Connecting Chemical Enrichment with Resolved Star Formation Histories

Abstract

We present a new framework for modeling the chemical enrichment histories of galaxies by integrating the chemical evolution with resolved star formation histories (SFHs) derived from color-magnitude diagrams. This novel approach links the time evolution of the metallicity of the star-forming ISM to the cumulative stellar mass formed in the galaxy, enabling a physically motivated, self-consistent description of chemical evolution. We apply this methodology to four isolated, gas-rich Local Group dwarf galaxies -- WLM, Aquarius, Leo A, and Leo P -- using deep HST and JWST imaging. For WLM, Aquarius, and Leo A, we independently validate our metallicity evolution results using ages and metallicities of individual red giant stars with spectroscopic measurements, finding good agreement. We quantify systematic uncertainties by repeating our analysis with multiple stellar evolution and bolometric correction libraries. We then compare the observed chemical enrichment histories to predictions from the TNG50 and FIREbox cosmological hydrodynamic simulations and the Galacticus semi-analytic model. We find that the enrichment history of WLM is best reproduced by the FIREbox simulation, while TNG50 and Galacticus predict higher metallicities at early times. Our results suggest that differences in stellar feedback and metal recycling prescriptions drive significant variation in the predicted chemical enrichment of dwarf galaxies, particularly at early times. This work demonstrates the power of combining resolved SFHs with physically motivated chemical evolution models to constrain galaxy formation physics and highlights the need for further observational and theoretical studies of metal retention and recycling in low-mass dwarf galaxies.

Figures (14)

• Figure 1: Comparison of [Fe/H] measurements of single red giants from Aquarius, WLM, and Leo A. The vertical axis shows the spectroscopic [Fe/H] measurements, which are used as a prior when measuring the metallicity and ages from the photometry in § \ref{['sec:ages']}. The resulting photometrically-measured [Fe/H] values are shown on the horizontal axis. The line where [Fe/H]$_\text{spec} =$ [Fe/H]$_\text{phot}$ is overplotted for comparison. Agreement between the two is generally excellent, although the photometric technique prefers marginally higher metallicities for stars with [Fe/H]$_\text{spec} < -1.8$ dex.
• Figure 2: Comparison of the Hess diagram of WLM with our best-fit model. (a) Hess diagram of WLM constructed from the JWST/NIRCam photometric catalog of Weisz2024. (b) Best-fit model Hess diagram using our MZH metallicity evolution model with PARSEC stellar tracks and YBC bolometric corrections. (c) Residual between the observed Hess diagram and the best-fit model in raw star counts. (d) Residual between the observed Hess diagram and the best-fit model in units of standard deviations (i.e., the residual significance).
• Figure 3: Top row: The cumulative SFHs (left) and AMRs (right) of WLM derived from the JWST/NIRCam imaging. Included are measurements with a linear AMR (green line) and with our hierarchical MZH model (orange line), both of which assume the PARSEC stellar models (§ \ref{['subsec:stellar_tracks']}) with the YBC bolometric corrections (§ \ref{['subsec:bcs']}). For comparison we also show the result of McQuinn2024 (blue line), who used the match code on the same data. The systematic uncertainty on the MZH SFH and metallicity evolution is shown as a light shaded region (see § \ref{['sec:systematics']} for more details). The (much smaller) random uncertainty is shaded more darkly. Overplotted on the right panel are age and metallicity measurements for individual red giant stars obtained by combining the spectroscopic metallicities of Leaman2009 with our photometry (see § \ref{['sec:ages']}). Bottom row: The six individual measurements of the SFH of WLM under the MZH model for different combinations of stellar tracks (PARSEC, MIST, and BaSTIv2) and bolometric corrections (MIST, YBC) that were used to define the systematic uncertainty region in the upper row. The PARSEC and BaSTIv2 tracks give fairly consistent results, while the MIST stellar tracks prefer more early star formation. The measurements under the MIST bolometric corrections prefer higher present-day metallicities (by $\sim0.1$ dex) compared to the solutions with the YBC bolometric corrections.
• Figure 4: Comparison of the Hess diagram of Aquarius with our best-fit model. The panels follow the same layout as in Figure \ref{['fig:wlm_hess']}.
• Figure 5: The results of our resolved SFH and metallicity evolution fit for Aquarius based on the HST/ACS data following the same format as Figure \ref{['fig:wlm_sfh']}. In the top row the result of McQuinn2024b (blue line), who used the match code on the same data, is shown for comparison. Overplotted on the upper right panel are age and metallicity measurements for individual red giant stars obtained by combining the spectroscopic metallicities of HermosaMunoz2020 (HM+20, red) and Kirby2017a (K+17, blue) with our photometry (see § \ref{['sec:ages']}).