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

Abstract

We report on the discovery of a new Milky Way (MW) satellite in Boötes based on data from the on-going 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 (32.3$\sigma$) overdensity of stars having characteristics of a metal-poor, old stellar population. The distance to this stellar system is $D_{\odot }=209_{-18}^{+20}$ kpc with a $V$-band absolute magnitude of $M_V=-4.53_{-0.21}^{+0.23}$ mag. Boötes IV has a half-light radius of $r_h=462_{-84}^{+98}$ 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 $M_V=-0.20_{-0.83}^{+0.59}$ mag and $r_h=5.9_{-1.3}^{+1.5}$ pc located at $D_{\odot }=46_{-4}^{+4}$ kpc. Adopting the recent prediction for the total population of satellites in a MW-sized halo by Newton et al. (2018), which combined the characteristics of the observed satellites by SDSS and DES with the subhalos obtained in $\Lambda$CDM models, we estimate that there should be about two MW satellites at $M_V\le 0$ in the $\sim 676$ deg${}^2$ covered by HSC-SSP, whereas that area includes six satellites. 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 $\Lambda$CDM theory overpredicts the number of satellites in a MW-sized halo. This may imply that the models need more refinements for the assignment of subhalos to satellites such as considering those found by the current deeper survey. [abridged]

arXiv comment13 pages, 11 figures, 2 tables, accepted for publication in PASJ

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