(Proposal ID) S14B-098

(PI) Takada Masahiro

(Proposal Title)
 Constraining primordial black hole dark matter with HSC observation of M31

(Abstract)
We propose to conduct a dense-cadence HSC r-band observation of M31, 450 
visits of 1 min exposures over 10 hours, in order to search for microlensing 
events on stars in M31 due to primordial black holes (PBHs) in the Milky Way 
halo, which are one of viable dark matter (DM) candidates. A combination of 
the wide HSC FoV, matching the entire region of M31, and the large aperture of 
Subaru allows us to monitor about 10^{10} stars of M31, even with 1 min 
exposure (the depth 22 Vega mag/arcsec^2). In addition the microlensing event 
search towards M31 (750~kpc) is sensitive to PBHs in a larger volume in the 
distance range of 10kpc to 100kpc from the Earth. These make one-night HSC-M31 
observation complementary to and even more powerful than the Kepler PBHs 
search, which is monitoring about 10^5 stars every 30 minutes towards an open 
star cluster at a distance of about 5kpc (therefore probing PBHs in a few kpc 
distance) over 2 years (Griest et al. 2013, PRL). If PBHs of mass 10^{-7}M_sun 
make up the entirety of DM in the Milky Way halo, we expect about 10^4 
microlensing events with a light curve of 30min to a few hours, through pixel 
lensing, thereby allowing us to constrain the fraction of PBHs to the DM 
amount to an accuracy of Omega_{PBH}/Omega_{m}~ 0.01, better than the Kepler 
constraint.