Exploring the ultra-faint dwarf Bootes I using JWST and HST: Metallicity distribution and binaries

Ultra-faint dwarf galaxies (UFDs) are among the oldest and most metal-poor stellar systems in the Universe. Their metallicity distribution encodes the fossil record of the earliest star formation, feedback, and chemical enrichment, providing crucial tests of models of the first stars, galaxy assembly, and dark matter halos. However, due to their faint luminosities and the limited number of bright giants, spectroscopic studies of UFDs typically probe only small stellar samples. Here, we present an analysis of multi-epoch Hubble Space Telescope and James Webb Space Telescope observations of the UFD Bootes I. Using deep color-magnitude diagram in the F606W and F322W2 bands, extending from the subgiant branch to the M-dwarfs, and stellar proper motions to identify likely members, we obtained an unprecedentedly clean census of the system. The exquisite quality of the diagram, combined with the sensitivity of M-dwarf colors to metallicity, allowed us to constrain the metallicity distribution in a large stellar sample. As a first step, we derived the binary fraction in Bootes I. This is crucial, since binaries can bias kinematic mass estimates, affect stellar population analyses, and shape the photometric signatures used to infer metallicity. We find that 20$\pm$2% of stellar systems in Bootes I are binaries with mass ratios larger than 0.4, corresponding to a total binary fraction of $\sim$30%. This value is comparable to the binary fractions observed in globular clusters of similar stellar mass, suggesting that the presence of dark matter does not significantly affect the binary properties of Bootes I. We then exploited the metallicity sensitivity of M-dwarf colors to derive the metallicity distribution function. We find that most of the stars $\sim$85% have [Fe/H]<-2, and that roughly $\sim$17% have [Fe/H]<-3.

💡 Research Summary

This paper presents a comprehensive photometric study of the ultra‑faint dwarf galaxy Boötes I using multi‑epoch observations from the Hubble Space Telescope (HST) and the James Webb Space Telescope (JWST). The authors combine deep optical imaging in the HST/ACS F606W filter with near‑infrared imaging in the JWST/NIRCam F322W2 filter, complemented by HST F814W and JWST F150W data for proper‑motion measurements. By processing all exposures with the KS2 software, they obtain high‑precision photometry across a wide magnitude range, employing three specialized measurement strategies for bright, intermediate, and faint stars. Artificial‑star tests are used to quantify photometric uncertainties and to validate the quality cuts applied to the real catalog.

Proper motions derived from the multi‑epoch data allow the authors to separate Boötes I members from foreground/background contaminants, rejecting stars that deviate by more than three times the galaxy’s internal proper‑motion dispersion. The resulting clean sample contains several thousand main‑sequence (MS) stars extending from the turn‑off down to the faint M‑dwarf regime (approximately four magnitudes below the turn‑off in F322W2).

The color–magnitude diagram (CMD) in the (F606W − F322W2) versus F322W2 plane shows a well‑defined MS, a clear binary sequence offset to the red, and a sparsely populated sub‑giant branch. The authors exploit the binary sequence to measure the fraction of binaries with mass ratios q > 0.4, finding 20 ± 2 % of stellar systems in this regime. Accounting for lower‑q binaries, they infer a total binary fraction of roughly 30 %, comparable to values reported for globular clusters of similar mass. This result is important because unresolved binaries can inflate the observed line‑of‑sight velocity dispersion, leading to overestimates of the dark‑matter content if not properly accounted for.

To investigate the metallicity distribution, the authors focus on the faint MS region (24.5 < m_F322W2 < 25.5 mag) where the (F606W − F322W2) color is highly sensitive to iron abundance. They adopt BaSTI isochrones spanning