The Tucana dwarf spheroidal: a distant backsplash galaxy of M31?

Abstract

We use the APOSTLE suite of Local Group (LG) cosmological hydrodynamical simulations to examine the properties of "backsplash" galaxies, i.e, dwarfs outside the virial radius, r 200 , of the Milky Way (MW) or M31 which may have been within their virial boundaries in the past. More than half of all dwarfs at distances between one and two virial radii of each primary are backsplash galaxies. More distant backsplash systems, i.e., those that reach distances well beyond 2 r 200 , are typically systems close to apocentre of nearly radial orbits, and, therefore, essentially at rest relative to their primary. We use this result to investigate which LG dwarfs beyond ∼ 500 kpc of either primary could be a backsplash satellite of either MW or M31. The Tucana dwarf spheroidal (dSph), one of the few known quiescent LG field dwarfs, at d M31 ≈ 1350 kpc and d MW ≈ 880 kpc, is a promising candidate. Tucana's radial velocity suggests it is at rest relative to M31. Further, Tucana is located close to the orbital plane of M33 around M31, and simple orbit integrations indicate that Tucana may have been ejected during an early pericentric passage of M33 about ∼ 11 Gyr ago, a timing which approximately coincides with Tucana's last episode of star formation. We suggest that Tucana may have been a satellite of M31 or M33, providing a compelling explanation for its puzzling lack of gas and ongoing star formation despite its isolated nature.

Discovery of a red backsplash galaxy candidate near M81

“…The right panel of Figure 6 shows the implications of the measured SBF distance for Blobby's local environment. The uncertainties on the distances to both Blobby and M81 suggest that Blobby is likely between 1 − 2.7𝑅 𝑣𝑖𝑟 with a central measured value of approximately 590 kpc from M81 (compared to the M81-M82 system virial radius of 𝑅 𝑣𝑖𝑟 ∼ 353 kpc), meaning Blobby lies in a region where simulations predict that the abundance of backsplash galaxies around similar mass hosts is roughly 50% (Gill et al 2005;Garrison-Kimmel et al 2014;Simpson et al 2018;Buck et al 2019;Haggar et al 2020;Applebaum et al 2021;Bakels et al 2021;Santos-Santos et al 2022).…”