Halo dust detection around NGC 891
  Observations of edge-on galaxies allow us to investigate the vertical extent
and properties of dust, gas and stellar distributions. NGC 891 has been studied
for decades and represents one of the best studied cases of an edge-on galaxy.
We use deep PACS data together with IRAC, MIPS and SPIRE data to study the
vertical extent of dust emission around NGC 891. We also test the presence of a
more extended, thick dust component. By performing a convolution of an
intrinsic vertical profile emission with each instrument PSF and comparing it
with observations we derived the scaleheight of a thin and thick dust disc
component. For all wavelengths considered the emission is best fit with the sum
of a thin and a thick dust component. The scaleheight of both dust components
shows a gradient passing from 70 $\mu$m to 250 $\mu$m. This could be due to a
drop in dust heating (and thus dust temperature) with the distance from the
plane, or to a sizable contribution ($\sim 15 - 80%$) of an unresolved thin
disc of hotter dust to the observed surface brightness at shorter wavelengths.
The scaleheight of the thick dust component, using observations from 70 $\mu$m
to 250 $\mu$m has been estimated to be $(1.44\pm 0.12)$ kpc, consistent with
previous estimates (extinction and scattering in optical bands and MIR
emission). The amount of dust mass at distances larger than $\sim 2$ kpc from
the midplane represents $2 - 3.3$ % of the total galactic dust mass and the
relative abundance of small grains with respect to large grains is almost
halved comparing to that in the midplane. The paucity of small grains high
above the midplane might indicate that dust is hit by interstellar shocks or
galactic fountains and entrained together with gas. The halo dust component is
likely to be embedded in an atomic / molecular gas and heated by a thick
stellar disc.
