  One of the biggest mysteries in the modern cosmology and galaxy formation is
the hideout of the "missing baryons". The leading theory of galaxy formation
predicts that a huge amount of baryons resides around galaxies extending out to
their virial radii in the form of diffuse and hot gas of $10^6-10^7\,$K, which
is also known as the major component of the circumgalactic medium (CGM).
Studies by various groups via different techniques, however, have not reached a
consensus on the role of CGM in accounting for the missing baryons, with the
estimated contribution ranging from a minor fraction to enclosing the baryon
budget of the galaxy. In this work we attempt to measure the mass of missing
baryons in CGM with a novel method based on the gamma-ray observations of the
extended halo of the Andromeda Galaxy. Since cosmic-ray particles that are
generated inside the galaxy will eventually escape to the CGM, they will
produce gamma-ray emission via the proton-proton collision with CGM. Different
from some traditional measurements which are sensitive only to gas in certain
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.
Our result suggests that the total baryon mass contained within the virial
radius is less than $(1.4-5)\times 10^{10}M_\odot$ 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 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.
