Not so dead after all: young stars in the massive S0 NGC 3115
Abstract
Context: Massive early-type galaxies (ETGs) are traditionally considered "red-and-dead" based on optical observations. However, ultraviolet (UV) spectroscopy can uncover a hidden population of young stars with a sub-one-per cent mass fraction. Aims: We measure the young stellar mass fraction in NGC 3115, a massive isolated lenticular galaxy, to test the ubiquity and spatial distribution of this component beyond previously studied ellipticals. Methods: Using SOAR/Goodman spectra and line-strength fitting, we modelled the star-formation history with a single bulk stellar population plus a young component (constant star-formation over the last 1 Gyr). We used ultranest to derive posterior distributions for age, metallicity, [Mg/Fe], and the young stellar mass fraction. Results: By tracking spatial variations in U-band indices, we found the young stellar mass fraction peaks at 0.69+0.09−0.15% in the centre, dropping to zero beyond 1 kpc. This young component correlates with kinematic structures, dominating in the pressure-supported region. We interpret this as residual central star formation fueled by internal processes, such as recycled stellar material. Alternative UV-bright models (e.g., extreme horizontal branch stars) yielded poorer fits and unphysically high mass fractions. Conclusions: Young stars in old, "red-and-dead" galaxies are more common than expected. Their presence across various ETGs with similar gradients indicates that residual star formation is a ubiquitous feature, offering key constraints on the baryon cycle in these galaxies.
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BibTeX
@misc{benedetti2026dead,
title = {{Not so dead after all: young stars in the massive S0 NGC 3115}},
author = {João P. V. Benedetti and Alexandre Vazdekis and Anna Ferré-Mateu and Elham Eftekhari and Alberto Rodríguez-Ardila and Rogério Riffel},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.37774},
eprint = {2610.37774},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.37774},
note = {CC BY-NC-SA 4.0},
}
RIS
TY - UNPB
TI - Not so dead after all: young stars in the massive S0 NGC 3115
AU - João P. V. Benedetti
AU - Alexandre Vazdekis
AU - Anna Ferré-Mateu
AU - Elham Eftekhari
AU - Alberto Rodríguez-Ardila
AU - Rogério Riffel
PY - 2026
DA - 2026/10/27
PB - arXiv
DO - 10.48550/arxiv.2610.37774
UR - https://arxiv.org/abs/2610.37774
AB - Context: Massive early-type galaxies (ETGs) are traditionally considered "red-and-dead" based on optical observations. However, ultraviolet (UV) spectroscopy can uncover a hidden population of young stars with a sub-one-per cent mass fraction. Aims: We measure the young stellar mass fraction in NGC 3115, a massive isolated lenticular galaxy, to test the ubiquity and spatial distribution of this component beyond previously studied ellipticals. Methods: Using SOAR/Goodman spectra and line-strength fitting, we modelled the star-formation history with a single bulk stellar population plus a young component (constant star-formation over the last 1 Gyr). We used ultranest to derive posterior distributions for age, metallicity, [Mg/Fe], and the young stellar mass fraction. Results: By tracking spatial variations in U-band indices, we found the young stellar mass fraction peaks at 0.69^+0.09_−0.15% in the centre, dropping to zero beyond 1 kpc. This young component correlates with kinematic structures, dominating in the pressure-supported region. We interpret this as residual central star formation fueled by internal processes, such as recycled stellar material. Alternative UV-bright models (e.g., extreme horizontal branch stars) yielded poorer fits and unphysically high mass fractions. Conclusions: Young stars in old, "red-and-dead" galaxies are more common than expected. Their presence across various ETGs with similar gradients indicates that residual star formation is a ubiquitous feature, offering key constraints on the baryon cycle in these galaxies.
ER -