Discovery of a new Local Group Dwarf Galaxy Candidate in UNIONS: Boötes V

Abstract

We present the discovery of Boötes 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 ± 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 ± 0.10 dex and a heliocentric radial velocity of v r = 5.1 ± 13.4 km s −1 . Boötes 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 Boötes V is an ultra-faint dwarf galaxy, though future spectroscopic studies will be necessary to definitively classify this object.

Six More Ultra-faint Milky Way Companions Discovered in the DECam Local Volume Exploration Survey

“…During the preparation of this manuscript, the authors of Smith et al (2022) informed us of their independent discovery of the stellar system referred to here as Boötes V using data from the Ultraviolet Near-Infrared Optical Northern Survey (UNIONS). 37 We have not altered our analysis of this system in light of their work, nor have we reviewed their manuscript.…”

“…Based on the likelihood that three of the four systems with similar morphological properties to DELVE J1415+3254 are dwarf galaxies, we conclude that DELVE J1415+3254 is also most likely a dwarf galaxy. Accordingly, we name the system Boötes V following the convention that dwarf galaxy candidates are named after the constellation within which they reside; this name matches that given by the contemporaneous work by Smith et al (2022).…”