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Structural Parameters and Possible Association of the Ultra-faint Dwarfs Pegasus III and Pisces II from Deep Hubble Space Telescope Photometry

Richstein, Hannah and Patel, Ekta and Kallivayalil, Nitya and Simon, Joshua D. and Zivick, Paul and Tollerud, Erik and Fritz, Tobias and Warfield, Jack T. and Besla, Gurtina and van der Marel, Roeland P. and Wetzel, Andrew and Choi, Yumi and Deason, Alis and Geha, Marla and Guhathakurta, Puragra and Jeon, Myoungwon and Kirby, Evan N. and Libralato, Mattia and Sacchi, Elena and Sohn, Sangmo Tony (2022) Structural Parameters and Possible Association of the Ultra-faint Dwarfs Pegasus III and Pisces II from Deep Hubble Space Telescope Photometry. Astrophysical Journal, 933 (2). Art. No. 217. ISSN 0004-637X. doi:10.3847/1538-4357/ac7226.

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We present deep Hubble Space Telescope (HST) photometry of the ultra-faint dwarf (UFD) galaxies Pegasus III (Peg III) and Pisces II (Psc II), two of the most distant satellites in the halo of the Milky Way (MW). We measure the structure of both galaxies, derive mass-to-light ratios with newly determined absolute magnitudes, and compare our findings to expectations from UFD-mass simulations. For Peg III, we find an elliptical half-light radius of aₕ = 1.′88_(-0.33)^(+0.42) (118₋₃₀⁺³¹ pc) and Mᵥ = -4.17_(-0.22)^(+0.19); for Psc II, we measure aₕ = 1.′31_(−0.09)^(+0.10) (69 ± 8 pc) and Mᵥ = −4.28_(-0.16)^(+0.19). We do not find any morphological features that indicate a significant interaction between the two has occurred, despite their close separation of only ∼40 kpc. Using proper motions (PMs) from Gaia early Data Release 3, we investigate the possibility of any past association by integrating orbits for the two UFDs in an MW-only and a combined MW and Large Magellanic Cloud (LMC) potential. We find that including the gravitational influence of the LMC is crucial, even for these outer-halo satellites, and that a possible orbital history exists where Peg III and Psc II experienced a close (∼10–20 kpc) passage about each other just over ∼1 Gyr ago, followed by a collective passage around the LMC (∼30–60 kpc) just under ∼1 Gyr ago. Considering the large uncertainties on the PMs and the restrictive priors imposed to derive them, improved PM measurements for Peg III and Psc II will be necessary to clarify their relationship. This would add to the rare findings of confirmed pairs of satellites within the Local Group.

Item Type:Article
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URLURL TypeDescription Paper
Richstein, Hannah0000-0002-3188-2718
Patel, Ekta0000-0002-9820-1219
Kallivayalil, Nitya0000-0002-3204-1742
Simon, Joshua D.0000-0002-4733-4994
Zivick, Paul0000-0001-9409-3911
Tollerud, Erik0000-0002-9599-310X
Fritz, Tobias0000-0002-3122-300X
Warfield, Jack T.0000-0003-1634-4644
Besla, Gurtina0000-0003-0715-2173
van der Marel, Roeland P.0000-0001-7827-7825
Wetzel, Andrew0000-0003-0603-8942
Choi, Yumi0000-0003-1680-1884
Deason, Alis0000-0001-6146-2645
Geha, Marla0000-0002-7007-9725
Guhathakurta, Puragra0000-0001-8867-4234
Jeon, Myoungwon0000-0001-6529-9777
Kirby, Evan N.0000-0001-6196-5162
Libralato, Mattia0000-0001-9673-7397
Sacchi, Elena0000-0001-5618-0109
Sohn, Sangmo Tony0000-0001-8368-0221
Additional Information:© 2022. The Author(s). Published by the American Astronomical Society. Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Received 2022 April 4; revised 2022 May 9; accepted 2022 May 19; published 2022 July 15. We would like to thank the anonymous referee for their thorough reading of this manuscript and comments that have led to its improvement and clarification. These data are associated with the HST Treasury Program 14734 (PI: N. Kallivayalil). Support for this program was provided by NASA through grants from the Space Telescope Science Institute. This material is based upon work supported by the National Science Foundation under grant No. AST-1847909. H.R. acknowledges support from the Virginia Space Grant Consortium Graduate Research STEM Fellowship. E.P. acknowledges support from the Miller Institute for Basic Research in Science at the University of California Berkeley. This research has made use of NASA's Astrophysics Data System. Facility: HST (ACS, WFC3), Software: Aplpy (Robitaille & Bressert 2012); Astrodrizzle (Fruchter & Hook 2002); Astropy (Astropy Collaboration et al. 2013, 2018); (Foreman-Mackey 2016); dustmaps (Green 2018); emcee (Foreman-Mackey et al. 2013); Jupyter Notebook (Kluyver et al. 2016); Matplotlib (Hunter 2007); NumPy (Harris et al. 2020); photutils (Bradley et al. 2020); scikitlearn (Pedregosa et al. 2012); SciPy (Virtanen et al. 2020); stsynphot (STScI Development Team 2010); synphot (STScI Development Team 2018),
Group:Astronomy Department
Funding AgencyGrant Number
Virginia Space Grant ConsortiumUNSPECIFIED
Miller Institute for Basic Research in ScienceUNSPECIFIED
Subject Keywords:Dwarf galaxies; Local Group; Galaxy structure
Issue or Number:2
Classification Code:Unified Astronomy Thesaurus concepts: Dwarf galaxies (416); Local Group (929); Galaxy structure (622)
Record Number:CaltechAUTHORS:20220727-37608000
Persistent URL:
Official Citation:Hannah Richstein et al 2022 ApJ 933 217
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:115908
Deposited By: George Porter
Deposited On:29 Jul 2022 14:58
Last Modified:29 Jul 2022 14:58

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