Alignments of galaxies within cosmic filaments from SDSS DR7
  Using a sample of galaxy groups selected from the Sloan Digital Sky Survey
Data Release 7 (SDSS DR7), we examine the alignment between the orientation of
galaxies and their surrounding large scale structure in the context of the
cosmic web. The latter is quantified using the large-scale tidal field,
reconstructed from the data using galaxy groups above a certain mass threshold.
We find that the major axes of galaxies in filaments tend to be preferentially
aligned with the directions of the filaments, while galaxies in sheets have
their major axes preferentially aligned parallel to the plane of the sheets.
The strength of this alignment signal is strongest for red, central galaxies,
and in good agreement with that of dark matter halos in N-body simulations.
This suggests that red, central galaxies are well aligned with their host
halos, in quantitative agreement with previous studies based on the spatial
distribution of satellite galaxies. There is a luminosity and mass dependence
that brighter and more massive galaxies in filaments and sheets have stronger
alignment signals. We also find that the orientation of galaxies is aligned
with the eigenvector associated with the smallest eigenvalue of the tidal
tensor. These observational results indicate that galaxy formation is affected
by large-scale environments, and strongly suggests that galaxies are aligned
with each other over scales comparable to those of sheets and filaments in the
cosmic web.
