Structural analysis of massive galaxies using HST deep imaging at z < 0.5

¹ Instituto de Astrofísica e Ciências do Espaço, Universidade de Lisboa, OAL, Tapada da Ajuda, 1349-018 Lisboa, Portugal
² Departamento de Física, Faculdade de Ciências da Universidade de Lisboa, Edifício C8, Campo Grande, 1749-016 Lisboa, Portugal
e-mail: sreis@oal.ul.pt
³ Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
⁴ Departamento de Física e Astronomia, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre 687, 4169-007 Porto, Portugal

Received: 9 July 2019
Accepted: 26 November 2019

Abstract

Context. The most massive galaxies (M_stellar ≥ 10¹¹ M_⊙) in the local Universe are characterized by a bulge-dominated morphology and old stellar populations, in addition to being confined to a tight mass-size relation. Identifying their main components can provide insights into their formation mechanisms and subsequent mass assembly.

Aims. Taking advantage of Hubble Space Telescope (HST) CANDELS data, we analyze the lowest redshift (z <  0.5) massive galaxies in the H and I band in order to disentangle their structural constituents and study possible faint non-axisymmetric features.

Methods. Our final sample consists of 17 massive galaxies. Due to the excellent HST spatial resolution for intermediate redshift objects, they are hard to model by purely automatic parametric fitting algorithms. We performed careful single and double (bulge-disk decompositions) Sérsic fits to their galaxy surface brightness profiles. We compare the model color profiles with the observed ones and also derive multi-component global effective radii attempting to obtain a better interpretation of the mass-size relation. Additionally, we test the robustness of our measured structural parameters via simulations.

Results. We find that the Sérsic index does not offer a good proxy for the visual morphological type for our sample of massive galaxies. Our derived multi-component effective radii give a better description of the size of our sample galaxies than those inferred from single Sérsic models with GALFIT. Our galaxy population lies on the scatter of the local mass-size relation, indicating that these massive galaxies have not experienced a significant growth in size since z ∼ 0.5. Interestingly, the few outliers are late-type galaxies, indicating that spheroids must reach the local mass-size relation earlier. For most of our sample galaxies, both single- and multi-component Sérsic models with GALFIT show substantial systematic deviations from the observed surface brightness profiles in the outskirts. These residuals may be partly due to several factors, namely a nonoptimal data reduction for low surface brightness features or the existence of prominent stellar haloes for massive galaxies, or they could also arise from conceptual shortcomings of parametric 2D image decomposition tools. They consequently propagate into galaxy color profiles. This is a significant obstacle to the exploration of the structural evolution of galaxies, which calls for a critical assessment and refinement of existing surface photometry techniques.