Structural parameters and possible association of the Ultra-Faint Dwarfs\n Pegasus III and Pisces II from deep Hubble Space Telescope photometry

Abstract

We present deep Hubble Space Telescope (HST) photometry of the ultra-faint\ndwarf (UFD) galaxies Pegasus III (Peg III) and Pisces II (Psc II), two of the\nmost distant satellites in the halo of the Milky Way (MW). We measure the\nstructure of both galaxies, derive mass-to-light ratios with newly determined\nabsolute magnitudes, and compare our findings to expectations from UFD-mass\nsimulations. For Peg III, we find an elliptical half-light radius of\n$a_h=1.88^{+0.42}_{-0.33}$ arcminutes ($118^{+31}_{-30}$ pc) and\n$M_V{=}{-4.17}^{+0.19}_{-0.22}$; for Psc II, we measure\n$a_h{=}1.31^{+0.10}_{-0.09}$ arcminutes ($69\\pm8$ pc) and\n$M_V{=}{-4.28}^{+0.19}_{-0.16}$. We do not find any morphological features that\nindicate a significant interaction between the two has occurred, despite their\nclose separation of only $\\sim$40 kpc. Using proper motions (PMs) from Gaia\nearly Data Release 3, we investigate the possibility of any past association by\nintegrating orbits for the two UFDs in a MW-only and a combined MW and Large\nMagellanic Cloud (LMC) potential. We find that including the gravitational\ninfluence of the LMC is crucial, even for these outer-halo satellites, and that\na possible orbital history exists where Peg III and Psc II experienced a close\n($\\sim$10-20 kpc) passage about each other just over $\\sim$1 Gyr ago, followed\nby a collective passage around the LMC ($\\sim$30-60 kpc) just under $\\sim$1 Gyr\nago. Considering the large uncertainties on the PMs and the restrictive priors\nimposed to derive them, improved PM measurements for Peg III and Psc II will be\nnecessary to clarify their relationship. This would add to the rare findings of\nconfirmed pairs of satellites within the Local Group.\n