Indirect Detection of Decaying Dark Matter with High Angular Resolution: The Case for Axion Search by IRCS on the Subaru Telescope

Recent advances in photon detectors have provided exceptional sensitivities to dark matter with high angular resolution. Motivated by this, we present a detailed study of photon flux from dark matter decay in dwarf spheroidal galaxies (dSphs) by focusing on the detectors with arcsecond-level field of view and/or angular resolution. We propose to use differential D -factors since such detectors are sensitive to their dark matter distributions. We carefully estimate the differential D -factors of 35 dSphs. By using the differential D -factors, it turns out that the resulting signal flux can have a more than ${ \mathcal O }$ (1–10) enhancement compared to conventional estimations. Based on this analysis, we find that the Infrared Camera and Spectrograph (IRCS) installed on the 8.2 m Subaru Telescope can be an excellent dark matter detector for the mass in the eV range, particularly axion-like particles (ALPs). Observing the Draco or Ursa Major II dSphs with the IRCS for ${ \mathcal O }(1)$ night will enable us to place the most stringent bound for the ALP dark matter in the mass range of 1 eV ≲ m _a ≲ 2 eV.