The onset of warps in Spitzer observations of edge-on spiral galaxies
  We analyze warps in the nearby edge-on spiral galaxies observed in the {\em
Spitzer/IRAC} 4.5 micron band. In our sample of 24 galaxies we find evidence of
warp in 14 galaxies. We estimate the observed onset radii for the warps in a
subsample of 10 galaxies. The dark matter distribution in each of these
galaxies are calculated using the mass distribution derived from the observed
light distribution and the observed rotation curves. The theoretical
predictions of the onset radii for the warps are then derived by applying a
self-consistent linear response theory to the obtained mass models for 6
galaxies with rotation curves in the literature. By comparing the observed
onset radii to the theoretical ones, we find that discs with constant thickness
can not explain the observations; moderately flaring discs are needed. The
required flaring is consistent with the observations. Our analysis shows that
the onset of warp is not symmetric in our sample of galaxies. We define a new
quantity called the onset-asymmetry index and study its dependence on galaxy
properties. The onset asymmetries in warps tend to be larger in galaxies with
smaller disc scale lengths. We also define and quantify the global asymmetry in
the stellar light distribution, that we call the edge-on asymmetry in edge-on
galaxies. It is shown that in most cases the onset asymmetry in warp is
actually anti-correlated with the measured edge-on asymmetry in our sample of
edge-on galaxies and this could plausibly indicate that the surrounding dark
matter distribution is asymmetric.
