The Next Generation Fornax Survey (NGFS). III. Revealing the Spatial Substructure of the Dwarf Galaxy Population Inside Half of Fornax's Virial Radius

We report the discovery of 271 previously undetected dwarf galaxies in the outer Fornax cluster regions at radii rvir/4 < r < rvir/2 using data from the Next Generation Fornax Survey (NGFS) with deep coadded u′, g′, and i′ images obtained with Blanco/DECam at Cerro Tololo Interamerican Observatory. From the 271 dwarf candidates, we find 39 to be nucleated. Together with our previous study of the central Fornax region, the new dwarfs detected with NGFS data number 392, of which 56 are nucleated. The total Fornax dwarf galaxy population from NGFS and other catalogs rises, therefore, to a total of 643 with 181 being nucleated, yielding an overall nucleation fraction of 28%. The absolute i′-band magnitudes for the outer NGFS dwarfs are in the range −18.80 ≤ Mi′ ≤ −8.78 with effective radii reff,i′ = 0.18–2.22 kpc and an average Sérsic index . Nonnucleated dwarfs are found to be fainter and smaller by mag and than the nucleated dwarfs. We demonstrate a significant clustering of dwarf galaxies on scales ≲100 kpc, and projected surface number density profile estimates, ΣN(r), show a concentration of dwarfs in the Fornax core region within r ≲ 350 kpc. ΣN(r) has a flat distribution up to ∼350 kpc, beyond which it declines for the nonnucleated dwarfs. The nucleated dwarfs have a steeper ΣN(r) distribution, are more concentrated toward NGC 1399, and are decreasing rapidly outwards. This is the first time the transition from cluster to field environment has been established for the very faint dwarf galaxy population with robust sample statistics.