Rejuvenated galaxies with very old bulges at the origin of the bending of the main sequence and of the "green valley"

Chiara Mancini, Emanuele Daddi, Stéphanie Juneau, Alvio Renzini, Giulia Rodighiero, Michele Cappellari, Lucía Rodríguez-Muñoz, Daizhong Liu, Maurilio Pannella, Ivano Baronchelli, Alberto Franceschini, Pietro Bergamini, Chiara D'Eugenio, Annagrazia Puglisi

Published 2019-01-14Version 1

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 disk decomposition. 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 disks are young ($\langle$ T$_{50}\rangle \sim 1.5$ Gyr). We conclude that 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 (ETGs) 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.