The Small Magellanic Cloud through the lens of the James Webb Space Telescope : binaries and mass function within the galaxy outskirts

TL;DR

This study uses deep JWST NIRCam photometry of the Small Magellanic Cloud outskirts to simultaneously measure the unresolved binary fraction and the present-day stellar mass function down to 0.22 M⊙. The binary fraction for mass ratios q>0.6 is found to be $f_{ m bin}^{q>0.6}=0.14\pm0.01$, robust to metallicity variations, and the blue straggler population is modest with $f_{ m BSS}=0.04\pm0.01$. After correcting the luminosity function for binaries and completeness, the MF slope is $α=-1.99\pm0.08$ (MLE: $α_{MLE}=-1.85\pm0.01$), with no decisive need for a broken power law. The results indicate IMF variation in this low-metallicity, low-density environment, contrasting with the canonical Salpeter slope and aligning with MF measurements in other low-density Galactic and extragalactic systems, thereby informing models of star formation and chemical evolution in dwarf galaxies.

Abstract

The stellar initial mass function (IMF) and the fraction of binary systems are fundamental ingredients that govern the formation and evolution of galaxies. Whether the IMF is universal or varies with environment remains one of the central open questions in astrophysics. Dwarf galaxies such as the Small Magellanic Cloud (SMC), with their low metallicity and diffuse star-forming regions, offer critical laboratories to address this issue. In this work, we exploit ultra-deep photometry from the James Webb Space Telescope to investigate the stellar populations in the field of the SMC. Using the $m_{\rm F322W2}$ versus $m_{\rm F115W}-m_{\rm F322W2}$ color-magnitude diagram (CMD), we derive the luminosity function and measure the fraction of unresolved binary systems. We find a binary fraction of $f_{\rm bin}^{q>0.6}=0.14\pm0.01$, consistent with results from synthetic CMDs incorporating the metallicity distribution of the SMC. Additionally, the measured binary fraction in the SMC field is consistent with those observed in Galactic open clusters and Milky Way field stars of similar ages and masses, suggesting similar binary formation and evolutionary processes across these low-density environments. By combining the luminosity function with the best-fit isochrone, we derive the the mass function (MF) down to $0.22\,M_{\odot}$, the lowest mass limit reached for the SMC to date. The resulting MF follows a power-law with a slope of $α=-1.99\pm0.08$. This value is shallower than the canonical Salpeter slope of $α=-2.35$, providing new evidence for IMF variations in low-metallicity and low-density environments.

Figures (7)

• Figure 1: Image of the SMC obtained from the Digitized Sky Survey 2 (DSS2). The inset provides a close-up view of the region near 47 Tucanae where the observations for this study were carried out. The NIRCam field of view analyzed in this work is outlined in green, while the footprints of the observations used by kalirai2013a are shown in red. The white cross marks the position of the SMC center piatti2021. North is up and east is to the left.
• Figure 2: Illustration of the photometric data used in this work. Left panel. The $m_{\rm F322W2}$ vs. $m_{\rm F115W}-m_{\rm F322W2}$ CMD, showing all detected stars. Likely SMC members and 47 Tucanae stars are marked with black and gray points, respectively. Right panel. The same CMD, restricted to stars in the field of the SMC. The black dashed line corresponds to the 50 $\%$ completeness limit for SMC stars in this dataset. The inset zooms in on the upper MS ($20.4 < m_{\rm F322W2} < 23.8$), where three isochrones with an age of 7 Gyr, distance modulus $(m - M)_0 = 18.9$, and reddening $E(B-V) = 0.05$ are overplotted. The blue and red lines correspond to metallicities of [Fe/H] = $-2.2$ and $-0.5$, while the green line indicates the best-fitting isochrone with [Fe/H] = $-1.1$.
• Figure 3: Binary fraction estimation in the SMC field. Left panel.$m_{\rm F322W2}$ vs. $m_{\rm F115W}-m_{\rm F322W2}$ CMD of SMC stars. Region A (green solid line) includes MS stars and binaries with primary masses between 0.61 and 0.81 $M_{\odot}$. Region B (green-shaded area) contains binaries with $q>0.6$. MS and binary stars are indicated with black points and red crosses, respectively, while the remaining stars are colored in gray. Central panel. Same as left panel but for artificial and field stars. Azure starred symbols represent stars simulated with the TRILEGAL code girardi2005 within a Galactic field with the same area and coordinates as the one investigated in this work. Right panel. The best-fit simulated CMD. Colors and symbols follow the same scheme as in the left and central panels for comparison.
• Figure 4: Comparison of binary fractions in the SMC field and Galactic populations. Panel a. Binary fraction with $q>0.6$ as a function of the logarithm of cluster age. Galactic OCs are color-coded by their metallicity, as indicated by the color bar on the right, while the orange diamond marks the binary fraction in the field of the SMC. The binary fractions and ages for OCs are from cordoni2023 and dias2021, respectively. Panel b. Total binary fraction as a function of the mass of the primary star. Gray dots are taken from the review by offner2023, while the orange diamond indicates the binary fraction measured for the SMC field in this work.
• Figure 5: Identification of candidate BSSs in the field of the SMC. The gray dashed line indicates the zero-age MS isochrone, while the azure dashed vertical line marks the color of the MS turn-off. The blue and red solid line represent the fiducial sequence of MS stars shifted blueward by five times the color error, and the equal-mass binary fiducial, respectively. The azure-shaded area indicates region C, whereas region D is shown in gray. Candidate BSSs are highlighted with blue triangles, and field stars are shown as azure starred symbols. See the text for details.