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We have studied the extent of the Red Giant Branch stellar population in theFornax dwarf spheroidal galaxy using the spatially extended and homogeneousdata set from Gaia EDR3. Our preselection of stars belonging to Fornax is basedon their proper motions, parallaxes and color-magnitude diagram. The lattercriteria provide a Fornax star sample, which we further restrict by color andmagnitude to eliminate contaminations due to either Milky Way stars or QSOs.The precision of the data has been sufficient to reach extremely smallcontaminations (0.02 to 0.3%), allowing us to reach to a background level 12magnitudes deeper than the central surface brightness of Fornax. We discover abreak in the density profile, which reveals the presence of an additionalcomponent that extents 2.1 degree in radius, i.e. 5.4 kpc, and almost seventimes the half-light radius of Fornax. The extended new component represents10% of the stellar mass of Fornax, and behaves like an extended halo. Theabsence of tidally elongated features at such an unprecedented depth(equivalent to $37.94\pm0.16$ mag ${\rm arcsec}^{-2}$ in V-band) rules out apossible role of tidal stripping. We suggest instead that Fornax is likely atfirst infall, and has lost its gas very recently, which consequently leads to alack of gravity implying that residual stars have spherically expanded to formthe newly discovered stellar halo of Fornax.

An extended stellar halo discovered in the Fornax dwarf spheroidal using Gaia EDR3