Galaxy spin alignment in filaments and sheets: observational evidence
  Galaxies properties are known to be affected by their environment. One
important question is how their angular momentum reflects the surrounding
cosmic web. We use the SDSS to investigate the spin axes of spiral and
elliptical galaxies relative to their surrounding filament/sheet orientations.
To detect filaments a marked point process with interactions (the "Bisous
model") is used. Sheets are found by detecting "flattened" filaments. The minor
axes of ellipticals are found to be preferentially perpendicular to hosting
filaments. A weak correlation is found with sheets. These findings are
consistent with the notion that elliptical galaxies formed via mergers which
predominantly occurred along the filaments. The spin axis of spiral galaxies is
found to align with the host filament, with no correlation between spiral spin
and sheet normal. When examined as a function of distance from the filament
axis, a much stronger correlation is found in outer parts, suggesting that the
alignment is driven by the laminar infall of gas from sheets to filaments. When
compared with numerical simulations our results suggest that the connection
between DM halo and galaxy spin is not straightforward. Our results provide an
important input to the understanding of how galaxies acquire their angular
momentum.
