The Stellar Initial Mass Function down to 0.16M$_{\odot}$ Towards the Small Magellanic Cloud

Abstract

The presence (and nature) of variations in the stellar initial mass function (IMF) at substantially sub-solar masses and metallicities ($m$$<$0.5M$_{\odot}$, [M/H]$\lesssim$$-$1) remains poorly constrained. Predictions from simulations vary widely, while observationally, resolved star studies of ultra-faint dwarf galaxies (UFDs) suffer from small sample sizes and background galaxy contamination due to low projected stellar densities. As an alternative metal-poor target, we measure the IMF from resolved stars towards a carefully selected field in the Small Magellanic Cloud (SMC), leveraging a plethora of independent constraints on the target field stellar population including distributions of distance, %extinction, age and metallicity. We resolve $>$15,000 stars down to 0.16M$_{\odot}$ within a single pointing of NIRCam onboard JWST, using an observing strategy that minimizes contamination from point-source-like background galaxies. We explore three different functional forms of the IMF, forward modeling observed color-magnitude diagrams (CMDs) and luminosity functions. We find a best-fit single power law IMF slope of $α$=$-$1.61$^{+0.03}_{-0.03}$, consistent with UFDs probed down to similar limiting masses. Fitting a broken power law IMF, we find low- and high-mass slopes of $α_{1}$=$-$1.44$^{+0.04}_{-0.04}$, $α_{2}$=$-$2.17$^{+0.11}_{-0.11}$ respectively, consistent with solar neighborhood values. Assuming a lognormal IMF, we find a characteristic mass and lognormal width of $m_{c}$=0.12$^{+0.03}_{-0.03}$M$_{\odot}$, $σ$=0.61$^{+0.07}_{-0.06}$M$_{\odot}$, allowing for characteristic masses lower than local values as seen in some simulations as well as low-metallicity Galactic clusters. Lastly, we quantify the impact of assumptions required in our analysis and discuss potential future improvements.

Figures (13)

• Figure 1: Left: The location of our NIRCam pointing (red diamond), overplotted on a DSS-2 image. Axes are labeled with tangent plane coordinates relative to our NIRCam field center in all panels. The APOGEE SMC-4 field is indicated by an orange circle, with individual SMC member targets shown using orange points. The optical center of the SMC crowl01subsub12 is indicated by a magenta cross. Center: The footprints of the individual NIRCam SW detectors, again overplotted on a DSS-2 image. Spectroscopic CaII targets from carrera08 are indicated with cyan circles, and the cyan box indicates the smc0100 optical imaging field used to infer the SFH presented in noel09 that we assume for our IMF analysis (see Sect. \ref{['inputsect']}). Right: A color image of our NIRCam field, where blue and red represent F150W and F322W2 respectively and green represents the mean of the two.
• Figure 2: Left: The photometric quality parameters sharp (top row) and crowd (bottom row) shown as a function of m$_{\rm F150W}$, magnitude in the F150W filter. Our chosen sharp and crowd cuts, shown using horizontal red lines, were set by comparing the distribution versus m$_{\rm F150W}$ seen in our catalog (left column) against the distribution seen for artificial stars (right column) to minimize the occurrence of spurious detections not present in the artificial star sample. Only sources passing the other photometric quality cuts (object type, photometric quality flag, signal-to-noise) are shown. Right: Same, but versus m$_{\rm F322W2}$.
• Figure 3: Left: CMD of all sources passing our photometric quality cuts. Sources in the light blue shaded region were excluded from our IMF analysis to mitigate contamination by both Galactic halo sources (with m$_{\rm 150W}$-m$_{\rm F322W2}$$\sim$0.6 and 17$\lesssim$m$_{\rm 150W}$$\lesssim$24) and background galaxies appearing as faint red point sources (see Sect. \ref{['methodsect']}). Photometric errors in magnitude bins are shown along the left side in magenta, and the 50% faint completeness limit (assessed from artificial star tests) is indicated by a dashed blue line. BaSTI isochrones are overplotted assuming the extinction from skowron21 and mean line-of-sight distance from elyoussoufi21 to illustrate the lack of stars younger than $\sim$500 Myr. Stellar masses (in M$_{\odot}$) are indicated along the main sequence of the ancient metal-poor isochrone, demonstrating that we achieve 50% completeness at $\sim$0.16 M$_{\odot}$. Top Right: CMD of sources from SMASH in our NIRCam field of view. Isochrones are shown with the same representative ages and metallicities as in the left panel, confirming the lack of young ($\lesssim$500 Myr) stars. Bottom Right: A map of photometric completeness over the CMD assessed from the artificial star tests. The 50% completeness limit and our observed catalog are shown as in the left panel.
• Figure 4: Left: Fractional contamination by Galactic foreground sources as a function of CMD location. Purple contours show the density of foreground contaminants as indicated in the upper right corner. The black area indicates the CMD region we use for our IMF analysis, avoiding CMD loci with statistically significant foreground contamination fractions. The CMD region indicated by the dotted blue rectangle, devoid of true SMC sources, is used as a check on the model-predicted foreground contamination (see Sect. \ref{['fgsect']}). Right: The CMD distribution of background galaxies from the CANUCS NIRCam flanking fields (NFFs). Again, the contamination fraction, plotted versus m$_{\rm F150W}$ in the rightmost panel, is estimated by comparing the number of observed versus predicted sources in a CMD region devoid of true SMC stellar detections (1.5$\leq$m$_{\rm F150W}$$\leq$2.0), indicted by dotted brown vertical lines.
• Figure 5: Illustration of our correction for the color difference between observed and model-predicted color. The observed fiducial sequence (blue; panel a) is compared to the fiducial sequence obtained from synthetic photometry (red; panel b), and the difference (magenta; panel c) is applied to the synthetic photometry, with the results shown in panel (d).