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Over the last few decades there have been many significant discoveries by Observatories astronomers: Allan Sandage’s heroic efforts to measure the Hubble constant and the ages of globular clusters; Sandage, Leonard Searle, and collaborators’ pioneering work on the formation of the Milky Way; Wendy Freedman's work as a principal investigator on the Hubble Space Telescope Key Project on the Extragalactic Distance Scale; and Steve Shectman, Alan Dressler, and their colleagues’ mapping of distant galaxies, and the discovery of giant voids and immense structures in the distribution of galaxies—the cosmic web.
Carnegie astronomers continue to play a pivotal role in studying and understanding the early universe. Pat McCarthy, Eric Persson, Alan Dressler, Dan Kelson, Gus Oemler, John Mulchaey, and Carnegie Fellow Mike Gladders have been using the telescopes at Las Campanas, the Hubble Space Telescope, and the Chandra X-ray telescope to study distant galaxies, groups of galaxies, and clusters of galaxies. These and other studies have shown that galaxies appear to have started to form very early, with galaxy clusters and groups assembling more slowly over time. This picture is broadly consistent with theoretical models for the assembly of galaxies and structure in the universe, models that incorporate the early initial conditions from inflation and include dark matter. The process of forming galaxies is very dynamic—galaxies merge and collide (as Carnegie’s François Schweizer has studied for many years), transform in appearance, have episodic bursts of star formation, and emit copious amounts of radiation spanning the X-ray, ultraviolet, through the optical, infrared, and radio parts of the spectrum. However, to observe directly the first light in the universe, the formation of the first stars, and first galaxies we must await higher resolution and sensitivity (one of the goals of Carnegie’s proposed Giant Magellan Telescope). We know almost nothing of this epoch; it has been dubbed the “dark ages.”
Some galaxies have nuclei with luminosities that can exceed those of ordinary galaxies by factors of hundreds. Strong evidence over the past four decades suggests that the engines powering the various kinds of observed active nuclei (the most luminous of which are quasars) are black holes with masses ranging from a million to a billion times the mass of our Sun. Most likely these black holes grew by mergers and the accretion of gas onto high-density regions of matter. Work by Carnegie staff members Luis Ho and Alan Dressler and their collaborators has shown that these supermassive black holes are found in the nuclei of all galaxies so far studied. Unrecognized until recently, it appears that somehow the black hole knows about the galaxy environment in which it is situated—the mass of the black hole is closely related to the properties of its host galaxy. These observations have important implications for understanding how galaxies assemble and how this process is related to black hole formation. The details of the formation of such supermassive objects at high redshift remain to be understood and must again await future telescopes.
Two other interesting probes of the chemical and dynamical evolution of the universe come from studies of distant quasars, as well as detailed studies of our own Milky Way galaxy. Quasars can be used as bright-light sources against which to study the distribution and chemical composition of matter throughout the universe—the research of staff member Michael Rauch and staff member emeritus Ray Weymann. These and other studies have helped to reveal both the vast cosmic web of galaxies, which has evolved into our present-day universe, and the detailed compositions and physical conditions of the gas in the early universe. The study of distant galaxies and quasars is one way to probe the evolution of the universe. The inverse and complementary approach is to study the chemical and dynamical history of the Milky Way by observing nearby stars of differing chemistries and ages. George Preston, Steve Shectman, Andy McWilliam, and Ian Thompson have an ambitious program at Las Campanas surveying for and discovering stars of very low chemical abundances, which are among the earliest-formed objects in the Galaxy.
A new class of extragalactic objects—gamma-ray bursts (GRBs)—has emerged, with the exciting recent discovery that many of these objects lie at cosmological distances. These extremely luminous objects emit most of their extraordinary energy at gamma-ray frequencies, and do so in the space of only a few seconds. Because they are so bright, they provide a new beacon for probing the earliest epochs of star formation. The study of these objects is being conducted at Carnegie by Edo Berger, our first Carnegie-Princeton Fellow. The field has advanced rapidly with the capability of satellites to measure the positions of these sources accurately and rapidly, allowing follow-up with ground-based telescopes. In a very recent advance, optical spectra of some objects have shown a connection between GRBs and supernovae. In October 2004, NASA launched a new gamma-ray mission called Swift. Swift is now providing very accurate locations of the targets so that they can immediately be observed from the ground. Edo Berger, in collaboration with a number of Carnegie and Caltech astronomers, is using the unique rapid instrument-changing capability of Carnegie’s Magellan telescopes to interrupt the observing on nights when new GRBs are detected by Swift and immediately take data before the bursts fade. His goal is to study the star formation history early in the universe and use the bright GRBs as backlights to illuminate the distribution of objects and measure their chemical composition in the early universe, similarly to quasar studies. A whole new window is opening with this burgeoning field. In fact, the 40-inch Swope telescope provided immediate follow-up observations of the first gamma-ray burst detected by Swift.