Apr 12 – 17, 2021
Africa/Johannesburg timezone

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

Apr 15, 2021, 4:05 PM


Yi Zhang


It has been suggested that a huge amount of baryons 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 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 CGM and produce gamma rays. Different from some traditional measurements which are sensitive 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 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)e10 M_sun according to the gamma-ray observation. 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 diffusion coefficient of the CRs 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.

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