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Дата изменения: Thu May 24 13:19:47 2007 Дата индексирования: Mon Oct 1 22:40:04 2012 Кодировка: Поисковые слова: universe |
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An ESF exploratory workshop on:Tracing Dust
in
Spiral Galaxies:
|
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).
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. |
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