# Searches for New Milky Way Satellites from the First Two Years of Data of the Subaru/Hyper Suprime-Cam Survey: Discovery of Cetus~III

## TL;DR

This paper reports the discovery of a new ultra-faint dwarf galaxy candidate, Cetus III, from the Subaru Hyper Suprime-Cam survey, demonstrating the survey's effectiveness in detecting faint Milky Way satellites beyond previous limits.

## Contribution

The study presents the first detection of Cetus III, a new ultra-faint dwarf galaxy candidate, and updates parameters for Virgo I, highlighting the survey's potential to find missing Milky Way satellites.

## Key findings

- Cetus III is located at ~251 kpc with an absolute magnitude of -2.4 mag.
- The survey detected Cetus III with high statistical significance (10.7σ).
- Predictions suggest 1-2 new satellites could be found in 300 deg² of data.

## Abstract

We present the results from a search for new Milky Way (MW) satellites from the first two years of data from the Hyper Suprime-Cam (HSC) Subaru Strategic Program (SSP) $\sim 300$~deg$^2$ and report the discovery of a highly compelling ultra-faint dwarf galaxy candidate in Cetus. This is the second ultra-faint dwarf we have discovered after Virgo~I reported in our previous paper. This satellite, Cetus~III, has been identified as a statistically significant (10.7$\sigma$) spatial overdensity of star-like objects, which are selected from a relevant isochrone filter designed for a metal-poor and old stellar population. This stellar system is located at a heliocentric distance of 251$^{+24}_{-11}$~kpc with a most likely absolute magnitude of $M_V = -2.4 \pm 0.6$~mag estimated from a Monte Carlo analysis. Cetus~III is extended with a half-light radius of $r_h = 90^{+42}_{-17}$~pc, suggesting that this is a faint dwarf satellite in the MW located beyond the detection limit of the Sloan Digital Sky Survey. Further spectroscopic studies are needed to assess the nature of this stellar system. We also revisit and update the parameters for Virgo~I finding $M_V = -0.33^{+0.75}_{-0.87}$~mag and $r_h = 47^{+19}_{-13}$~pc. Using simulations of $\Lambda$-dominated cold dark matter models, we predict that we should find one or two new MW satellites from $\sim 300$~deg$^2$ HSC-SSP data, in rough agreement with the discovery rate so far. The further survey and completion of HSC-SSP over $\sim 1,400$~deg$^2$ will provide robust insights into the missing satellites problem.