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We report on the discovery of a new Milky Way (MW) satellite in Bo\"otesbased on data from the on-going Hyper Suprime-Cam (HSC) Subaru StrategicProgram (SSP). This satellite, named Bo\"otes IV, is the third ultra-faintdwarf that we have discovered in the HSC-SSP. We have identified astatistically significant (32.3$\sigma$) overdensity of stars havingcharacteristics of a metal-poor, old stellar population. The distance to thisstellar system is $D_{\odot}=209^{+20}_{-18}$ kpc with a $V$-band absolutemagnitude of $M_V=-4.53^{+0.23}_{-0.21}$ mag. Bo\"otes IV has a half-lightradius 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 satellitegalaxy. We also found another overdensity that appears to be a faint globularcluster with $M_V=-0.20^{+0.59}_{-0.83}$ mag and $r_h=5.9^{+1.5}_{-1.3}$ pclocated at $D_{\odot}=46^{+4}_{-4}$ kpc. Adopting the recent prediction for thetotal population of satellites in a MW-sized halo by Newton et al. (2018),which combined the characteristics of the observed satellites by SDSS and DESwith the subhalos obtained in $\Lambda$CDM models, we estimate that thereshould be about two MW satellites at $M_V\le0$ in the $\sim676$ deg$^2$ coveredby HSC-SSP, whereas that area includes six satellites. Thus, the observednumber of satellites is larger than the theoretical prediction. On the face ofit, we have a problem of too many satellites, instead of the well-known missingsatellites problem whereby the $\Lambda$CDM theory overpredicts the number ofsatellites in a MW-sized halo. This may imply that the models need morerefinements for the assignment of subhalos to satellites such as consideringthose found by the current deeper survey. [abridged]

Bo\"otes IV: A New Milky Way Satellite Discovered in the Subaru Hyper Suprime-Cam Survey and Implications for the Missing Satellite Problem