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 (UFD) 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 data set. It has a physical half-light radius of 26.9 − 5.4 + 7.5 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 UFD, though future spectroscopic studies will be necessary to definitively classify this object.

Metallicity Distribution Functions of 13 Ultra-faint Dwarf Galaxy Candidates from Hubble Space Telescope Narrowband Imaging

“…Nearly half of known MW UFDs lack any metallicity information, while the remainder only have a handful of stars bright enough to observe with current ground-based facilities. Next-generation photometric surveys are already delivering on their promise to discover more faint satellites and out to larger distances (e.g., Homma et al 2019;Mutlu-Pakdil et al 2021;Cerny et al 2021;Smith et al 2023), and even observations with next-generation spectrographs on extremely large telescopes (ELTs) will not be able to observe adequate numbers of stars in these galaxies to sufficiently populate the galaxy's MDF. For example, Figure 9 from Simon (2019) projects that medium-resolution spectroscopy on forthcoming ELTs may only be able to reach 10 stars at best in a Ret II equivalent UFD at 250 kpc.…”

The Discovery of the Faintest Known Milky Way Satellite Using UNIONS

“…Our particular implementation of the algorithm will be described here in brief, though we refer the reader to Smith et al (2023) for a more detailed view of the search method. A matched filter seeks to isolate stars that belong to a particular stellar population, in order to create contrast in stellar density between the member stars of a putative satellite and the Milky Way stellar background.…”

“…In Smith et al (2023), we developed a similar method to estimate the absolute magnitude of the Boötes V UFD. This methodology was found to produce an excess in the number of bright, late-stage stars (RGB, HB), which resulted in an overestimation of the total absolute magnitude.…”

“…This methodology was found to produce an excess in the number of bright, late-stage stars (RGB, HB), which resulted in an overestimation of the total absolute magnitude. We rederive the total stellar mass and absolute magnitude of Boötes V following the methodology described in this section by producing 1000 realizations using the stellar population and structural parameters found in Smith et al (2023). We measure the stellar mass of Boötes V to be 3) and note that the intrinsic velocity dispersion drops to -+ 1.9 1.1…”

“…Ongoing surveys, such as the Dark Energy Local Volume Explorer (DELVE; Drlica-Wagner et al 2021 and the Ultraviolet Near Infrared Optical Northern Survey (UNIONS; Ibata et al 2017), have continued to bolster the known Milky Way satellite dwarf galaxy population (Mau et al 2020;Cerny et al 2021Cerny et al , 2023aCerny et al , 2023bSmith et al 2023). The Gaia space telescope (Gaia Collaboration et al 2016 has also been revolutionary, with its proper-motion measurements providing additional constraints for characterizing and discovering both dwarf galaxies (e.g., Pace & Li 2019;Torrealba et al 2019b;McConnachie & Venn 2020a, 2020b and globular clusters (e.g., Torrealba et al 2019a;Pace et al 2023).…”