Context : Outskirts of large galaxies contain important information about galaxy formation and assembly .
Resolved star count studies can probe the extremely low surface brightness of the outer halos .

Aims : NGC 253 is a nearly edge-on disk galaxy in the Sculptor group where we resolved the halo stars from ground-based images , with the aim of studying its stellar population content , the structure and the overall extent of the halo .

Methods : We use Z and J-band images from the VIRCAM camera mounted on the VISTA telescope to construct the spatially resolved J vs. Z-J colour-magnitude diagrams ( CMDs ) .
The very deep photometry and the wide area covered allows us to trace the red giant branch ( RGB ) and asymptotic giant branch ( AGB ) stars that belong to the halo of NGC 253 out to 50 kpc along the galaxy minor axis .

Results : We confirm the existence of an extra planar stellar component of the disk , with a very prominent southern shelf and a symmetrical feature on the north side .
The only additional visible sub-structure is an overdensity in the north-west part of the halo \sim 28 kpc distant from the plane and extending over 20 kpc parallel with the disk .
We measure the transition from the disk to the halo at a radial distance of about 25 kpc with a clear break in the number density profile .
The isodensity contours show that the inner halo is a flattened structure that blends with a more extended , diffuse , rounder outer halo .
Such external structure can be traced to the very edge of our image out to 50 kpc from the disk plane .
The number density profile of the stars in the stellar halo follows a power law with index -1.6 , as function of radius .
The CMD shows a very homogeneous stellar population across the field ; by comparing with isochrones we conclude that the RGB stars are \sim 8 Gyr old or more , while the AGB stars trace a population of about 2 \times 10 ^ { 8 } M _ { \odot } formed from \sim 0.5 to a few Gyr ago .
Surprisingly , part of this latter population appears scattered over a wide area .
We explore several ideas to explain the origin of this relatively young component in the inner halo of NGC 253 .

Conclusions :