Pegasus IV: Discovery and Spectroscopic Confirmation of an Ultra-faint Dwarf Galaxy in the Constellation Pegasus

Abstract

We report the discovery of Pegasus IV, an ultra-faint dwarf galaxy found in archival data from the Dark Energy Camera processed by the DECam Local Volume Exploration Survey. Pegasus IV is a compact, ultra-faint stellar system ( r 1 / 2 = 41 − 6 + 8 pc; M V = −4.25 ± 0.2 mag) located at a heliocentric distance of 90 − 6 + 4 kpc . Based on spectra of seven nonvariable member stars observed with Magellan/IMACS, we confidently resolve Pegasus IV’s velocity dispersion, measuring σ v = 3.3 − 1.1 + 1.7 km s−1 (after excluding three velocity outliers); this implies a mass-to-light ratio of M 1 / 2 / L V , 1 / 2 = 167 − 99 + 224 M ⊙ / L ⊙ for the system. From the five stars with the highest signal-to-noise spectra, we also measure a systemic metallicity of [Fe/H] = − 2.63 − 0.30 + 0.26 dex, making Pegasus IV one of the most metal-poor ultra-faint dwarfs. We tentatively resolve a nonzero metallicity dispersion for the system. These measurements provide strong evidence that Pegasus IV is a dark-matter-dominated dwarf galaxy, rather than a star cluster. We measure Pegasus IV’s proper motion using data from Gaia Early Data Release 3, finding (μ α*, μ δ ) = (0.33 ± 0.07, −0.21 ± 0.08) mas yr−1. When combined with our measured systemic velocity, this proper motion suggests that Pegasus IV is on an elliptical, retrograde orbit, and is currently near its orbital apocenter. Lastly, we identify three potential RR Lyrae variable stars within Pegasus IV, including one candidate member located more than 10 half-light radii away from the system’s centroid. The discovery of yet another ultra-faint dwarf galaxy strongly suggests that the census of Milky Way satellites is still incomplete, even within 100 kpc.

Pegasus W: An Ultrafaint Dwarf Galaxy Outside the Halo of M31 Not Quenched by Reionization

“…UFD galaxies went mostly undetected due to their low surface brightness, faint luminosities, and small physical sizes until ∼20 yr ago. With the advent of wide-field, deeper optical surveys, such as the Sloan Digital Sky Survey (York et al 2000), the Panoramic Survey Telescope and Rapid Response System (Chambers et al 2016), the Pan-Andromeda Archeological Survey (PAndAS; McConnachie et al 2009), the Dark Energy Survey (Drlica-Wagner et al 2020), the DECam Local Volume Exploration (Cerny et al 2022(Cerny et al , 2023, and the DESI Legacy Imaging Surveys (Dey et al 2019), searches for verylow-mass galaxies have been incredibly fruitful, evidenced by the growing number of UFD galaxies now known (e.g., Simon 2019, and references therein). The majority of the galaxies discovered lie in close proximity to the Milky Way (MW) and the Large Magellanic Cloud, as intrinsically faint systems are more readily detected at closer distances.…”

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

“…Nonetheless, the final six candidates that we report were each detected in 2 of the search iterations (see Section 5). In addition to these 54 overdensities, we also reidentified a large number of previously reported systems, including, for example, all of those discovered in earlier searches of the DELVE data (Mau et al 2020;Cerny et al 2021aCerny et al , 2021bCerny et al , 2023.…”

“…The census of ultra-faint dwarf galaxies has now advanced to more than 60 systems, nearly all of which have been identified as resolved satellites of the Milky Way, M31/Andromeda, and a select few nearby hosts in the Local Volume (McConnachie 2012;Simon 2019, and references therein). These systems continue to be discovered at a rapid pace, with roughly 10 ultra-faint dwarf galaxy candidates discovered in the last few years alone (e.g., see Homma et al 2019;Mau et al 2020;Cerny et al 2021aCerny et al , 2023Martínez-Delgado et al 2022;Mutlu-Pakdil et al 2022;Collins et al 2022;Sand et al 2022;Bell et al 2022).…”

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

“…The discovery and study of dwarf galaxies in the Local Group has prospered due to the great advances in wide-field imaging surveys over the last two decades (e.g., Willman et al 2005aWillman et al , 2005bBelokurov et al 2006Belokurov et al , 2007Belokurov et al , 2008Koposov et al 2015a;Laevens et al 2015aLaevens et al , 2015bTorrealba et al 2016Torrealba et al , 2018Torrealba et al , 2019Mau et al 2020;Cerny et al 2021a, 2021b, Cerny et al 2023bSand et al 2022). The Sloan Digital Sky Survey (SDSS; York et al 2000;Abazajian et al 2009) led the early charge, with the Pan-STARRS consortium (Chambers et al 2016), the Dark Energy Survey (DES; Abbott et al 2018), and the DECam Local Volume Exploration Survey (DELVE; Drlica-Wagner et al 2021) contributing mightily to the discovery of Milky Way satellites, while the Pan-Andromeda Archaeological Survey (McConnachie et al 2009) has been prolific in finding dwarf galaxies around M31 (e.g., McConnachie et al 2008;Richardson et al 2011;Martin et al 2013).…”