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

TL;DR

This study uses Euclid Early Release Observations to map the extended old stellar component of IC 10 with deep, wide-field resolved-star photometry. By constructing a robust RGB-based radial profile and foreground subtraction, the authors reveal a two-component structure: a compact inner Sérsic core and a more extended outer exponential, implying an old-star mass of about $(6.7$–$8.1) \times 10^8\ M_\odot$ and an outer component containing roughly 13–26% of the old population. A precise TRGB distance of $D=762 \pm 20$ kpc is derived, and the outer structure’s alignment with the M31 orbit and HI features points to tidal or accretion processes shaping IC 10’s outskirts. The results underscore Euclid’s capability to probe extremely faint, extended stellar components in nearby dwarfs, with implications for dwarf galaxy assembly and comparisons with cosmological simulations.

Abstract

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 $(m-M)_0=24.41\pm 0.05$.

Figures (17)

• Figure 1: colour-combined ( in blue, in green, and in red) image of IC 10 with the stretch optimized to highlight the morphology of the old stellar component. North is up and east is left.
• Figure 2: Our source selection based on the distribution in the ${\IE({\rm 1.5\,pix})}-{\IE({\rm 2\,pix})}$ versus $\IE{\rm (1.5\,pix)}$ plane. Point sources populate a narrow region around $\Delta\IE\sim0.18$, while bright saturated stars and extended background galaxies tend to have larger values. We retained all sources within $2\,\sigma$ of the mode of the distribution, as indicated by the dashed green line, and furthermore discarded bright stars with $\IE{\rm(1.5\,pix)}\le19.5$.
• Figure 3: Top: spatial map of $E(B-V)$ in IC 10 over the entire FoV as derived from the dustmaps code assuming the schlegel98 dust maps. Foreground extinction increases from south-east to north-west. The high $E(B-V)$ values at the centre of the field are due to spurious contamination from IC 10 in the dust maps. Bottom:$E(B-V)$ masked spatial map, where we have assigned to a central 8-radius circular region a constant reddening value equal to the average $E(B-V)$ computed in an adjacent external annulus.
• Figure 4: Panel (a): versus $I_\sfont{E}-H_\sfont{E}$ CMD of IC 10 before reddening correction. The red polygon indicates our selection of RGB stars for the dust distribution, while the yellow curve is the reference RGB line with $E(B-V)\sim0.65$. The orange arrow is the reddening vector for $E(B-V)=0.5$. Panel (b): $E(B-V)$ map derived from the RGB resolved stars, as described in Sect. \ref{['sc:ebv']}. The dashed ellipse denotes the region used to infer the reference RGB line displayed in (a). Panel (c): FIR dust emission map from Herschel SPIRE/250 taken from Madden2013. The footprint is indicated for reference as the dotted square.
• Figure 5: Panel (a): $H_\sfont{E,0}$ versus $I_\sfont{E,0}-H_\sfont{E,0}$ reddening-corrected CMD of 336957.0 sources with four-band photometry in the entire FoV after selection cuts (see Sect. \ref{['sc:photometry']}). The CMD was corrected for spatially variable reddening as described in Sect. \ref{['sc:ebv']}. Indicated is the contribution from MW disc stars (both blue stars, and red M dwarf stars); yellow dots denote the 7724.0 MW stars removed from Gaia DR3 proper motions and parallaxes. On the right side, we indicate the photometric errors for $0\leq I_\sfont{E,0}-H_\sfont{E,0}\leq1$, while the black dashed curve denotes the average 50% completeness level, both from the results of the artificial stars over the entire field. Panel (b): cleaned CMD (227128.0 sources) after removal of Galactic M dwarfs and compact red galaxies as described in Sect. \ref{['sc:mw']}. The main stellar evolutionary sequences are indicated: RSGs with ages from about 20 Myr to 50 Myr; bright and red AGB stars with ages from $\sim\,0.1$ to a few Gyrs; RGB stars, with ages older than 1--2 Gyr; at the faintest magnitudes is a hint of the RC, with ages $>$1--2 Gyr. The faint cloud of blue stars at $I_\sfont{E,0}-H_\sfont{E,0}\lesssim-1$, $H_\sfont{E,0}\gtrsim22$ is likely dominated by Galactic WDs. Faint red sources at $I_\sfont{E,0}-H_\sfont{E,0}\gtrsim2.5$, $H_\sfont{E,0}\gtrsim21$ are residual background galaxies. Panel (c): same CMD as in (b) with superimposed PARSEC stellar isochrones for a metallicity of $Z=0.006$ ($\sim$ 40% solar) and ages in the range of 10 Myr--1 Gyr, and for lower metallicities of $Z=0.004$ and 0.001 (25%, and 6% solar, respectively) and ages of 5 and 10 Gyr. Panel (d): $J_\sfont{E,0}$ versus $Y_\sfont{E,0}-H_\sfont{E,0}$ CMD, where the oxygen-rich and the carbon-rich AGB stars (O-AGB and C-AGB) appear well separated and define vertical and horizontal sequences, respectively. Indicated is the average 50% completeness level and the photometric errors at $-0.5\leq Y_\sfont{E,0}-H_\sfont{E,0}\leq0.5$.