Back to Home

An ESF exploratory workshop on:

Tracing Dust in Spiral Galaxies:

radiative transfer studies in the dawn
of a new generation of observing facilities

May 14-16, 2007
Het Pand - Ghent - Belgium

Scientific case

Background: The interstellar medium of spiral galaxies is filled by gas and small solid particles, dust grains. Despite constituting only a minor fraction of the galactic mass (between 0.1% and 0.01% for the Milky Way), dust grains have a major role in shaping the appearance of a galaxy. Because of their dimension (typically smaller than a few tenth of a micron), they are very effective in absorbing and scattering the radiation emitted by stars in the ultraviolet, optical and near-infrared. This is evident in the extinction lanes seen in galaxies at high inclination (like the Milky Way or NGC891 in the Figure - top).

Figure: The edge-on spiral galaxy NGC 891: dust extinction is clearly seen in an optical image (top picture), where
the intensity of the stellar radiation is reduced along the disk (brighter tones in the picture means dimmer radiation output);
dust emission at longer wavelengths, as FIR or the submm  (bottom picture) allows a direct detection of the grains.

As extinction is wavelength dependent, dust affects not only the shape of a galaxy, but also modifies its spectrum: thus, corrections are needed to retrieve the intrinsic luminosities of each stellar population. Because of this, in the past dust was often simply treated as a nuisance that hindered optical observations. Nowadays, instead, its importance is fully recognised. In particular, dust grains are a major contributor to a galaxy's radiation output, by emitting at far-infrared (FIR) and sub-millimetre (submm) wavelengths the energy they have absorbed (See Figure - bottom). Dust influence not only limits to the energy redistribution, but is a major ingredient in a wide range of studies, from planet and star formation in the interstellar medium (where grains regulate the chemistry and the heating/cooling mechanisms) to cosmology (where dust hides the strong UV emission of star forming galaxies and allows, with its emission, the detection of distant objects).

Observing the dust in spiral galaxies: The interstellar dust medium of galaxies can be studied in two complementary ways. On the one hand, the properties of the stellar and dust distributions can in principle be derived by comparing images of galaxies with models for the transfer of stellar radiation through an opaque dusty medium. Since scattering cannot be neglected (in the optical, about 60% of the light impinging on a dust grain is di used) and the geometries of spiral galaxies are complex (exponential disks, spheroidal bulges, structures like arms and bars, inhomogeneities), time-consuming analytical or Monte Carlo techniques are necessary to produce realistic simulations. On the other hand, dust can be traced directly by observations at long wavelengths. Unfortunately, the FIR/submm data available today (mainly from the IRAS and ISO space missions) lack the necessary sensitivity and resolution to study the dust content of galaxies in detail. Due to the difficulties attached to each method, we still have no clear understanding of the interstellar dust medium in spiral galaxies. Only for a very limited number of nearby edge-on spiral galaxies, we have been able to study the dust medium in some detail. Radiative transfer modeling of the optical images suggests that the dust in spiral galaxies resides in a geometrically thin disk that is only moderately opaque, with less than 10% of starlight absorbed by dust grains. However, a strong discrepancy was found between such model predictions and the observed infrared uxes. Data from the ISO satellite have shown that normal spiral galaxies emit about 30% of their total bolometric luminosity in the FIR, meaning that almost one third of starlight is absorbed by dust. This is clearly in contrast with the models derived from optical studies. So far, the reason for this important discrepancy in energy balance is unclear, both because of the lack of a large multi-wavelength data set on spiral galaxies, and because of limitations in the current radiative transfer models (which prevent the modelling of features-rich objects such as the more abundant galaxies seen face-on). This means we can still not answer important questions such as: How much dust is present in a galaxy? How is dust distributed with respect to stars? What environments are responsible for dust extinction/emission in different wavelength regimes? What are the properties of dust responsible for the bulk of emission? Are there differences in the dust contents of galaxies of different morphological type?

A new era: At this very moment, an new era is opening up for the study of interstellar dust in galaxies. On the one hand, new powerful radiative transfer techniques are being explored (mainly driven by radiative transfer problems in other areas such as star formation and cosmology). By adapting and extending these techniques to galactic environments, we can now construct realistic and efficient 3D radiative transfer simulations for dusty galaxies that simulate in detail both the extinction of starlight and the emission of FIR/submm emission of dust grains. On the other hand, the capabilities of observing in the FIR/submm window are developing at an impressive pace. The recent advent of the Spitzer Space Telescope has marked a new landmark in the FIR astronomy. Within the next few years, an entirely novel generation of high-sensitivity FIR and submm instrumentation will revolutionise the study of dust in galaxies. These instruments include large and sensitive bolometer arrays on large ground-based submm telescopes (including LABOCA on the APEX telescope and SCUBA-2 on the JCMT telescope) as well as space-born missions (such as ESA's forthcoming Herschel Space Observatory and the Japanese-European Akari FIR mission).

An ESF workshop on dust in spiral galaxies: European astronomers have always been at the forefront of FIR astronomy, the study of interstellar dust and the development of radiative transfer codes. In order to maintain this leading position, a close collaborative effort between observers, modelers and theoreticians working on radiative transfer and dust in spiral galaxies (and beyond) is needed. With this ESF workshop, we propose to develop a broad pan-European platform for the observation, analysis and modeling of dust in spiral galaxies. The proposed workshop will take a multidisciplinary approach, with only part of the topics directly related to radiative transfer and dust emission and extinction. A good fraction of the time will be dedicated to other observables that trace the matter distribution in a spiral galaxy, as well as different ISM environments. We therefore invite specialists from other field in the extragalactic astronomical community and foresee a lively discussion and interaction between theoreticians and observers in virtually all fields of observational astronomy (including UV, optical, infrared, submm and radio astronomy). It will hopefully create a working group of European researchers, which will share their expertise on different disciplines in galactic studies. In particular, observers and modelers will discuss strategies for planning observations with current and future instrumentation, defining together a galaxy sample which is the most suitable for radiative transfer studies. To continue the collaboration, we will also explore the possibility of participating to funding opportunities offered by the ESF and EC Framework Programmes. The numerous collaborations that can sprout from the workshop will constitute a valuable background to fully exploit the observing capabilities of future high sensitivity FIR and submm instrumentation to which European nationals will have access: the PACS and SPIRE instruments aboard the Herschel Space Observatory which is planned to be launched in 2007 by the European Space Agency; the LABOCA large field bolometer array, which will operate at the APEX telescope (a German, Swedish and European Southern Observatory collaboration) in the southern hemisphere, scheduled for mid 2006; the SCUBA-2 array at the UK - Dutch - Canadian James Clerk Maxwell Telescope, operative from 2007; the recently launched Akari satellite, a FIR satellite launched by the Japanese Space Agency with the support of the European Space Agency; the Atacama Large Millimetric Array, a huge (sub)millimetric interferometric observatory to be built in northern Chile by a European, American and Japanese consortium.

Back to Home