(Proposal ID) S16B-014

(PI) Tanaka, Masayuki

(Proposal Title)
First Observational Test of the Missing Satellite Problem Beyond Local Group

(Abstract)
We propose to observe 20 nearby (<20 Mpc) galaxies to their virial radii with 
HSC for the first observational test of the missing satellite problem beyond 
the Local Group (LG). The problem has been a big challenge to the widely 
accepted LambdaCDM paradigm, and by its nature, the problem must be addressed 
with a statistical rigor as proposed here; however, it has been tested only 
within the LG so far. In fact, both our pilot observation and recent hydro-
dynamical simulations have revealed a large diversity of Milky Way-sized halos 
in terms of the abundance of dwarf satellite galaxies. We propose to observe 
galaxies outside of the LG with HSC and perform a statistical test of the 
missing satellite problem for the very first time. In addition to this primary 
goal, we will use the same data for additional tests of LambdaCDM; spatial 
alignments of dwarf galaxy orientations with respect to the central galaxies, 
which is recently reported as another challenge to LambdaCDM, galaxy merger 
rates from stellar tidal streams, and possibly constraining the subhalo mass 
function using spatial gaps within the streams. Furthermore, our data will 
become the golden calibrator for any future galaxy formation simulations down 
to the low-mass scale. With a small investment of 20 hours, we will make a 
significant impact in the field.