Stars on the edge: Galactic tides and the outskirts of the Sculptor dwarf spheroidal

Abstract

The formation of ”stellar halos” in dwarf galaxies have been discussed in terms of early mergers or Galactic tides, although fluctuations in the gravitational potential due to stellar feedback is also a possible candidate mechanism. A Bayesian algorithm is used to find new candidate members in the extreme outskirts of the Sculptor dwarf galaxy. Precise metallicities and radial velocities for two distant stars are measured from their spectra taken with the Gemini South GMOS spectrograph. The radial velocity, proper motion and metallicity of these targets are consistent with Sculptor membership. As a result, the known boundary of the Sculptor dwarf extends now out to an elliptical distance of ∼10 half-light radii, which corresponds to a projected physical distance of ∼3 kpc. As reported in earlier work, the overall distribution of radial velocities and metallicities indicate the presence of a more spatially and kinematically dispersed metal-poor population that surrounds the more concentrated and colder metal-rich stars. Sculptor’s density profile shows a ”kink” in its logarithmic slope at a projected distance of ∼25 arcmin (620 pc), which we interpret as evidence that Galactic tides have helped to populate the distant outskirts of the dwarf. We discuss further ways to test and validate this tidal interpretation for the origin of these distant stars.

Deep Hubble Space Telescope Photometry of Large Magellanic Cloud and Milky Way Ultrafaint Dwarfs: A Careful Look into the Magnitude–Size Relation

“…There has also been more interest in the stellar halos of dwarf galaxies, including how they could be built up through dwarfdwarf mergers (Deason et al 2022) Tidal interactions with the MW are also a strongly supported mechanism for the formation of stellar halos around dwarf galaxies. Classical dwarfs, such as Fornax, Sculptor, and Ursa Minor I, have had member stars found at large radii, which dynamical models are able to explain by invoking the effects of Galactic tides, whether the target is on first infall (e.g., Fornax; Yang et al 2022), was recently at pericenter (e.g., Sculptor, Sestito et al 2023a), or has had multiple orbits around the MW (e.g., Ursa Minor I, Sestito et al 2023b). Member stars at large galactocentric distances have also been studied in other UFDs, including Boötes I, Coma Berenices, Hercules, and Ursa Major I (Longeard et al 2023;Waller et al 2023;Ou et al 2024).…”

GHOST Commissioning Science Results: Identifying a New Chemically Peculiar Star in Reticulum II

“…Members are selected based on their spatial positions, colors, magnitudes, and proper motions using photometry and astrometry from Gaia DR3, and adopting a two-component model for the distribution of stars in the satellite galaxies. This method has been shown to be efficient at identifying new members in both UFDs (Waller et al 2023) and classical dwarfs (Sestito et al 2023a(Sestito et al , 2023b. For Ret II, targets with membership probabilities >20% from Jensen (2023) are shown in Figure 1.…”

Extended Stellar Populations in Ultrafaint Dwarf Galaxies

“…They also suggest the existence of a stellar halo around that galaxy, which would be more extended than previously thought. Furthermore, Sestito et al (2023a) discovered two stars that belong to the Sculptor dSph and are located at 10 R h (∼3 kpc) from its center. Some studies focusing on the spatial distribution specifically of RRL in dSphs have also been undertaken.…”