This giant spiral disk of stars, dust and gas is 170,000 light-years across, or nearly twice the diameter of our Milky Way galaxy. (Credit for Hubble Image: NASA, ESA, K. Kuntz (JHU), F. Bresolin (University of Hawaii), J. Trauger (Jet Propulsion Lab), J. Mould (NOAO), Y.-H. Chu (University of Illinois, Urbana), and STScI)
Galaxies are gravitationally bound collections of stars, planets, gas, dust, dark matter and other objects, including black holes. They represent the standard "building blocks" of large-scale structure in the universe. Galaxies group hierarchically into clusters of galaxies and superclusters. These groupings in turn comprise larger structures of long, luminous filaments and sheets of galaxies, separated by gaping voids containing few galaxies. In general, galaxies come in three broad shapes and sizes: giant ellipticals, spirals, and small dwarfs. Astrophysicists study phenomena such as the mergers of galaxies, star formation rates and the influences from supernovae and black holes to understand the development of these and other galactic types over cosmic history. Cosmologists, meanwhile, use the evolution of the large-scale structures composed of galaxies to probe how the universe has changed since the Big Bang 13.8 billion years ago. A topic of major interest is the distribution of dark matter, which corresponds to the location of superclusters and filaments. Another is the effect of dark energy, which is accelerating the universe's expansion as evidenced by the speed of distant galaxies receding from our own galaxy, the Milky Way.
Andrea Merloni, Priyamvada Natarajan, Tommaso Treu and John Wise discuss the latest findings about black holes, including new questions about how supermassive black holes at the centers of galaxies came into existence.
John Carlstrom, Dan Marrone and Joaquin Vieira discuss how the world’s most powerful radio telescope revealed that the most vigorous bursts of star birth in the cosmos took place much earlier than previously thought.
As the search for dark matter intensifies, a colloquium brought together cosmologists, particle physicists and observational astrophysicists --- three fields now united in the hunt to determine what is dark matter.
Dark galaxies – galaxies with few if any stars and made predominately of dense gas – have been impossible to detect directly until now. Members of an international team of astronomers discuss their discovery and the place these galaxies hold in the universe.