The Hidden Story of Chemical Evolution in Local Star-Forming Nuclear Rings

TL;DR

This study tackles how chemical evolution in nuclear star-forming rings is inferred from integrated light by separating young and old stellar populations using full spectral fitting of VLT/MUSE data. It introduces a physically meaningful metallicity definition, Z_{ ext{phys}}, and a young/old age split to overcome biases inherent in luminosity- or mass-weighted metallicities, demonstrated across four barred galaxies with prominent circumnuclear rings. The analysis shows that young stars in the rings are typically solar to supersolar in metallicity, while the old population exhibits a broad metallicity range shaped by long-term inflow of metal-poor gas; dust lanes traced by extinction maps support the inflow scenario. Crucially, the work reveals that omitting very young templates biases metallicity inferences, underscoring the need for a comprehensive age grid. Collectively, the results provide a galaxy-by-galaxy narrative of circumnuclear chemical evolution and establish a robust framework for studying nuclear rings with IFU data.

Abstract

A VLT/MUSE population synthesis study of metallicities in the nuclear star-forming rings of four disk galaxies (NGC 613, NGC 1097, NGC 3351, NGC 7552) is presented. Disentangling the spectral contributions of young and old stellar populations, we find a large spread of ages and metallicities of the old stars in the nuclear rings. This indicates a persistent infall of metal-poor gas and ongoing episodic star formation over many gigayears. The young stars have metallicities a factor two to three higher than solar in all galaxies except NGC 3351, where the range is from half to twice solar. Previously reported detections of extremely metal poor regions at young stellar age on the rings of these four galaxies are a methodological artifact of the average over all stars, young and old. In addition, it is important to include contributions of very young stars ($<6$ Myr) in this environment. For each of the four galaxies, the extinction maps generated through our population synthesis analysis provide support for the infall scenario. They reveal dust lanes along the leading edges of the stellar bars, indicating the flow of interstellar material towards the circumnuclear zone. Prominent stellar clusters show little extinction, most likely because of the onset of stellar winds. Inside and on the nuclear rings, regions that are largely free of extinction are detected.

Figures (13)

• Figure 1: First column: B-Band images from the CTIO 1.5m telescope (NGC 7552), CTIO 0.9m telescope (NGC 613), du Pont 2.5m telescope (NGC 1097) and CTIO 1m telescope (NGC 3351). The MUSE FOV used for our FSF fit is marked in white. North is to the top, east is to the left. The following columns show the FOV with results from the FSF fit: mean light-weighted Age, visual extinction $A_V$ and light-weighted total metallicity [Z]$^{\mathrm{lw}}$. The color bar at the top holds for all the subplots in the column.
• Figure 2: Metallicity difference $\Delta [Z]$ between individual stellar probes and population synthesis (see Section \ref{['M83_test']}) as a function of the luminosity fraction b$_y$ of young stars (top) and the angular distance from the center (bottom). The different symbols represent differences with respect to BSG (blue circles), SSC (red triangles), YMC with optical analysis (dark green squares), YMC with UV analysis (light green circles). Errors result from the addition of stellar source and population synthesis errors in quadrature. The shaded gray strip indicates a difference $\leq 0.1$ dex and is added for orientation.
• Figure 3: The central region of NGC 3351: (top) map of mean ages of the young stellar population; (bottom) interstellar reddening E(B-V). The circles indicate the locations of the three most prominent young stellar clusters highlighted in Fig. 24 of Emsellem2022 and discussed in the text.
• Figure 4: Mean ages of the old stellar population as a function of galactocentric distance. The nuclear star forming regions are indicated in gray. From top to bottom: NGC 7552, NGC 613, NGC 1097, NGC 3351.
• Figure 5: Metallicity maps of the young stellar population (top) and radial metallicity distribution including uncertainties (bottom).