Discovery of a new Local Group Dwarf Galaxy Candidate in UNIONS: Bo\"otes V

TL;DR

This paper reports the discovery of Boötes V, a new ultra-faint dwarf galaxy candidate in the Local Group, identified through star overdensity in the UNIONS dataset and confirmed with Gaia and Gemini spectroscopy.

Contribution

It introduces Boötes V as a newly identified ultra-faint dwarf galaxy candidate with detailed astrometric and spectroscopic characterization, expanding knowledge of faint satellite galaxies.

Findings

Boötes V has a half-light radius of ~27 pc.

It is located at ~100 kpc from the Sun.

Spectroscopy shows a metallicity of [Fe/H] = -2.85 and radial velocity of 5.1 km/s.

Abstract

We present the discovery of Bo\"otes V, a new ultra-faint dwarf galaxy candidate. This satellite is detected as a resolved overdensity of stars during an ongoing search for new Local Group dwarf galaxy candidates in the UNIONS photometric dataset. It has a physical half-light radius of 26.9$^{+7.5}_{-5.4}$ pc, a $V$-band magnitude of $-$4.5 $\pm$ 0.4 mag, and resides at a heliocentric distance of approximately 100 kpc. We use Gaia DR3 astrometry to identify member stars, characterize the systemic proper motion, and confirm the reality of this faint stellar system. The brightest star in this system was followed up using Gemini GMOS-N long-slit spectroscopy and is measured to have a metallicity of [Fe/H] $=$ $-$2.85 $\pm$ 0.10 dex and a heliocentric radial velocity of $v_r$ = 5.1 $\pm$ 13.4 km s$^{-1}$. Bo\"otes V is larger (in terms of scale radius), more distant, and more metal-poor than the vast majority of globular clusters. It is likely that Bo\"otes V is an ultra-faint dwarf galaxy, though future spectroscopic studies will be necessary to definitively classify this object.