Paper detail

Euclid: Early Release Observations -- The extended stellar component of the IC10 dwarf galaxy

We present a detailed analysis of the old, extended stellar component of the Local Group dwarf galaxy IC 10 using deep resolved-star photometry in the VIS and NISP bands of the Euclid Early Release Observations. Leveraging Euclid's unique combination of wide field of view and high spatial resolution, we trace red giant branch (RGB) stars out to $\sim$8 kpc from the galaxy centre, reaching azimuthally-averaged surface brightness levels as faint as $μ_{HE}\sim$29 mag arcsec$^{-2}$. Our analysis reveals that IC 10's stellar distribution is significantly more extended than previously thought. After correcting for foreground extinction and subtracting contamination from Milky Way stars and background galaxies, we derive a radial stellar density profile from RGB star counts. The profile shows a marked flattening beyond $\sim$5 kpc, and is best fit by a two-component (Sersic + exponential) model, yielding a total stellar mass in old (age $>$1 Gyr) stars of $M_{\star}=(6.7-8.1)\times10^8 M_{\odot}$. The origin of the outer stellar component is unclear. It may be accreted, even possibly associated with the counter-rotating HI gas in the outer regions of IC 10, or it may represent an ancient in-situ stellar halo. We tentatively detect two symmetric stellar overdensities at the edge of our imagery. These roughly align with the direction of IC 10's orbit around M31, suggesting that they may be signatures of tidal stripping. As part of our analysis, we derive a new distance to IC 10 based on the RGB tip, finding $D=762\pm 20$ kpc and the distance modulus is $(m-M)_0=24.41\pm 0.05$.