Nearby galaxies supply us with the opportunity to study galaxy dynamics
and star formation on large scales, yet are close enough to reveal the
details needed to make connections to the phenomena we observe around us
in the Milky Way. Nearby galaxy studies therefore provide a better
understanding of the physical properties of our own Galaxy, in which
observations are often hampered by high obscuration from dust and by our
location inside its disk. Additionally, studying a diverse sample of
nearby galaxies allows us to determine the evolution of galaxies with
time, and to better constrain the conditions and evolution of high
redshift galaxies that we currently cannot resolve.
Radio waves offer a particularly powerful means to probe the cool
interstellar medium in nearby galaxies. Much of the interstellar medium in
typical star-forming galaxies is comprised of neutral atomic hydrogen (HI)
which can be detected via its spectral line transition at 21-cm. This
21-cm transition was first detected by Harold Ewen and Edward Purcell in
1951, using a simple horn antenna located on the Harvard campus. Today the
HI 21-cm line remains one of the most valuable tracers of the kinematics
of nearby galaxies, since it is not obscured by dust or starlight, and
often it can trace the disks of galaxies beyond where their outermost
stars are observed.
The interstellar medium of galaxies also contains a denser, molecular gas
component that emits radiation at millimeter and submillimeter wavelengths.
Clouds of molecular gas collapse to form new stars and planets and are
also thought to be one of the major fuel sources in the nuclei of active
galaxies. The figure on the right shows an optical image of the spiral
galaxy NGC 7331 compared with the warm molecular gas distribution observed
with the SMA. The molecular gas is wrapped into a ring-like structure,
with no convincing evidence for gas in the very center. Thus the fuel
source for this active galaxy remains a mystery.
The molecular gas distribution can be deconvolved into a series of
individual clumps with sizes of the order of giant molecular clouds (GMCs)
with a mass spectrum similar to that seen in other nearby galaxies such as
M33 and M31. Detailed analyses such as this will lead to a better
understanding of the processes involved in the evolution of star formation
and AGN activity and their impact on galaxy evolution in general.
The Submillimeter Array
Christine Wilson, Kazushi Sakamoto,