms that postulate the internal formation of bulges at late times .
The very old stellar ages of our bulges suggest a number density of Early Type galaxies at z = 1 - 3 higher than actually observed .
If confirmed , this might represent one of the first direct validations of hierarchical assembly of bulges at high redshifts .