  As the millimeter wavelength range remains a largely unexplored spectral
region for galaxies, the IMEGIN large program aims to map the millimeter
continuum emission of 22 nearby galaxies at 1.15 and 2 mm. Using the
high-resolution maps produced by the NIKA2 camera, we explore the existence of
very cold dust and take possible contamination by free-free and synchrotron
emission into account. We study the IR-to-radio emission coming from different
regions along the galactic plane and at large vertical distances. New
observations of NGC 891, using the NIKA2 camera on the IRAM 30m telescope,
along with a suite of observations at other wavelengths were used to perform a
multiwavelength study of the spectral energy distribution in the interstellar
medium in this galaxy. This analysis was performed globally and locally, using
the advanced hierarchical Bayesian fitting code, HerBIE, coupled with the
THEMIS dust model. Our dust modeling is able to reproduce the near-IR to
millimeter emission of NGC 891, with the exception of an excess at a level of
25% obtained by the NIKA2 observations in the outermost parts of the disk. The
radio continuum and thermal dust emission are distributed differently in the
disk and galaxy halo. Different dusty environments are also revealed by a
multiwavelength investigation of the emission features. Our detailed
decomposition at millimeter and centimeter wavelengths shows that emission at 1
mm is purely originated by dust. Radio components become progressively
important with increasing wavelengths. Finally, we find that emission arising
from small dust grains accounts for ~ 9.5% of the total dust mass, reaching up
to 20% at large galactic latitudes. Shock waves in the outflows that shatter
the dust grains might explain this higher fraction of small grains in the halo.
