Boötes. IV. A new Milky Way satellite discovered in the Subaru Hyper Suprime-Cam Survey and implications for the missing satellite problem

We report on the discovery of a new Milky Way (MW) satellite in Boötes based on data from the ongoing Hyper Suprime-Cam (HSC) Subaru Strategic Program (SSP). This satellite, named Boötes IV, is the third ultra-faint dwarf that we have discovered in the HSC-SSP. We have identified a statistically significant (32.3σ) overdensity of stars with characteristics of a metal-poor, old stellar population. The distance to this stellar system is D_{⊙}=209^{+20}_{-18}kpc with a V-band absolute magnitude of M_V=-4.53^{+0.23}_{-0.21}mag. Boötes IV has a half-light radius of r_h=462^{+98}_{-84}pc and an ellipticity of 0.64^{+0.05}_{-0.05}, which clearly suggests that this is a dwarf satellite galaxy. We also found another overdensity that appears to be a faint globular cluster with M_V=-0.20^{+0.59}_{-0.83}mag and r_h=5.9^{+1.5}_{-1.3}pc located at D_{⊙}=46^{+4}_{-4}kpc. Adopting the recent prediction for the total population of satellites in a MW-sized halo by Newton et al. (2018, MNRAS, 479, 2853), which combined the characteristics of the satellites observed by the Sloan Digital Sky Survey and the Dark Energy Survey with the subhalos obtained in ΛCDM models, we estimate that there should be about two MW satellites at M_V ≤ 0 in the ∼676 deg² covered by HSC-SSP, whereas that area includes six satellites (Sextans, Leo IV, Pegasus III, Cetus III, Virgo I, and Boötes IV). Thus, the observed number of satellites is larger than the theoretical prediction. On the face of it, we have a problem of too many satellites, instead of the well-known missing satellites problem whereby the ΛCDM theory overpredicts the number of satellites in a MW-sized halo. This may imply that the models need more refinement for the assignment of subhalos to satellites, such as considering those found by the current deeper survey. More statistically robust constraints on this issue will be brought by further surveys of HSC-SSP over the planned ∼1400 deg² area.