[Paper Review] Structural parameters and possible association of the Ultra-Faint Dwarfs Pegasus III and Pisces II from deep Hubble Space Telescope photometry

Hannah Richstein, Ekta Patel+18 more|arXiv (Cornell University)|Apr 4, 2022

Galaxies: Formation, Evolution, Phenomena92 references20 citations

TL;DR

This study presents deep Hubble Space Telescope photometry of the ultra-faint dwarfs Pegasus III and Pisces II, measuring their structural parameters and mass-to-light ratios. It finds no morphological evidence of interaction despite their close projected separation (~40 kpc), but orbital integration with Gaia eDR3 proper motions suggests a possible past close passage (~10–20 kpc) and collective passage near the Large Magellanic Cloud ~1 Gyr ago, implying a potential bound association that requires improved proper motion measurements for confirmation.

ABSTRACT

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 $a_h=1.88^{+0.42}_{-0.33}$ arcminutes ($118^{+31}_{-30}$ pc) and $M_V{=}{-4.17}^{+0.19}_{-0.22}$; for Psc II, we measure $a_h{=}1.31^{+0.10}_{-0.09}$ arcminutes ($69\pm8$ pc) and $M_V{=}{-4.28}^{+0.19}_{-0.16}$. We do not find any morphological features that indicate a significant interaction between the two has occurred, despite their close separation of only $\sim$40 kpc. Using proper motions (PMs) from Gaia early Data Release 3, we investigate the possibility of any past association by integrating orbits for the two UFDs in a MW-only and a combined MW and Large Magellanic Cloud (LMC) potential. We find that including the gravitational influence of the LMC is crucial, even for these outer-halo satellites, and that a possible orbital history exists where Peg III and Psc II experienced a close ($\sim$10-20 kpc) passage about each other just over $\sim$1 Gyr ago, followed by a collective passage around the LMC ($\sim$30-60 kpc) just under $\sim$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.

Motivation & Objective

• To measure the structural parameters (half-light radius, absolute magnitude) of the ultra-faint dwarfs Pegasus III and Pisces II using deep HST photometry.
• To derive mass-to-light ratios using newly determined absolute magnitudes and assess their consistency with UFD simulations.
• To investigate the dynamical history of Peg III and Psc II using Gaia eDR3 proper motions and orbital integration in both MW-only and MW+LMC gravitational potentials.
• To test whether the two UFDs could be a gravitationally bound pair based on their kinematic and spatial proximity.
• To evaluate the precision needed in future proper motion measurements to definitively determine if Peg III and Psc II are associated.

Proposed method

• Acquired deep HST imaging with ACS and WFC3 to perform high-precision photometry of Peg III and Psc II.
• Used isodensity contour mapping and stellar density profile fitting to measure half-light radii and structural parameters.
• Calculated absolute magnitudes (MV) using heliocentric distances and corrected for extinction to derive mass-to-light ratios.
• Integrated orbits of both UFDs backward in time using Gaia eDR3 proper motions in both MW-only and MW+LMC potential models.
• Performed Monte Carlo simulations to assess the required precision in transverse velocity measurements to distinguish between associated and unassociated systems.
• Compared results from different proper motion measurements (Li et al. 2021 vs. McConnachie & Venn 2020) to assess consistency and sensitivity to input uncertainties.

Experimental results

Research questions

• RQ1 Do Pegasus III and Pisces II show morphological features indicating a past interaction?
• RQ2 What is the orbital history of Peg III and Psc II, and is there a plausible past close passage between them?
• RQ3 How does the inclusion of the Large Magellanic Cloud's gravitational potential affect the orbital solutions for these outer-halo UFDs?
• RQ4 What level of proper motion precision is required to confidently determine whether Peg III and Psc II are a bound pair?
• RQ5 Are the observed kinematic and spatial properties of Peg III and Psc II consistent with predictions from UFD-mass simulations?

Key findings

• Pegasus III has a half-light radius of 1.′88+0.42−0.33 (118+31−30 pc) and an absolute magnitude MV = −4.17+0.19−0.22.
• Pisces II has a half-light radius of 1.′31+0.10−0.09 (69 ± 8 pc) and an absolute magnitude MV = −4.28+0.19−0.16.
• No morphological features indicate a significant interaction between Peg III and Psc II, despite their projected separation of ~40 kpc.
• Orbital integration with the LMC included reveals a possible past close passage (~10–20 kpc) between the two UFDs ~1 Gyr ago, followed by a collective passage near the LMC (~30–60 kpc) shortly thereafter.
• The probability of a chance association is low (4.7%) if future proper motion measurements achieve ~50 km s−1 precision, especially when combined with small line-of-sight velocity differences.
• A velocity difference of less than ~50 km s−1 between the two UFDs would suggest with ~95% confidence that they are gravitationally bound, requiring improved proper motion data from HST or future Gaia releases.