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

http://arxiv.org/abs/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]

Read this paper on arXiv…

D. Homma, M. Chiba, Y. Komiyama, et. al.
Wed, 19 Jun 19
18/60

Comments: 13 pages, 11 figures, 2 tables, accepted for publication in PASJ