Boötes IV: A New Milky Way Satellite Discovered in the Subaru Hyper Suprime-Cam Survey and Implications for the Missing Satellite Problem (1906.07332v1)

Abstract: 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]

• The paper reports the discovery of Boötes IV, an ultra-faint Milky Way satellite with a 32.3σ stellar overdensity.
• It details Boötes IV’s key properties: M_V = -4.53 mag, a 462 pc half-light radius, 0.64 ellipticity, and a 209 kpc distance, indicating an old, metal-poor population.
• The study finds six satellites in 676 sq deg versus the predicted two, challenging ΛCDM models and underscoring the need for deeper surveys.

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

This paper introduces the discovery of a newly identified satellite galaxy of the Milky Way, designated as Boötes IV, detected through data acquired from the Hyper Suprime-Cam (HSC) Subaru Strategic Program (SSP). This discovery contributes to the ongoing investigation into the "missing satellite problem" within the framework of the $\Lambda$CDM cosmological model.

The paper details the characteristics of Boötes IV as an ultra-faint dwarf galaxy, denoted by a significant ($32.3\sigma$) stellar overdensity, indicative of a metal-poor, old stellar population. The satellite resides at a distance of $209$ kpc from the Sun, with an absolute magnitude of $M_V = -4.53$ mag, a half-light radius of $462$ pc, and notable ellipticity of $0.64$. Additionally, the paper identifies another stellar overdensity associated with a faint globular cluster, dubbed HSC 1, located at a closer distance of $46$ kpc.

Implications for $\Lambda$CDM and the Missing Satellite Problem

The identification of Boötes IV in the current survey area of approximately $676$ square degrees provides new evidence for revisiting the "missing satellite problem." Classical understanding within the $\Lambda$CDM framework anticipates a larger number of dark matter subhalos than the corresponding count of observable satellite galaxies. However, this research argues for an excess of observed satellites in the footprint of the HSC-SSP compared to theoretical predictions. According to the model by Newton et al., based on a combination of data from the Sloan Digital Sky Survey (SDSS) and the Dark Energy Survey (DES) with $\Lambda$CDM simulations, only about two satellites were expected in the surveyed area, yet six satellites, including Boötes IV, were detected.

Practical and Theoretical Significance

The discovery of Boötes IV and HSC 1 enhances our understanding of the Milky Way's satellite system and its dark matter content. This research suggests that the current theoretical models of subhalo distribution in a Milky Way-sized halo may require refinement, particularly in the allocation of dark subhalos to visible satellite galaxies. It emphasizes the need to consider deeper surveys that account for faint satellites that were not detectable with previous surveys like SDSS and DES.

Future Research Directions

This paper sets the stage for future large-scale sky surveys, such as the Legacy Survey of Space and Time (LSST), to further investigate the population and distribution of ultra-faint dwarf galaxies. The continued HSC-SSP will also increment our knowledge of faint satellite galaxies. Additionally, spectroscopic follow-up studies are necessary to confirm the membership and characterize the dynamics of these systems to delineate their dark matter content definitively.

In sum, the paper underscores a potential shift in understanding the Milky Way's satellite population, highlighting the need for deeper observational data to refine cosmological models, particularly regarding small-scale structures and their dark matter distributions.