The Next Generation Fornax Survey (NGFS): IV. Mass and Age Bimodality of
  Nuclear Clusters in the Fornax Core Region
  We present the analysis of 61 nucleated dwarf galaxies in the central regions
($<R_{\rm vir}/4$) of the Fornax galaxy cluster. The galaxies and their nuclei
are studied as part of the Next Generation Fornax Survey (NGFS) using optical
imaging obtained with the Dark Energy Camera (DECam) mounted at Blanco/CTIO and
near-infrared data obtained with VIRCam at VISTA/ESO. We decompose the
nucleated dwarfs in nucleus and spheroid, after subtracting the surface
brightness profile of the spheroid component and studying the nucleus using PSF
photometry.In general, nuclei are consistent with colors of confirmed
metal-poor globular clusters, but with significantly smaller dispersion than
other confirmed compact stellar systems in Fornax. We find a bimodal nucleus
mass distribution with peaks located at $\log({\cal M_*}/M_\odot)\!\simeq\!5.4$
and $\sim\,6.3$. These two nucleus sub-populations have different stellar
population properties, the more massive nuclei are older than $\sim\!2$ Gyr and
have metal-poor stellar populations ($Z\leq0.02\,Z_\odot$), while the less
massive nuclei are younger than $\sim\!2$ Gyr with metallicities in the range
$0.02\!<\!Z/Z_\odot\!\leq\!1$. We find that the nucleus mass (${\cal M}_{\rm
nuc}$) vs. galaxy mass (${\cal M}_{\rm gal}$) relation becomes shallower for
less massive galaxies starting around $10^8\,M_\odot$ and the mass ratio
$\eta_n\!=\!{\cal M}_{\rm nuc}/{\cal M}_{\rm gal}$ shows a clear
anti-correlation with ${\cal M}_{\rm gal}$ for the lowest masses, reaching
$10\%$. We test current theoretical models of nuclear cluster formation and
find that they cannot fully reproduce the observed trends. A likely mixture of
in-situ star formation and star-cluster mergers seem to be acting during
nucleus growth over cosmic time.
