Abstract
Abstract Studying 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 . They are relics of the early universe embedded in disk galaxies. A second wave, dominant in number, accounts for bulges formed at median redshift . 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 . 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.
Highlights
Integrated or spatially-resolved observations are typically used to infer the stellar content of galaxies at low-redshift, reconstructing their star formation history (SFH) with an “archaeological” approach (Thomas et al 2005; Rogers et al 2010; Gonzalez Delgado et al 2015)
We focus our analysis of these spectral energy distribution (SED) in the characterization of the spheroids at the mentioned redshifts
In this work we investigate the assembly history and evolutionary pathways of the spheroidal structures within massive galaxies at redshift z ≤ 1
Summary
Integrated or spatially-resolved observations are typically used to infer the stellar content of galaxies at low-redshift, reconstructing their star formation history (SFH) with an “archaeological” approach (Thomas et al 2005; Rogers et al 2010; Gonzalez Delgado et al 2015). To trace back the formation of stars in the Universe, it is possible to compare the stellar content of similar samples of galaxies at different redshifts (the so-called “look-back” approach; Schiavon et al 2006; Sanchez-Blazquez et al 2009; Gallazzi et al 2014) In these studies, stellar population models with different SFHs are usually compared to the best model either obtained from the fit of the spectral energy distribution (SED) or derived using key spectral features which are sensitive to fundamental physical parameters such as age, metallicity, or α enhancement (Kriek et al 2011; Domınguez Sanchez et al 2016). The abundance of bulgeless galaxies decreases with redshift until almost the entire population presents a significant bulge component (Huertas-Company et al 2016)
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