A census of massive eclipsing binaries in the Whirlpool Galaxy

We present a census of massive eclipsing binary (EB) candidates in the Whirlpool Galaxy (M51), a face-on spiral galaxy at 7.5 Mpc with Z about Z. Using 34 epochs of red-optical Hubble Space Telescope (HST) photometry and a single epoch of broadband UV-optical photometry, we identify 173 massive EB candidates (>5 M) with orbital periods P orb =1-30 days. The sample traces young stellar populations across the spiral arms of M51. Joint light-curve and spectral energy distribution fits for 136 systems yield binaries spanning T eff=10, 000 - 50, 000 K, R=7 - 66 R, and masses of 5 - 120 M. We use detailed injection-recovery simulations to quantify the survey's completeness and infer intrinsic properties of the massive binary population. At the population level, we infer a close-binary fraction of f close=27.4 plus or minus7.1% for 2 at most P orb/ d <20 and 0.1 at most q at most1. Over the same period range, the intrinsic period distribution is consistent with being log-uniform, d N/ d P orb proportional to P orb π with π=0.29 plus or minus0.32. These demographics broadly match those of massive-star populations in the Milky Way, LMC, and SMC, with tentative evidence for a lower close-binary fraction at lower metallicity. Across the about0.2 - 1.1 Z range spanned by these galaxies, the overall similarities suggest that the formation of close massive binaries depends only weakly on metallicity. Our survey establishes EBs as a powerful tool for measuring massive-binary demographics beyond the Local Group and provides a rich panchromatic census of about 100,000 luminous stars and variables in M51.