2022ApJ...928...14B

Most of the baryons in L* galaxies are unaccounted for and are predicted to lie in hot gaseous halos (T ∼ 10^6.5 K) that may extend beyond R₂₀₀. A hot gaseous halo will produce a thermal Sunyaev-Zeldovich signal that is proportional to the product of the gas mass and the mass-weighted temperature. To best detect this signal, we used a Needlet Independent Linear Combination all-sky Planck map that we produced from the most recent Planck data release, also incorporating WMAP data. The sample is 12 L* spiral galaxies with distances of 3-10 Mpc, which are spatially resolved so that contamination from the optical galaxy can be excluded. One galaxy, NGC 891, has a particularly strong SZ signal, and when excluding it, the stack of 11 galaxies is detected at about 4σ (declining with radius) and is extended to at least 250 kpc (≃R₂₀₀) at >99% confidence. The gas mass within a spherical volume to a radius of 250 kpc is 9.8 ± 2.8 x 10¹⁰M_☉, for T_avg = 3 x 10⁶ K. This is about 30% of the predicted baryon content of the average galaxy (3.1 x 10¹¹M_☉), and about equal to the mass of stars, disk gas, and warm halo gas. The remaining missing baryons (≃1.4 x 10¹¹M_☉, 40%-50% of the total baryon content) are likely to be hot and extend to the 400-500 kpc volume, if not beyond. The result is higher than predictions, but within the uncertainties.