[Paper Review] The CALIFA survey across the Hubble sequence: Spatially resolved stellar population properties in galaxies

R. M. González Delgado, R. García-Benito+33 more|arXiv (Cornell University)|Jun 12, 2015

Galaxies: Formation, Evolution, PhenomenaPhysics and Astronomy135 references148 citations

TL;DR

This study uses spatially resolved integral field spectroscopy from the CALIFA survey to analyze stellar population properties—age, metallicity, mass surface density, and extinction—across 300 galaxies spanning all Hubble types. It reveals that galaxy morphology, not just stellar mass, governs quenching and structural gradients, with inside-out growth evident in spiral galaxies and major mergers shaping early-type galaxies' shallow metallicity gradients.

ABSTRACT

This paper characterizes the radial structure of stellar population properties of galaxies in the nearby universe, based on 300 galaxies from the CALIFA survey. The sample covers a wide range of Hubble types, and galaxy stellar mass. We apply the spectral synthesis techniques to recover the stellar mass surface density, stellar extinction, light and mass-weighted ages, and mass-weighted metallicity, for each spatial resolution element in our target galaxies. To study mean trends with overall galaxy properties, the individual radial profiles are stacked in seven bins of galaxy morphology. We confirm that more massive galaxies are more compact, older, more metal rich, and less reddened by dust. Additionally, we find that these trends are preserved spatially with the radial distance to the nucleus. Deviations from these relations appear correlated with Hubble type: earlier types are more compact, older, and more metal rich for a given mass, which evidences that quenching is related to morphology, but not driven by mass. Negative gradients of ages are consistent with an inside-out growth of galaxies, with the largest ages gradients in Sb-Sbc galaxies. Further, the mean stellar ages of disks and bulges are correlated, with disks covering a wider range of ages, and late type spirals hosting younger disks. The gradients in stellar mass surface density depend mostly on stellar mass, in the sense that more massive galaxies are more centrally concentrated. There is a secondary correlation in the sense that at the same mass early type galaxies have steeper gradients. We find mildly negative metallicity gradients, shallower than predicted from models of galaxy evolution in isolation. The largest gradients occur in Sb galaxies. Overall we conclude that quenching processes act in manners that are independent of mass, while metallicity and galaxy structure are influenced by mass-dependent processes.

Motivation & Objective

• To characterize radial gradients in stellar population properties across diverse galaxy types in the local universe.
• To disentangle the roles of stellar mass and morphology in shaping galaxy evolution.
• To test theoretical models of galaxy formation using high-resolution spatially resolved data.
• To investigate the connection between quenching, metallicity gradients, and structural properties in early and late-type galaxies.
• To assess the impact of feedback and merger histories on observed radial profiles of age, metallicity, and mass surface density.

Proposed method

• The CALIFA survey provides spatially resolved, high signal-to-noise ratio integral field spectroscopy for 300 galaxies across the Hubble sequence.
• Spectral synthesis techniques are applied to derive stellar population parameters at each spatial resolution element: stellar mass surface density (μ⋆), extinction (AV), light- and mass-weighted ages, and mass-weighted metallicity (⟨log Z⋆⟩M).
• Radial profiles of these properties are extracted and stacked in seven Hubble type bins (E to Sd) to identify mean trends.
• The fossil record method is used to reconstruct the star formation and chemical enrichment history from observed spectra.
• Results are compared with cosmological simulations and theoretical models of galaxy evolution, including RaDES and Mollá & Díaz (2005) models.
• Statistical stacking allows robust detection of radial gradients independent of individual galaxy noise and resolution effects.

Experimental results

Research questions

• RQ1 How do stellar population properties (age, metallicity, mass surface density, extinction) vary radially across the Hubble sequence?
• RQ2 To what extent is galaxy quenching driven by stellar mass versus morphology?
• RQ3 What is the origin of observed metallicity and age gradients in spiral and early-type galaxies?
• RQ4 How do observed radial profiles compare with predictions from cosmological simulations and chemical evolution models?
• RQ5 What role do major and minor mergers play in shaping the central regions of early-type galaxies?

Key findings

• More massive galaxies are more compact, older, more metal-rich, and less dusty, with these trends preserved radially across all Hubble types.
• Earlier-type galaxies (E/S0) are more compact, older, and more metal-rich than later types at the same stellar mass, indicating morphology-driven quenching independent of mass.
• Negative light-weighted age gradients (inside-out growth) are strongest in Sb–Sbc galaxies, with mild or flat gradients beyond 2 HLR, suggesting uniform star formation or significant stellar migration.
• Stellar mass surface density gradients depend primarily on stellar mass, with more massive galaxies more centrally concentrated; early types show steeper gradients at same mass, indicating morphology's secondary role.
• Metallicity gradients are mildly negative and shallower than predicted by isolated evolution models, with the largest gradients in Sb–Sbc galaxies, consistent with Galactic disk measurements and RaDES simulations.
• Extinction decreases outward in all spirals, independent of mass, and is redder than in early-type galaxies, but cannot reliably distinguish Hubble types.