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Public Seminar of MPhil Candidate:
Probing Roles of Dense Celestial Objects at Galactic Centers via Gravitational Wave and Gamma Ray

Speaker:Mr. Yuan GAO
Affiliation:The University of Hong Kong
Date:July 30, 2019 (Tuesday)
Time:1:00 p.m.
Venue:Rm 518, 5/F, Chong Yuet Ming Physics Building, HKU


Recent observations of gravitational way by the Laser Interferometer Gravitational Wave Observatory and gamma-ray emission by the Fermi Gamma Ray Space Telescope have led to suspicion of interesting roles for two types of dense celestial objects at galactic centers - black holes and millisecond pulsars. While the frequent detection of ∼ 30 solar mass binary black hole mergers as gravitational wave emitters reminds people of the possibility that primordial black holes constitute dark matter, a diffuse residual gamma ray excess at the centers of the Milky Way and Andromeda might suggest a considerable population of millisecond pulsars as debris of disrupted globular clusters as they migrate into the galactic center. We have followed up and performed the most up to date investigation of the two scenarios and found very interesting results. Although the gravitational wave signals permit the possibility of dark matter with a primordial black hole nature, it was later refuted by a macro lensing observation featuring caustic crossing. This rules out primordial black holes in a narrow mass window as the dominant component of dark matter, and people will need to consider an extended mass spectrum or put more faith in the Weakly Interacting Massive Particles. On the other hand, we have improved the previous model in calculating the MSP luminosity in explaining the galactic center excess yet found no significant increase to account for the totality of the detected signal. Thus, the alternative explanation being DM annihilation is needed to account for the remaining signal strength, which sets constraints on its properties in the innermost region of the galaxy where their distribution is expected to be the densest. Together, the two parts of our research provide new insight into the intertwined astrophysical environment with dark matter and dense celestial objects at the galactic center. Besides, as a natural extension of our second research topic, we would like to extend our galactic evolution model obtained in evolving the MSP population to galaxies beyond the Milky Way and Andromeda, and systematically study the statistics of the MSP gamma ray emission across different halos with diverse assembly histories, and look at various galactic evolutionary consequences such as the globular cluster distribution in space and metallicity. In the end we would like to search for the best place to hunt for dark matter in the presence of such MSP population.

Anyone interested is welcome to attend.