The interstellar medium (ISM) plays an important role in the formation and
evolution of a galaxy. The ISM provides the material to form stars and stars in
turn inject radiation, metals, and mechanical energy into the ISM, altering the
physical conditions, abundances, and distribution of the ISM and affecting
future generations of star formation. It is thus essential that we understand
the physical structure of the ISM, the physical processes that operate in the
ISM, and the interplay between stars and ISM.
The physical structure and processes of the ISM are best studied in the Galaxy
and nearby galaxies, and this starts with large-scale surveys for different
components of the ISM. For example, the International Galactic Plane Survey
(IGPS) of the HI component presents the backbone of the interstellar structure
in the Galactic plane, while the HI Nearby Galaxy Survey (THINGS) maps out the
HI in nearby galaxies. Spitzer and Herschel have revealed the dust component at
high angular resolution, and the Bolocam Galactic Plane Survey (BGPS) and the
APEX Telescope Large Area Survey of the Galaxy (ATLASGAL) are mapping out the
colder dust in the Galaxy. The Atacama Large Millimeter Array (ALMA) will
provide clear view of dust and star formation in the Galaxy and nearby galaxies.
The polarimetric capabilities of Institut de Radioastronomie Millimetrique
(IRAM) in lines, Planck in continuum, and the upcoming ALMA in mm-submm
wavelengths are opening a new window to probe magnetic fields. The Wisconsin
H-Alpha Mapper (WHAM) has mapped the distribution and velocities of warm ionized
gas in the Galaxy from the north and is extending to the south. Chandra and
XMM-Newton Observatories have been used to investigate the distribution and
physical properties of the 10^6 K hot ionized gas in star forming regions as
well as diffuse fields in the Galaxy, Magellanic Clouds, and nearby galaxies.
The Far UV Spectroscopic Explorer (FUSE) and the Hubble Space Telescope (HST)
STIS and COS have been used to probe the 10^5 K hot gas at interfaces and in the
Galactic halo. The Fermi Gamma-Ray Observatory has resolved the gamma-ray
emission and revealed the sites of cosmic-ray production in the Galaxy as well
as the Magellanic Clouds, and, most recently, the existence of huge bubbles in
the Galactic Center of mysterious origin.
During the past one and a half decades, Considerable progress has been made in
the numerical modelling of local and global conditions of the ISM, its
morphology and its time-dependent evolution, owing to a rapid development of
suitable hard- and software. It is now possible to follow the full non-linear
evolution of a plasma by solving the hydro- or MHD equations in high resolution
simulations with adaptive mesh refinement. One of the key results of the past
years was to recognize, and quantitatively describe, the role of compressible
turbulence in the ISM and its impact on the distribution of gas into phases, on
the mixing of chemically enriched material, on the volume and mass filling
factors of the ISM plasma, on its heating and cooling history, amongst others.
However, there is still a long way to go, as the requirements of resolution and
evolution time are extremely challenging, and analytical models are needed to
help us understand complex numerical results. Thus, it is time to review where
we stand, and to examine further steps for the future that would lead to a
meaningful interpretation of the observations.
A great deal has already been learned from the recent advances in observations
and theories. Most meetings have been organized to focus on specialized topics
or missions, for example, "Stormy Cosmos: The Evolving ISM from Spitzer to
Herschel and Beyond" held at Pasadena in November 2010. There has not been a
general meeting for the ISM as a whole in the last 15 years. The most "recent"
general ISM meetings were held in 1995 "Physics of the ISM and IGM" and "The
Interplay between Massive Star Formation, the ISM, and Galaxy Evolution" and in
2002 "From Observations to Self-Consistent Modeling of the ISM in Galaxies".
None of these were IAU meetings. It is time to hold an IAU Symposium to update
people on recent advances in the ISM observations and theories, discuss a
comprehensive picture of the ISM in a galaxy, connect the ISM in the Galaxy to
those of nearby galaxies and even high-z galaxies, especially since recent
cosmological simulations have shown the importance of feedback mechanisms on the
cluster IGM and galaxy ISM scale. Our proposed IAU Symposium to be held at the
IAU GA meeting will maximize the participation of both ISM and extragalactic
researchers and stimulate discussions across disciplines.
Another important goal of our proposed symposium is to produce a proceeding
volume containing a comprehensive set of proceedings papers that can adequately
represent our current understanding of the ISM and be used as a desk reference
or a textbook. Note that a concurrent symposium proposal "Molecular Gas, Dust,
and Star Formation in Galaxies" has been submitted by Drs. Martin Bureau and
Yasuo Fukui. Our proposed symposium emphasizes the physical structure/conditions
and dynamical processes of the ISM in general, including all phases, both
observationally and theoretically, while their proposed symposium emphasizes the
cold phases of the ISM and the star formation process. These two symposia will
cover very different topics, but are complementary to each other and are
proposed to be held in two consecutive weeks during the IAU GA in Beijing in
August 2012. It is likely that most of those attending our symposium will attend
their symposium, and vice versa.
Finally, we would like to dedicate this symposium to the late Professor John
Dyson, the president of Div VI in 2003-2006. John has pioneered the dynamical
interactions of stellar winds and outflows with the interstellar gas. He
authored the book "The Physics of the Interstellar Medium", which is widely used
as a textbook for ISM courses. His enthusiasm and vision has guided many of us
in the study of interstellar dynamics. He is sorely missed by all of us.