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

• Published in: arXiv.org
• Main Authors: Homma, Daisuke, Chiba, Masashi, Komiyama, Yutaka, Tanaka, Masayuki, Okamoto, Sakurako, Tanaka, Mikito, Ishigaki, Miho N, Hayashi, Kohei, Arimoto, Nobuo, Carlsten, Scott G, Lupton, Robert H, Strauss, Michael A, Miyazaki, Satoshi, Torrealba, Gabriel, Shiang-Yu, Wang, Murayama, Hitoshi
• Format: Paper
• Language: English
• Published: Ithaca Cornell University Library, arXiv.org 18.06.2019
• Subjects: Dwarf galaxies; Ellipticity; Metallicity; Milky Way Galaxy; Satellite observation
• ISSN: 2331-8422
• DOI: 10.48550/arxiv.1906.07332

We report on the discovery of a new Milky Way (MW) satellite in Bo\"otes based on data from the on-going Hyper Suprime-Cam (HSC) Subaru Strategic Program (SSP). This satellite, named Bo\"otes 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 \(D_{\odot}=209^{+20}_{-18}\) kpc with a \(V\)-band absolute magnitude of \(M_V=-4.53^{+0.23}_{-0.21}\) mag. Bo\"otes 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 \(D_{\odot}=46^{+4}_{-4}\) 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 \(M_V\le0\) in the \(\sim676\) 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]