A multiwavelength infrared study of NGC 891

We present a multiwavlength infrared (IR) study of the nearby, edge-on, spiral galaxy NGC 891. We have examined 20 independent, spatially resolved IR images of this galaxy, 14 of which are newly reduced and/or previously unpublished images. These images span a wavelength regime from lambda 1.2 mu m in which the emission is dominated by cool stars, through the mid-IR, in which emission is dominated by polycyclic aromatic hydrocarbons (PAHs), to lambda 850 mu m, in which emission is dominated by cold dust in thermal equilibrium with the radiation field. The changing morphology of the galaxy with wavelength illustrates the changing dominant components. We detect extraplanar dust emission in this galaxy, consistent with previously published results, but now show that PAH emission is also in the halo, to a vertical distance of z >= 2.5 kpc. We compare the vertical extents of various components and find that the PAHs (from lambda 7.7 and 8 mu m data) and warm dust (lambda 24 mu m) extend to smaller z heights than the cool dust (lambda 450 mu m). For six locations in the galaxy for which the signal-to-noise ratio was sufficient, we present spectral energy distributions (SEDs) of the IR emission, including two in the halo - the first time a halo SED in an external galaxy has been presented. We have modelled these SEDs and find that the PAH fraction, f(PAH), is similar to Galactic values (within a factor of 2), with the lowest value at the galaxy's centre, consistent with independent results of other galaxies. In the halo environment, the fraction of dust exposed to a colder radiation field, f(cold), is of the order of unity, consistent with an environment in which there is no star formation. The source of excitation is likely from photons escaping from the disc.