The galaxy you are looking at is the result of an N-body+SPH simulation of galaxy formation and evolution performed using the code GASOLINE. This simulation has been post-processed using the dust radiation transfer code DART-Ray in order to calculate galaxy images at the different wavelengths and viewing angles. This spiral galaxy has been formed out of primordial gas, embedded in a dark matter halo, that has been let to evolve in isolation for 10 Gyr. During this time, new stars have been created out of dense gas regions within the galaxy simulation. These stars have affected the subsequent evolution of the gas through both energy feedback, due to stellar winds and supernova explosions, but also by enriching the gas with “metals”, chemical elements heavier than Helium. Dynamical interaction involving stars, gas and dark matter have also determined the current structure of the galaxy. Note its numerous features such as the stellar disk, the spiral arms, the bulge, the bar, the central nuclear disk.
Using this tool, you are able to visualize the galaxy simulation at a set of wavelengths spanning the ultraviolet, optical and infrared ranges. The emission that you see at each wavelength regime is usually dominated by very specific galaxy components.
In the UV the emission comes mainly from young stars, that is, stars with ages less than 100 million years (Myr). These stars are predominantly concentrated within the galaxy disk and in particular along the spiral arms. This galaxy also contains a central nuclear disk, where substantial star formation is ongoing. You can also notice some bright spots overlaid on top of the spiral arms. These appear at the locations of star formation regions, the places where new stars are forming.
In the optical and near-infrared, you can see the emission from old stars, that is, stars with ages higher than 100 Myr. Generally, the longer the wavelength, the older is the stellar population that dominate the emission at that wavelength. You can notice that the old stars are more widely distributed compared to the young stars. They are largely found in a disk but this disk is thicker compared to the one occupied by the young stars. In addition, the old stars are the main stellar population within the galaxy bulge and halo.
In the mid- and far-infrared the emission you are seeing is mainly produced by interstellar dust particles. These particles are cold (typical equilibrium temperatures in the range 1-100 K) and they are well mixed with the more metal rich gas. Although in terms of mass the dust particles are only a tiny fraction of the total galaxy baryonic mass (about 0.1%), their emission dominates the galaxy infrared luminosity. However, the energy powering this emission still comes from stars! In fact, dust absorbs part of the radiation produced by the stars, in the UV and optical ranges, and it re-emits it in the infrared range. In particular, the mid-infrared emission is mainly powered through the absorption of light produced by young stellar populations. For this reason, the morphology of the mid-infrared emission resembles the one you can see in the UV for the young stars. In the far-infrared (>70 um) the dust emission is powered by both young and old stellar populations. For this reason, the emission appears more smooth and diffuse compared to the mid-infrared emission. Note that you can realize the presence of dust also from the UV and optical images. The dark patches that you see, in particular when the galaxy is more inclined, are due to the absorption of stellar light by the interstellar dust!
Giovanni Natale, Cristina Popescu, Richard Tuffs
Thanks to Benjamin Thompson and Adam Clarke for technical support.
We acknowledge financial support from:
Leverhulme Trust research project grant RPG-2013-41