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The Passage of Light within Spiral Galaxies
Description and Aims
The transport of light through a spiral galaxy is one of the new generation of complex problem in Astrophysics. The interplay between the light emitted by different stellar populations with the interstellar matter’s molecules --dust and gas-- results in a spectrum where stellar light and dust are equally important; it is an observational fact that some 30-50% of the blue stellar light is absorbed by interstellar dust and re-emitted in longer wavelengths. Current models of energy transport through a spiral disk fail to explain how this exactly happens (the “Spiral Disk Energy Budget Crisis”, Baes et al. 2010). A particular useful perspective on spiral disks is edge-on: it allows us to characterize the vertical structure of the interstellar matter (dust) and stellar disks, a virtual unknown in current models. This workshop aimed to bring together as many authors of Spectral Energy Distribution (SED) models to work towards a common solution and move beyond results on single objects.
A stated aim of this workshop was to get every group to model the 28 galaxies for which full observational coverage had been obtained with their model. It was quickly realized that the Gent groups’s efforts to generate fits and models were far more suitable and advanced to this particular aim than those by the Florence or Leicester groups. The latter two will serve as “sanity checks” in particular cases to verify the results of the Gent Group’s FitSKIRT results. Significant progress has been made to move away from a single canonical example galaxy (NGC 891) towards results on small samples of galaxies. In this respect the push from this workshop was very successful: the data was circulated round to several members of the Gent groups and fits and analysis were started already during the workshop.
Collective epiphanies were that (a) generalizing from a single galaxy has significant weaknesses and, while observationally and computationally expensive, one needs samples to arrive at general conclusions, (b) a template Southern Galaxy was desirable (NGC 5746 seems to have won), (c) two groups got very similar results on the Sombrero galaxy independently (Bendo and Peletier), adding a lot of confidence to two completely different approaches. The next two observational steps are IFU and ALMA observations.
Format of the workshop
The published program was kept tight on time on purpose (typically 20 min talks, no scheduled room for discussion right after) with a large block of time in the afternoon for targeted discussion. This is of course never how discussions happen (they happen during or right after a talk) but in the published program forced everyone to prepare a 20 min talk (rather than 30 min…). Thus talks were on time and the program casually used up the discussion time in the afternoon as discussions started during/after talks. It kept talks and discussions short and to the point. For the Snellius environment, this may be a useful approach.
The workshop progressed smoothly with the above informal format (for example, a tally of who mentioned NGC 891 was kept on one side). The Junior/Senior ratio was good but the gender balance reflected the small number statistics for the tiny sub-field (Christina Popescu had to beg off at the last minute due to illness). Another tangible result was the idea to hold a Radiative Transfer Summer School at the Lorentz Center Oort location as a bridge between those who model light through galaxies and the passage of light though tissue.
Maarten Baes (Gent, Belgium)
Simone Bianchi (Florence, Italy)
Benne W. Holwerda (Leiden, Netherlands)