Nearby Galaxies

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. Scientists in the R&G division are using nearby galaxies to study the connections between atomic gas, molecular gas, star formation (traced by ultraviolet, optical, and infrared emission), and fueling of active galactic nuclei.