• DOI:10.3847/1538-4357/aca1c3
• Corpus ID: 247597301

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

@article{Cerny2022PegasusID,
  title={Pegasus IV: Discovery and Spectroscopic Confirmation of an Ultra-faint Dwarf Galaxy in the Constellation Pegasus},
  author={William Cerny and Joshua D. Simon and T. S. Li and Alex Drlica-Wagner and Andrew B. Pace and Clara E. Mart{\'i}nez-V{\'a}zquez and Alexander H. Riley and B. Mutlu-Pakdil and Sidney Mau and P. S. Ferguson and Denis Erkal and R. R. Mu{\~n}oz and Clecio Roque De Bom and Jeffrey L. Carlin and Daniela Carollo and Y. Choi and Alexander P. Ji and Viraj Manwadkar and David Martinez-Delgado and Annaliese Miller and Noelia E. D. No{\"e}l and Joanna D. Sakowska and David. J. Sand and Guy S. Stringfellow and Erik J. Tollerud and Anna Katherina Vivas and Julio A. Carballo-Bello and D. Hernandez-Lang and David J. James and David L. Nidever and J. L. Nilo Castell{\'o}n and K. A. G. Olsen and Alfredo Zenteno},
  journal={The Astrophysical Journal},
  year={2022},
  volume={942}
}

• W. Cerny, J. Simon, +30 authors A. Zenteno
• Published 22 March 2022
• Physics
• The Astrophysical Journal

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 ( r1/2=41−6+8 pc; M V = −4.25 ± 0.2 mag) located at a heliocentric distance of 90−6+4kpc . Based on spectra of seven nonvariable member stars observed with Magellan/IMACS, we confidently resolve Pegasus IV’s velocity dispersion, measuring σv=3.3−1.1+1.7 km s−1 (after excluding three velocity outliers); this implies a mass-to-light ratio of M1/2/LV,1/2=167−99+224M⊙/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.