Gas and galaxies in filament between clusters of galaxies: The study of
  A399-A401
  We have performed a multi-wavelength analysis of two galaxy cluster systems
selected with the thermal Sunyaev-Zel'dovich (tSZ) effect and composed of
cluster pairs and an inter-cluster filament. We have focused on one pair of
particular interest: A399-A401 at redshift z~0.073 seperated by 3 Mpc. We have
also performed the first analysis of one lower significance newly associated
pair: A21-PSZ2 G114.09-34.34 at z~0.094, separated by 4.2 Mpc. We have
characterised the intra-cluster gas using the tSZ signal from Planck and, when
this was possible, the galaxy optical and infra-red (IR) properties based on
two photometric redshift catalogs: 2MPZ and WISExSCOS. From the tSZ data, we
measured the gas pressure in the clusters and in the inter-cluster filaments.
In the case of A399-A401, the results are in perfect agreement with previous
studies and, using the temperature measured from the X-rays, we further
estimate the gas density in the filament and find n0=4.3+-0.7x10^-4 cm-3. The
optical and IR colour-colour and colour-magnitude analyses of the galaxies
selected in the cluster system, together with their Star Formation Rate, show
no segregation between galaxy populations, in the clusters and in the filament
of A399-A401. Galaxies are all passive, early type, and red and dead. The gas
and galaxy properties of this system suggest that the whole system formed at
the same time and corresponds to a pre-merger, with a cosmic filament gas
heated by the collapse. For the other cluster system, the tSZ analysis was
performed and the pressure in the clusters and in the inter-cluster filament
was constrained. However, the limited or nonexistent optical and IR data
prevent us from concluding on the presence of an actual cosmic filament or from
proposing a scenario.
