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Authors
Abstract(s)
The most massive (Mstellar > 1011M_) galaxies of the Universe experience a dramatic evolution in their observational properties (size, morphology, kinematics, star formation rates) across cosmic time. This metamorphosis, which implies an accelerated evolution of this galaxy population in comparison with lower mass galaxies, is not well understood.
This thesis explores the structural properties and spectroscopic evolution of massive galaxies in the nearby Universe (z < 0:5) by conducting two different but complementary studies: one employing surface photometry using deep photometric data from the Hubble Space Telescope (HST), and another one exploiting spectral synthesis using state-of-the-art integral field spectroscopy (IFS) from MUSE.
The first study makes use of HST CANDELS data to analyze a sample of 17 nearby (z < 0:5) massive (Mstellar > 1011M_) galaxies, with the aim to disentangle the structural components of these systems and study possible faint non-axissymmetric features. The excellent HST spatial resolution for intermediate redshift objects justifies a detailed structural analysis that combines two parametric components for the disk and bulge, instead of a purely automatic application of one single Sérsic model. Therefore careful single Sérsic fits and bulge-disk decompositions are performed to model the galaxy surface brightness profiles. The model color profiles are compared with the observed ones, and multi-component global effective radii are derived in order to obtain more accurate measurements of the size of galaxies and their position on the mass-size relation. It was found that the Sérsic index does not offer a good proxy for the visual morphology of our sample of massive galaxies. The derived multi-component effective radii offer a better description of the size of the sample galaxies than those inferred from single Sérsic models with GALFIT. The galaxies in our sample lie on the scatter of the local masssize relation, indicating that these massive objects have not experienced a significant growth in size since z ~ 0.5. Interestingly, the few outliers in the local mass-size relation are late-type galaxies, suggesting that spheroids must have reached the local mass-size relation earlier. For most of our sample, both single and multi-component Sérsic models retrieved with GALFIT show considerable systematic deviations from the observed surface brightness profiles in the galaxy peripheries that consequently propagate into significant uncertainties in the Sérsic fit parameters. In several cases they also indicate that colors inferred from Sersic fits show strong discrepancies from the observed colors.
The second study explores the build-up process of massive galaxies by examining IFS MUSE data. We conduct a direct comparison between the spatially resolved stellar populations of the largest galaxy known in the Universe – IC1101 – and four other nearby early-type massive galaxies (ETGs), which represent the predominant morphological massive class in the nearby Universe. The IFS data were spatially-binned using Voronoi tesselation and modeled with our in-house spectral fitting pipeline Porto3D, enabling us to study the galaxy stellar populations, and focusing on the stellar age and metallicity radial profiles and their corresponding gradients. It was found a diversity in the radial distribution of equivalent width (EW) of H α) supporting a subdivision of ETGs into two groups: type i and type ii ETGs. The ones classified as type ii ETGs show an outwardly increasing EW(H α) whereas type i show a nearly constant EW(Hα) with values being consistent with gas photoionization by post-AGB stellar populations. All but one of the analyzed galaxies show LINER-specific spectroscopic properties out to several kpc from their nucleus. Old stellar ages (above 9 Gyr) and negative radial gradients are found for all galaxies, supporting an inside-out formation scenario. Stellar metallicities are over-solar for all galaxies with only a single exception. The main difference in the comparison of IC1101 with the rest of the sample lies on its positive stellar metallicity gradient. This result is atypical for ETGs in the low-redshift Universe and may be an indication of different evolutionary mechanisms for this galaxy. We tentatively propose plausible scenarios to explain the metal-enriched periphery of IC1101, being (i) a contribution of metal-rich intra-cluster stellar
light, and (ii) a self-enrichment by “wet” mergers of accreted satellites that fed star formation.
We also conjecture that this might partly be an artifact created by the astonishing large velocity dispersion determined in the outskirts of IC1101.
Description
Keywords
galáxias: evolução galáxias: estrutura galáxias: fotometria galáxias: elíptica e lenticular cD – galáxias: conteúdo estelar galaxies: evolution galaxies: structure galaxies: photometry galaxies: elliptical and lenticular cD – galaxies: stellar content