© 2021. The American Astronomical SocietyConstantin, L.Pérez González, P. G.Méndez Abreu, J.Huertas Company, M.Dimauro, P.Alcalde Pampliega, B.Buitrago, F.Ceverino, D.Daddi, E.Domínguez Sánchez, H.Espino Briones, N.Hernán Caballero, A.Koekemoer, A. M.Rodighiero, G.2022-02-152022-02-152021-06-02The Astrophysical Journal 913(2): 125(2021)0004-637Xhttps://iopscience.iop.org/article/10.3847/1538-4357/abef72http://hdl.handle.net/20.500.12666/605Studying the resolved stellar populations of the different structural components that build massive galaxies directly unveils their assembly history. We aim at characterizing the stellar population properties of a representative sample of bulges and pure spheroids in massive galaxies (M⋆ > 1010 M⊙) in the GOODS-N field. We take advantage of the spectral and spatial information provided by SHARDS and Hubble Space Telescope data to perform the multi-image spectrophotometric decoupling of the galaxy light. We derive the spectral energy distribution separately for bulges and disks in the redshift range 0.14 < z ≤ 1 with spectral resolution R ∼ 50. Analyzing these spectral energy distributions, we find evidence of a bimodal distribution of bulge formation redshifts. We find that 33% of them present old mass-weighted ages, implying a median formation redshift ${z}_{\mathrm{form}}={6.2}_{-1.7}^{+1.5}$. They are relics of the early universe embedded in disk galaxies. A second wave, dominant in number, accounts for bulges formed at median redshift ${z}_{\mathrm{form}}={1.3}_{-0.6}^{+0.6}$. The oldest (first-wave) bulges are more compact than the youngest. Virtually all pure spheroids (i.e., those without any disk) are coetaneous with the second-wave bulges, presenting a median redshift of formation ${z}_{\mathrm{form}}={1.1}_{-0.3}^{+0.3}$. The two waves of bulge formation are distinguishable not only in terms of stellar ages but also in star formation mode. All first-wave bulges formed fast at z ∼ 6, with typical timescales around 200 Myr. A significant fraction of the second-wave bulges assembled more slowly, with star formation timescales as long as 1 Gyr. The results of this work suggest that the centers of massive disk-like galaxies actually harbor the oldest spheroids formed in the universe.engAttribution-NonCommercial-NoDerivatives 4.0 Internationalhttps://creativecommons.org/licenses/by-nc-nd/4.0/Galaxy bulgesGalaxy structureGalaxy disksGalaxy spheroidsGalaxy formationGalaxy evolutionGalaxy quenchingGalaxy stellar contentGalaxy propertiesGalaxy agesinfo:eu-repo/semantics/article10.3847/1538-4357/abef721538-4357info:eu-repo/semantics/openAccess