Abstract

Context: Substructures in stellar haloes are a strong prediction of galaxy formation models in ΛCDM. Cold streams such as those from small satellite galaxies are extremely difficult to detect and kinematically characterize. The COld STream finder Algorithm (COSTA) is a novel algorithm able to find streams in the phase space of planetary nebulae (PNe) and globular cluster (GC) populations. COSTA isolates groups of (N) particles with small velocity dispersion (between 10 km s⁻¹ and ∼120 km s⁻¹) using an iterative (n) sigma-clipping over a defined number of (k) neighbor particles.

Aims: We applied COSTA to a catalog of PNe and GCs from the Fornax Cluster VLT Spectroscopic Survey (FVSS) within ∼200 kpc from the cluster core in order to detect cold substructures and characterize their kinematics (mean velocity and velocity dispersion).

Methods: We selected more than 2000 PNe and GCs from the FVSS catalogs and adopted a series of optimized setups of the COSTA parameters based on Montecarlo simulations of the PN and GC populations to search for realistic stream candidates. We find 13 cold substructures with velocity dispersion ranging from ∼20 to ∼100 km s⁻¹, which are likely associated either to large galaxies or to ultra-compact dwarf (UCD) galaxies in the Fornax core.

Results: The luminosities of these streams show a clear correlation with internal velocity dispersion, and their surface brightness correlates with their size and distance from the cluster center, which is compatible with the dissipative processes producing them. However, we cannot exclude that some of these substructures formed by violent relaxation of massive satellites that finally merged into the central galaxy. Among these substructures we have: (1) a stream connecting NGC 1387 to the central galaxy, NGC 1399, previously reported in the literature; (2) a new giant stream produced by the interaction of NGC 1382 with NGC 1380 and (possibly) NGC 1381; (3) a series of streams kinematically connected to nearby UCDs; and (4) clumps of tracers with no clear kinematical association to close cluster members.

Conclusions: We show evidence for a variety of cold substructures predicted in simulations. Most of the streams are kinematically connected to UCDs, supporting the scenario that they can be remnants of disrupted dwarf systems. However, we also show the presence of long coherent substructures connecting cluster members and isolated clumps of tracers possibly left behind by their parent systems before these merged into the central galaxy. Unfortunately, the estimated low-surface brightness of these streams does not allow us to find their signatures in the current imaging data and deeper observations are needed to confirm them.