Defense
Student: Geraldo Gonçalves dos Santos Junior
Program: Astronomy
Title: "A study of young stellar populations in spiral galaxies with TIMER"
Advisor: Profa. Dra. Paula Rodrigues Teixeira Coelho
Judging Comitee:
- Profa. Dra. Paula Rodrigues Teixeira Coelho (Chair and Advisor)
- Prof. Dr. Dimitri Alexei Gadotti - Durham University (videoconference)
- Dra. Camila de Sá Freitas - ESO Chile (videoconference)
- Prof. Dr. Ricardo Piorno Schiavon - Liverpool John Moores University (videoconference)
- Profa. Dra. Marina Trevisan - UFRGS (videoconference)
- Profa. Dra. Lucimara Pires Martins - UNICID
Other Members:
- Prof. Dr. Marcos Perez Diaz - IAG/USP
- Profa. Dra. Beatriz Leonor Silveira Barbuy - IAG/USP
- Profa. Dra. Maria Luisa Gomes Buzzo - Yale University
- Profa. Dra. Marie Martig - Liverpool John Moores University
- Prof. Dr. Laerte Sodré Junior - IAG/USP
- Prof. Dr. Roberto Cid Fernandes Junior - UFSC
Abstract:
Stellar populations in the central regions of disk galaxies offer essential clues to the formation and evolutionary pathways of galactic structures. Bars, nuclear rings, inner disks, and pseudo-bulges are all shaped by the dynamical redistribution of angular momentum, a process that channels gas toward the central kiloparsec and triggers recurrent episodes of star formation. Understanding the ages, metallicities, and star formation histories of the stars that reside in these environments is therefore fundamental for constraining how secular evolution operates across cosmic time. To achieve this, modern studies rely on stellar population synthesis models, which translate integrated-light spectra into physical properties such as age and chemical composition. These models vary widely in their ingredients—stellar evolutionary tracks, spectral libraries, and interpolation schemes—making it crucial to evaluate how different assumptions shape the inferred properties of young stellar populations. In this work, we investigate the young stellar populations in the central regions of barred galaxies from the Local Universe as observed by the TIMER survey. Our analysis employs four simple stellar population (SSP) model suites: two based on empirical stellar libraries (MILES and X-Shooter) and two based on theoretical libraries (Starburst99 and HR-pyPopStar). To extract stellar population parameters, we use full spectral fitting with pPXF, adopting regularization to recover smooth star formation histories and masking nebular lines when appropriate. We complement these stellar metallicities with independently derived gas-phase metallicities obtained from strong-line emission diagnostics, allowing a direct comparison between the chemical compositions of the ionized gas and the young stars presumably formed from it. Our results show that inferred stellar metallicities depend strongly on the choice of stellar population model. Empirical-library models systematically yield extremely metal-poor young populations—up to 2 dex below the metallicities of their surroundings—which is physically implausible. In contrast, the theoretical models produce more moderate variations: Starburst99 typically yields metallicities differing by only ~0.5 dex relative to the gas phase, consistent with expectations from chemical evolution. Indeed, the gas-phase metallicities themselves are lower in the central star-forming regions, supporting a picture in which bars funnel metal-poor gas inward from the outer disk. However, the magnitude of these inflows cannot account for the extreme metallicities recovered by empirical-library models, reinforcing the conclusion that such models introduce substantial biases for very young stellar populations. Despite these systematic discrepancies, all galaxies in our sample show clear evidence of young stellar populations in their central regions, often arranged in nuclear rings or compact inner disks. Their age–metallicity distributions reveal two chemically distinct components: an older population likely predating the formation of the bar, and a younger population formed from bar-driven inflows with a different enrichment history. Taken together, our findings show that while the qualitative presence of young stars is robust across models, the inferred metallicities are highly model-dependent. Empirical-library models tend to produce unrealistically low values, whereas theoretical models yield results that are both astrophysically plausible and consistent with the gas metallicities.
Keywords: galaxies, abundances, galaxy evolution, star formation