Galaxies infalling into groups: filaments vs. isotropic infall
  We perform a comparative analysis of the properties of galaxies infalling
into groups classifying them accordingly to whether they are: falling along
filamentary structures; or they are falling isotropically. For this purpose, we
identify filamentary structures connecting massive groups of galaxies in the
SDSS. We perform a comparative analysis of some properties of galaxies in
filaments, in the isotropic infall region, in the field, and in groups. We
study the luminosity functions (LF) and the dependence of the specific star
formation rate (SSFR) on stellar mass, galaxy type, and projected distance to
the groups that define the filaments. We find that the LF of galaxies in
filaments and in the isotropic infalling region are basically indistinguishable
between them, with the possible exception of late-type galaxies. On the other
hard, regardless of galaxy type, their LFs are clearly different from that of
field or group galaxies. Both of them have characteristic absolute magnitudes
and faint end slopes in between the field and group values. More significant
differences between galaxies in filaments and in the isotropic infall region
are observed when we analyse the SSFR. We find that galaxies in filaments have
a systematically higher fraction of galaxies with low SSFR as a function of
both, stellar mass and distance to the groups, indicating a stronger quenching
of the star formation in the filaments compared to both, the isotropic
infalling region, and the field. Our results suggest that some physical
mechanisms that determine the differences observed between field galaxies and
galaxies in systems, affect galaxies even when they are not yet within the
systems.
