Measuring the Mass of Missing Baryons in the Halo of Andromeda Galaxy with Gamma-Ray Observations

It has been suggested that a huge amount of baryon resides in the circumgalactic medium (CGM) extending out to the virial radii of galaxies. In this work we attempt to measure the baryon mass in the CGM with a novel method based on the gamma-ray observations of the extended halo of the Andromeda galaxy Messier 31 (M31). Since cosmic-ray particles generated inside the galaxy will eventually escape to the CGM, they will produce gamma-ray emission via the proton–proton collision with the CGM and produce gamma-rays. Different from some traditional measurements that are sensitive only to certain metallic ions in specific temperature range, the hadronic gamma-ray flux is sensitive to baryonic gases in all phases and does not rely on the metallicity in the halo, hence the measured gamma-ray flux from the galaxy’s halo can be used to constrain the mass of the CGM. By dealing with the cosmic-ray transport in the halo and calculating the hadronic gamma-ray intensity, we find that the total baryon mass contained within the virial radius is less than (1.4–5) × 1010 M ⊙ according to the gamma-ray intensity obtained with a model-dependent analysis. It implies that the CGM of Andromeda galaxy may not account for more than 30% of the missing baryons, but the result is subject to uncertainties from the gamma-ray intensity upper limit, diffusion coefficient of the cosmic-rays in the halo, as well as the stellar mass and dark matter halo mass of the galaxy. This method will become more constraining provided better understandings on these issues and more sensitive gamma-ray telescopes in the future