Final Results of Search for New Milky Way Satellites in the Hyper Suprime-Cam Subaru Strategic Program Survey: Discovery of Two More Candidates (2311.05439v2)

Abstract: We present the final results of our search for new Milky Way (MW) satellites using the data from the Hyper Suprime-Cam (HSC) Subaru Strategic Program (SSP) survey over $\sim 1,140$ deg$^2$. In addition to three candidates that we already reported, we have identified two new MW satellite candidates in the constellation of Sextans at a heliocentric distance of $D_{\odot} \simeq 126$kpc, and Virgo at $D_{\odot} \simeq 151$kpc, named Sextans II and Virgo III, respectively. Their luminosities (Sext II:$M_V\simeq-3.9$mag; Vir III:$M_V\simeq-2.7$mag) and half-light radii (Sext II:$r_h\simeq154$ pc; Vir III:$r_h\simeq 44$ pc) place them in the region of size-luminosity space of ultra-faint dwarf galaxies (UFDs). Including four previously known satellites, there are a total of nine satellites in the HSC-SSP footprint. This discovery rate of UFDs is much higher than that predicted from the recent models for the expected population of MW satellites in the framework of cold dark matter models, thereby suggesting that we encounter a too many satellites problem. Possible solutions to settle this tension are also discussed.

• The paper reports the discovery of two new satellite candidates, Sextans II and Virgo III, using deep photometric data from HSC-SSP.
• Methodology involved isochrone filtering and maximum likelihood analysis to detect stellar overdensities and derive structural parameters.
• Results reveal a satellite count exceeding ΛCDM model predictions, prompting a re-evaluation of dark matter models and satellite formation theories.

An Analytical Overview of New Milky Way Satellites Search Using HSC-SSP

In this paper, the authors present the culmination of their search for new Milky Way satellites using data from the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP). The findings include the identification of two novel satellite candidates, Sextans II and Virgo III, along with a re-evaluation of previously reported candidates. These discoveries significantly contribute to addressing the "too many satellites" problem observed relative to predicted models within the framework of cold dark matter ($\Lambda$CDM) paradigms.

The key findings are based on deep photometric surveys over approximately 1,140 square degrees conducted by the HSC-SSP. The analysis led to the discovery of two satellite candidates in the Sextans and Virgo constellations. Sextans II is estimated at a distance of about 126 kpc with an absolute magnitude of $M_V \approx -3.9$ mag and a half-light radius $r_h \approx 154$ pc. Virgo III is located approximately 151 kpc away, with $M_V \approx -2.7$ mag and $r_h \approx 44$ pc. Both are positioned within the size-luminosity space typical of ultra-faint dwarf galaxies (UFDs).

Methodological Approach

The satellite search involved filtering out halo star candidates using isochrone filters on the color-magnitude diagrams, optimized for detecting metal-poor, old stellar populations typical of UFDs. Statistical significance of stellar overdensities was assessed to distinguish real from spurious detections. Structural parameters were derived using maximum likelihood analysis, contributing to the understanding of the spatial distribution of stars within these satellites.

Numerical Results and Observations

Including Sextans II and Virgo III, a total of five new candidate satellites were identified in HSC-SSP observations. This brings the total count within the survey's footprint to nine, which is notably higher than predictions by recent $\Lambda$CDM models, such as those by Nadler et al. (2020). These models combined observational data from the Dark Energy Survey (DES) and Pan-STARRS1 (PS1), suggesting $220 \pm 50$ satellites for the entire Milky Way halo. For the HSC region alone, the prediction was $3.9 \pm 0.9$ satellites, substantially fewer than observed.

Implications and Future Directions

The implications of these results accentuate discrepancies in current cosmological models concerning the Milky Way's satellite population, suggesting a potential revision in assumptions about satellite distribution and formation efficacy. Specifically, the "too many satellites" problem might point towards an inherent bias or underestimation of the spatial distribution extent predicted by cosmological simulations. Future observations, especially by upcoming surveys like the LSST, will be crucial for delineating whether these findings are consistent across larger footprints and more exhaustive surveys.

Further, high-resolution spectroscopy and space telescope campaigns (e.g., HST and Euclid) could help precisely determine stellar memberships and kinematics within these satellites, offering better constraints on dark matter models and the role of baryonic processes in satellite evolution.

This research bolsters the dialogue on small-scale discrepancies in $\Lambda$CDM models and highlights the power of deep surveys like HSC-SSP in enriching our understanding of galactic structure and dark matter's role therein. It sets a precedent for leveraging data from next-generation surveying platforms, poised to unlock deeper cosmic insights.