Galaxy mergers are extremely powerful mechanisms that drive the formation and evolution of galaxies in the Universe. Strong tidal forces are generated by such interactions, causing the redistribution and accumulation of baryonic matter (gas and dust inflows) in the interstellar medium (ISM), and triggering and/or enhancing the formation of new stars. However, mergers do not only interact gravitationally, but we also expect to interact radiatively. The exchange of radiation between the members of a merging system could significantly alter their spectral energy distribution.
In order to quantify the exchange of energy between the members of an interacting system, we constructed the 3D model of the radiation field of one of the most famous mergers in the local Universe, that of the grand-design spiral arm galaxy NGC 5194 (M51a), better known as the Whirlpool galaxy, and its companion, lenticular galaxy NGC 5195 (M51b). In the fifth paper of this radiative transfer paper series, we revisit and update the model of M51a (De Looze et al. 2014).
Main panel (bottom left): Optical image of M51, created by combining the SDSS u, g, r, i, z images. Small panels (top and right): 2D representations of the various model components. From left to right: a zoomed-in view of M51a's old stellar bulge, M51a's old stellar disc, a young non-ionizing stellar disc, and a young ionizing stellar disc. From top to bottom: a zoomed-in view of M51b's old stellar bulge, old stellar disc, and the combined dust distribution of M51a and M51b.
We used SKIRT, a state-of-the-art 3D Monte Carlo radiative transfer code, to construct the 3D model of the radiation field of M51, following the methodology defined in the DustPedia framework. In the interest of modelling, the assumed center-to-center distance separation between the two galaxies is 10kpc.
Dust heating maps of a face-on view of M51. Left panel: Dust heating fraction by the old stellar population; Middle panel: Dust heating fraction by the young stellar populations; and Right panel: Dust heating fraction by M51b. The rightmost map is shown on a log-scale, and the others are on a linear scale. The inner and outer green circles indicate the 3kpc and 8kpc radii, respectively. The red cross is the center of M51b. The gold circle indicates the extent of the dusty disc of M51b at 2kpc radius.
We quantify the contribution of the various dust heating sources in the system, and find that the young stellar population of M51a 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 M51b. Locally, we find that the regions of M51a closer to M51b are significantly affected by the radiation field of the latter, with the absorbed energy fraction rising up to 38%. The contribution of M51b remains under the percentage level in the outskirts of the disc of M51a. This is the first time that the heating of the diffuse dust by a companion galaxy is quantified in a nearby interacting system.
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