Wide binaries in an ultra-faint dwarf galaxy: discovery, population modeling, and a nail in the coffin of primordial black hole dark matter

TL;DR

This study uses deep JWST/NIRCam imaging to detect and characterize a wide binary population in the ultra-faint dwarf galaxy Boötes I, enabling a direct probe of binary formation and dynamical survival in a dark-matter–dominated, metal-poor environment. Through careful member selection, a two-point correlation analysis, and forward-modeling of selection effects and flyby disruptions, the authors infer a wide-binary fraction of $f_{wb} = 1.25\pm0.25\%$ for separations above $5{,}000$ au, with an apparent truncation near $15{,}000$ au explained by stellar flybys. They place new constraints on dark matter in the form of MACHOs, showing that compact objects with $M \gtrsim 5\,M_\odot$ cannot constitute more than $\sim1\%$ of the DM, while highlighting substantial uncertainties in using wide binaries to constrain the DM density profile due to initial-population assumptions and chance alignments. Comparisons with the Milky Way and Reticulum II indicate similar wide-binary fractions in metal-poor systems, supporting metallicity-invariant formation, but also reveal limitations in translating two-point statistics into definitive DM-profile constraints for dwarf galaxies.

Abstract

We report the discovery and characterization of a wide binary population in the ultrafaint dwarf galaxy Boötes I using deep JWST/NIRCam imaging. Our sample consists of 52 candidate binaries with projected separations of 7,000 - 16,000 au and stellar masses from near the hydrogen-burning limit to the main-sequence turnoff ($\sim0.1$ - $0.8~{\rm M_\odot}$). By forward-modeling selection biases and chance alignments, we find that $1.25\pm0.25\%$ of Boötes I stars are members of wide binaries with separations beyond 5,000 au. This fraction, along with the distributions of separations and mass ratios, matches that in the Solar neighborhood, suggesting that wide binary formation is largely insensitive to metallicity, even down to [Fe/H] $\approx -2.5$. The observed truncation in the separation distribution near 16,000 au is well explained by stellar flyby disruptions. We also discuss how the binaries can be used to constrain the galaxy's dark matter properties. We show that our detection places new limits on primordial black hole dark matter, finding that compact objects with $M \gtrsim 5~{\rm M_\odot}$ cannot constitute more than $\sim1\%$ of the dark matter content. In contrast to previous work, we find that wide binaries are unlikely to provide robust constraints on the dark matter profile of ultrafaint galaxies given the uncertainties in the initial binary population, flyby disruptions, and contamination from chance alignments. These findings represent the most robust detection of wide binaries in an external galaxy to date, opening a new avenue for studying binary star formation and survival in extreme environments.

Figures (15)

• Figure 1: Selection of main-sequence stars from NIRCam photometry. From left to right, we show (a) all photometric sources, (b) sources that pass the DOLPHOT star filters outlined in Section \ref{['subsec:star_selection']}, and (c) sources on the Boo I main sequence. The blue points show the CMD location of galaxies in the Hubble Ultra Deep Field, with JWST photometry synthesized from SED models. The last panel shows a 13 Gyr, [Fe/H] = -2.5 isochrone (purple dashed) with assumed distance modulus $\mu=19.11$DallOra06. Background galaxies rarely coincide with the main sequence, indicating that very few enter our final sample.
• Figure 2: Two-point correlation for stars in Boo I. We show the projected separation distribution for all Boo I stars (blue) compared to the expected distribution from chance alignments (black), derived using $30$ randomly shifted catalogs. Black points indicate the mean counts per separation bin, with $1\sigma$ error bars representing the spread across the shifted samples. We also plot a zoomed version of the distribution (with linear bins) in the range where there is an excess of wide binaries compared to chance alignments ($7000<s/{\rm au} < 16000$). The bottom panel shows the fractional excess of real binaries compared to chance alignments. A significant excess of pairs below $\sim16,000$ au ($100-400\%$) reveals the presence of an intrinsic wide binary population in Boo I.
• Figure 3: JWST/NIRCam images of Boo I wide binary candidates in the F150W filter. The primary star's source ID and masses of both components are displayed for each pair.
• Figure 4: Contrast sensitivity of close pairs with JWST/NIRCam and our adopted photometric cuts. Top: Angular separation ($\theta$) vs. the difference in F150W magnitude ($\Delta m_{\rm F150W}$) for all pairs in the sample. The solid black line shows an empirical fit to the detection limit, where pairs above the line are undetectable due to angular resolution limitations. Bottom: The minimum observable mass ratio ($q_{\rm min}$) as a function of separation ($s$) given the empirical contrast sensitivity for various values of primary mass $M_1$. At $\theta\mathrel{\hbox{$\sim$} \hbox{$<$}}0.23"$, where the wide binary candidates lie, the sample is only sensitive to pairs with relatively low contrast $\Delta m_{\rm F150W} \mathrel{\hbox{$\sim$} \hbox{$<$}}3$.
• Figure 5: Color-magnitude diagram (CMD) for the wide binary candidates in Boo I. Blue stars represent the primary (brighter) components, and yellow diamonds indicate secondary components. Each pair is connected by a thin red line. The magenta dashed curve shows a 13 Gyr isochrone with [Fe/H]$= -2.5$, consistent with the stellar population of Boo I. Average $1\sigma$ photometric uncertainties are shown on the left.