Publication: A duality in the origin of bulges and spheroidal galaxies
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2021-06
Authors
Costantin, Luca
Méndez Abreu, Jairo
Huertas Company, Marc
Dimauro, Paola
Buitrago, Fernando
Ceverino, Daniel
Daddi, Emanuele
Domínguez Sánchez, Helena
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IOP Publishing
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Abstract
Studying the resolved stellar populations of the different structural components which 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* > 10^(10) Mסּ ) in the GOODS-N field. We take advantage of the spectral and spatial information provided by SHARDS and HST data to perform the multi-image spectro-photometrical 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 SEDs, we find evidences 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_(form) = 6.2^(+1.5)_(−1.7). 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_(form) = 1.3^(+0.6)_(−0.6). The oldest (1st-wave) bulges are more compact than the youngest. Virtually all pure spheroids (i.e., those without any disk) are coetaneous with the 2nd-wave bulges, presenting a median redshift of formation z_(form) = 1.1^(+0.3)_(−0.3). The two waves of bulge formation are not only distinguishable in terms of stellar ages, but also in star formation mode. All 1st-wave bulges formed fast at z ∼ 6, with typical timescales around 200 Myr. A significant fraction of the 2nd-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.
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© 2021. The American Astronomical Society. We would like to thank the anonymous referee for improving the content of the manuscript. LC wishes to thank Cristina Cabello for the support provided while this project was devised and Michele Perna for the useful discussion. LC acknowledges financial support from Comunidad de Madrid under Atracción de Talento grant 2018-T2/TIC11612. LC and PGPG acknowledge support from Spanish Ministerio de Ciencia, Innovación y Universidades through grant PGC2018-093499-B-I00. JMA acknowledges support from the MCIU by the grant AYA2017- 83204-P and the Programa Operativo FEDER Andalucía 2014-2020 in collaboration with the Andalucian Office for Economy and Knowledge. DC is a RamonCajal Researcher and is supported by the Ministerio de Ciencia, Innovación y Universidades (MICIU/FEDER) under research grant PGC2018-094975-C21. This work has made use of the Rainbow Cosmological Surveys Database, which is operated by the Centro de Astrobiología (CAB/INTA), partnered with the University of California Observatories at Santa Cruz (UCO/Lick,UCSC).