Scylla at APOGEE: The Impact of Starbursts on the Chemical Evolution of the Magellanic Clouds

Abstract

Owing to their proximity to the Milky Way, the Large and Small Magellanic Clouds (L/SMC) uniquely probe the evolution of low-mass galaxies undergoing mutual interactions. In this work, we investigate the connection between the star formation histories (SFHs) of the L/SMC measured from HST imaging in the Scylla survey and APOGEE chemical abundances. We model the chemical evolution of the L/SMC in the [Mg/Fe]-[Fe/H] plane within a robust statistical framework to predict chemical abundance signatures resulting directly from starbursts in Scylla SFHs. Both the L/SMC rapidly enrich to high metallicity ([Fe/H] $\gtrsim$ $-1$) within 3 Gyr, followed by slower chemical evolution regulated by sequential starbursts, where the SMC may require higher Fe yields from Type Ia supernovae than the LMC. We also model the [Mg/Fe]-[Fe/H] plane to infer starburst properties across distinct spatial regions in the L/SMC. We identify dominant starbursts in the L/SMC with broadly similar timing, though the SMC may host an earlier burst, and larger burst strength in the LMC. The global starburst properties are nearly uniform across the LMC disk, whereas the dominant SMC population experiences a stronger and later-onset burst in its eastern wing compared to the main body. We also find evidence for a chemically distinct secondary population in the SMC that preferentially traces the foreground and may originate from the LMC. We discuss the implications of these results for the evolutionary history of the L/SMC and for starbursts in interacting low-mass galaxy pairs.

Figures (17)

• Figure 1: HST and APOGEE field locations in the LMC. Maps of the peak brightness temperature ($T_{B, {\rm peak}}$) of 21 cm emission along the line-of-sight to the LMC measured by the ATCA Kim1999, overlaid with the locations of HST and APOGEE fields (Section \ref{['sec:data']}). The maps are in tangent-plane coordinates, assuming an LMC center of ($\alpha, \delta$) = (78.77$^\circ$,$-69.01$$^\circ$), the dynamical center of the HI disk LuksRohlfs1992Kim1999. (Left panel) HST field sample from C24a, which includes WFC3 imaging from Scylla (blue squares; Murray2024) and METALS (red open squares; RomanDuval2019), WFPC2 imaging from the LGSPA (purple open triangles; Holtzman2006), and other archival HST ACS and WFC3 imaging (pink open circles). (Middle panel) HST field sample (black-outlined blue squares) compared to the footprint of 2$^\circ$ diameter APOGEE fields targeting the LMC (blue shaded circles; N20; Povick2024). (Right panel) HST fields compared to spatial regions defined in Section \ref{['sec:region_lmc']} based on de-projected in-plane radial distances and location in the northern versus southern halves of the LMC disk. Black contours delineating radial zones correspond to $R_{\rm inv, LMC}$/2 = 1.6 kpc and $R_{\rm inv, LMC}$ = 3.2 kpc, where $R_{\rm inv, LMC}$ is the inversion radius (C24a). We also show the adopted line-of-nodes PA, $\theta \sim 149$ deg E of N Choi2018 used to define the in-plane LMC coordinate system and its orthogonal line of maximum line-of-sight depth.
• Figure 2: HST and APOGEE field locations in the SMC. Similar to Figure \ref{['fig:lmc']}, except for the SMC (HI maps from the GASKAP survey; Pingel2022). We assumed an optical center for the SMC of ($\alpha, \delta$) = (13.19$^\circ$,$-72.83$$^\circ$) Crowl2001Subramanian2012. (Left panel) HST field sample from C24b, which includes WFC3 imaging from Scylla (blue squares; Murray2024), archival WFPC2 LGSPA imaging Holtzman2006, and other archival HST ACS and WFC3 imaging (pink open circles). (Right panel) HST field sample (black-outlined blue squares) compared to APOGEE fields targeting the SMC (blue shaded circles; Nidever2020Povick2024), where the APOGEE fields SMC3 and SMC5 that overlap with the HST fields designated as the SMC "body" and "wing" respectively are highlighted (Section \ref{['sec:region_smc']}).
• Figure 3: Age distributions predicted for APOGEE RGB populations in the L/SMC from Scylla SFHs. The ages are estimated using MATCH to generate synthetic JHK photometry from global Scylla SFHs for the L/SMC (Section \ref{['sec:fake']}) and applying the APOGEE RGB selection from Section \ref{['sec:apogee']}. Histograms assume bin sizes of 1 Gyr. The solid (dashed) vertical line and shaded vertical band represent the median and 1$\sigma$ ranges for the estimated age distribution of the LMC (SMC). Ages younger than 1 Gyr are present in the histograms due to minor contamination in the RGB selection region from red super giants. The median ages and 1$\sigma$ ranges for the LMC ($\tau_{\rm RGB}$ = 2.8$^{+4.3}_{-1.7}$) and SMC ($\tau_{\rm RGB}$ = 2.8$^{+2.8}_{-1.7}$) are consistent with each other.
• Figure 4: Scylla area-normalized SFRs in the LMC. Best-fit area-normalized SFRs as a function of lookback time derived from HST CMDs using PARSEC stellar evolutionary models (Section \ref{['sec:sfh']}). Shaded colored regions represent 1$\sigma$ uncertainties, including both systematic and random uncertainty contributions. The shaded grey vertical region in each panel represents the 68% percentile range on the age distribution of APOGEE RGB stars estimated from simulations with the global LMC SFH as input ($\tau_{\rm RGB}$ = 2.8$^{+4.3}_{-1.7}$ Gyr; Section \ref{['sec:fake']}). We show lookback times $\gtrsim$1 Gyr, where $\sim$1 Gyr corresponds to the age of the youngest RGB stars. (Top panels) We show SFRs along north/south (left) and radial (right) spatial divisions in the LMC (defined by the radius at which the LMC's age-radius relation inverts, $R_{\rm inv, LMC}$ = 3.2 kpc; C24a). The bottom panels show further radial subdivisions of the northern (left) and southern (right) halves of the LMC disk. The LMC experienced a global burst of star formation $\sim$3.2 Gyr ago (B25), which coincides with $\tau_{\rm RGB}$. The peak SFR enhancement during the burst is fairly uniform across the LMC (Section \ref{['sec:csfh']}).
• Figure 5: Scylla area-normalized SFRs in the SMC. Similar to Figure \ref{['fig:sfr_lmc']}, except for the SMC. The 68% percentile range on the stellar age distribution of APOGEE RGB stars in the SMC inferred from the global SFH (shaded grey vertical region) is $\tau_{\rm RGB}$ = 2.8$^{+2.8}_{-1.7}$ Gyr (Section \ref{['sec:fake']}). The main body of the SMC shows a starburst beginning $\sim$5 Gyr ago, with a localized burst occurring $\sim$3 Gyr ago (B25), both of which coincide with $\tau_{\rm RGB}$ in the SMC. In contrast, the SMC wing shows a short $\sim$4 Gyr ago burst followed by near-constant SF over the last $\sim$3 Gyr. Each burst shows an SFR enhancement of a factor of $\sim$3 (Section \ref{['sec:csfh']}).