COSTA: the COld STream finder Algorithm. Searching for kinematical
  substructures in the phase space of discrete tracers
  Context: We present COSTA (COld STream finder Algorithm), a novel algorithm
to search for cold kinematical substructures in the phase space of planetary
nebulae (PNe) and globular clusters (GCs) in the halo of massive galaxies and
intracluster regions. Aims: COSTA aims at detecting small sized, low velocity
dispersion streams, as the ones produced in recent interactions of dwarf
galaxies with the halo of more massive galaxies, including the ones sitting in
the central region of rich galaxy clusters. Methods: COSTA is based on a deep
friend-of-friend procedure that isolates groups of N particles with small
velocity dispersion (between 10 kms and $\sim$ 100 kms), using an iterative (n)
sigma-clipping over a defined number of (k) neighbor particles. The algorithm
has three parameters (k-n-N), plus a velocity dispersion cut-off, which defines
the "coldness" of the stream, that are set using Montecarlo realizations of the
sample under exam. Results: In this paper, we show the ability of COSTA to
recover simulated streams on mock data-sets of discrete kinematical tracers
with different sizes and measurement errors, from publicly available
hydrodynamical simulations. We also show the best algorithm set-up for a
realistic case of stream finding in the core of the Fornax cluster, for future
applications of COSTA to real populations of PNe and GCs. Conclusions: COSTA
can be generalized to all problems of finding small substructures in the phase
space of a limited sample of discrete tracers, provided that the algorithm is
trained on realistic mock observations reproducing the specific dataset under
exam.
