  Investigating the dust heating mechanisms in galaxies provides a deeper
understanding of how the internal energy balance drives their evolution. Over
the last decade, radiative transfer simulations based on the Monte Carlo method
have underlined the role of the various stellar populations heating the diffuse
dust. Beyond the expected heating through ongoing star formation, both older
stellar population (> 8Gyr) and even AGN can contribute energy to the infrared
emission of diffuse dust. Here, we examine how the radiation of an external
heating source, like the less massive galaxy NGC5195, in the M51 interacting
system, could affect the heating of the diffuse dust of its parent galaxy,
NGC5194, and vice versa. To quantify the exchange of energy between the two
galaxies we use SKIRT, a state-of-the-art Monte Carlo radiative transfer code.
In the interest of modelling, the assumed centre-to-centre distance separation
between the two galaxies is 10kpc. Our model reproduces the global SED of the
system, and it closely matches the observed images. In total, 40.7% of the
intrinsic stellar radiation of the combined system is absorbed by dust.
Furthermore, we quantify the contribution of the various dust heating sources
in the system, and find that the young stellar population of NGC5194 is the
predominant dust-heating agent, with a global heating fraction of 71.2%.
Another 23% is provided by the older stellar population of the same galaxy,
while the remaining 5.8% has its origin in NGC5195. Locally, we find that the
regions of NGC5194 closer to NGC5195 are significantly affected by the
radiation field of the latter, with the absorbed energy fraction rising up to
38%. The contribution of NGC5195 remains under the percentage level in the
outskirts of the disc of NGC5194. This is the first time that the heating of
the diffuse dust by a companion galaxy is quantified in a nearby interacting
system.
