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Outskirts of large galaxies contain important information about the galaxyformation and assembly process, and resolved star count studies can probe theextremely low surface brightness of the outer halos. We use images obtainedwith the VISTA telescope to construct spatially resolved J vs Z-Jcolour-magnitude diagrams (CMDs) of NGC 253, a nearly edge-on disk galaxy inthe Sculptor group. The very deep photometry, down to J ~ 23.5, and the widearea covered allows us to trace the red giant branch (RGB) and asymptotic giantbranch (AGB) stars that belong to the outer disk and the halo of NGC 253, outto 50 kpc along the galaxy minor axis. We confirm the existence of an extraplanar stellar component of the disk, with a very prominent southern shelf anda symmetrical feature on the north side. The only additional visiblesub-structure is an overdensity in the north-west part of the halo at about 28kpc from the plane and extending over ~ 20 kpc parallel with the disk of thegalaxy. From the stellar count profile along the major axis we measure thetransition from the disk to the halo at a radial distance of about 25 kpc,where a clear break appears in the number density profile. The isodensitycontours show that the inner halo is a flattened structure that blends with amore extended, diffuse, rounder outer halo. Such external structure can betraced to the very edge of our image out to 50 kpc from the disk plane. Thenumber density profile of the stars in the stellar halo follows a power lawwith index -1.6, as function of radius. The CMD shows a very homogeneousstellar population across the whole field; by comparison with theoreticalisochrones we conclude that the RGB stars are ~ 8 Gyr old or more, while theAGB stars trace a population of about 2 x 10^8 Mo, formed from ~ 0.5 to a fewGyr ago. Surprisingly, part of this latter population appears scattered over awide area.

A panoramic VISTA of the stellar halo of NGC 253