Oxygen isotopes reveal low-mass star dominance in the Small Magellanic Cloud

TL;DR

This work measures oxygen isotopic ratios in the metal-poor Small Magellanic Cloud by observing CO isotopologues with ALMA ACA toward the star-forming region LIRS 36, including the first detection of C$^{17}$O in the SMC. Through LTE+MCMC modeling of $^{13}$CO, C$^{18}$O, and C$^{17}$O, the study derives $^{18}$O/$^{17}$O = 0.87$\pm$0.26, along with $^{13}$C/$^{18}$O = 109.3$\pm$35.6 and $^{13}$C/$^{17}$O = 95.2$\pm$31.4, indicating enrichment dominated by low- and intermediate-mass stars. The exceptionally low $^{18}$O/$^{17}$O ratio supports a top-light IGIMF in the SMC, consistent with its long-term low star-formation rate, and suggests that such IMF characteristics may be common in metal-poor dwarf galaxies. The results establish a new isotopic benchmark for the SMC and motivate future surveys of oxygen isotopes in local dwarfs and high-redshift analogs to probe stellar-population and chemical-evolution histories across cosmic time.

Abstract

Oxygen isotope abundances and their ratios are fingerprints of stellar evolution and therefore provide a powerful tool in tracing the enrichment history of galaxies. However, their behavior in low-metallicity dwarf galaxies remains largely unexplored. The Small Magellanic Cloud (SMC), a nearby analog of young high-redshift galaxies, offers an ideal laboratory to investigate this regime. Using the Atacama Compact Array, we observed the $J=2\to 1$ transitions of $^{12}$CO, $^{13}$CO, C$^{18}$O, and C$^{17}$O from the massive star-forming region LIRS~36 (aka N12A), achieving the first detection of C$^{17}$O in the SMC. This detection enables the first direct measurement of the $^{18}$O/$^{17}$O abundance ratio of 0.87$\pm$0.26 in this galaxy, substantially lower than all values in the literature, including molecular clouds in the Milky Way and other galaxies. Such a low ratio of $^{18}$O/$^{17}$O, together with a high $^{13}$CO/C$^{18}$O ratio, indicates chemical enrichment dominated by low-mass stars, consistent with the observed paucity of high-mass stars in the SMC. We suggest that the SMC is governed by a top-light integrated galaxy-wide initial mass function, predicted by the SMC's persistently low star-formation activities.

Figures (4)

• Figure 1: Spatial distributions and spectra of the four $J=2\to 1$ transitions of CO isotopologues. Panels (a)–(d) show the integrated-intensity maps of $^{12}$CO $J=2\to1$, $^{13}$CO $J=2\to1$, C$^{18}$O $J=2\to1$, and C$^{17}$O $J=2\to1$, respectively, overlaid with their corresponding contours. The velocity integration ranges are 105--145 km s$^{-1}$ for $^{12}$CO $J=2\to1$ and 122--129 km s$^{-1}$ for the other three transitions. Contours start at 9.0 K km s$^{-1}$ and increase in steps of 4.5 K km s$^{-1}$ in panel (a), at 0.89 K km s$^{-1}$ with steps of 0.89 K km s$^{-1}$ in panel (b), and at 0.03 K km s$^{-1}$ with steps of 0.015 K km s$^{-1}$ in panels (c)–(d). Panels (e)--(h) show the $^{12}$CO $J=2\to 1$, $^{13}$CO $J=2\to 1$, C$^{18}$O $J=2\to 1$, and C$^{17}$O $J=2\to 1$ spectra of the position indicated by the red plus in panels (a)--(d). The red dashed curves in panels (e)--(h) show the Gaussian and hyperfine-structure fitting results, while the red vertical lines in panel (h) indicate the positions of the hyperfine-structure components. The $^{12}$CO $J=2\to1$ and $^{13}$CO $J=2\to1$ data are from ACA+TP combined observations, whereas the C$^{18}$O $J=2\to1$ and C$^{17}$O $J=2\to1$ data are from ACA-only observations. Flux-to-brightness temperature conversion factors are 2.4462, 2.2358, 2.2188, 2.3242 Jy K$^{-1}$ at an angular resolution of 7.5$^{\prime\prime}$ for $^{12}$CO $J=2\to1$, $^{13}$CO $J=2\to1$, C$^{18}$O $J=2\to1$, and C$^{17}$O $J=2\to1$, respectively.
• Figure 2: Oxygen isotope ratios.
• Figure 3: Comparison of the ACA-only and ACA+TP datasets toward LIRS 36. (a) Observed spectra of $^{12}$CO $J=2 \to 1$ and $^{13}$CO $J=2 \to 1$ toward the $^{13}$CO $J=2 \to 1$ peak emission, indicated by the plus signs in panels (b) and (c). (b) Intensity map of $^{12}$CO $J=2\to 1$ integrated from 105 km s$^{-1}$ to 145 km s$^{-1}$. Contours start at 9.0 K km s$^{-1}$ and increase by 4.5 K km s$^{-1}$. (c) Intensity map of $^{13}$CO $J=2\to 1$ integrated from 122 km s$^{-1}$ to 129 km s$^{-1}$. Contours start at 0.89 K km s$^{-1}$ and increase by 0.89 K km s$^{-1}$. In panels (b) and (c), the synthesized beam is shown in the lower left corner.
• Figure 4: Posterior probability distributions of $^{13}$CO column densities, C$^{18}$O column densities, C$^{17}$O column densities, and excitation temperatures for the Weeds LTE modeling, with the maximum posterior possibility point in the parameter space highlighted by orange lines and points. Contours represent the 0.5, 1.0, 1.5, and 2.0$\sigma$ confidence intervals. The vertical dashed black lines represent the 1$\sigma$ spread.