Galactic structure and evolution is the study of whole galaxies as coherent, self-contained systems of dark matter, stars, and gas and how those systems change over billions of years of time. Like a living organism, the history of a galaxy is shaped by its internal metabolic processes (star formation and death, gravitational interactions among all its components, and sometimes by an active black hole engine at its core) as well as by interactions with other galaxies, its environment, and the universe itself. Understanding how galaxies, especially the Milky Way, formed and evolved is key to understanding an ancient part of mankind's own origins.
Typically a hundred thousand light years across and weighing a few hundred billion suns, each galaxy is home to most of the universe's many basic building blocks. These are the abundant but mysterious dark matter that dominates a galaxy's mass, interstellar gas and dust, stars and planets, neutron stars and black holes, and, often at the galaxy's center, an active supermassive black hole engine that may outshine the combined light from all its stars.
On the "small' scale, galaxy evolution is influenced by the death of old stars, which expel newly-created elements forged in their central furnaces, as well as the formation of new stars from the enriched interstellar gas. This keeps the stellar population replenished over eons of time. A galaxy's shape is determined by how the stars and gas move within the gravitational field of the galaxy's (mostly dark) matter, and the rate at which it forms stars is determined by how much interstellar gas it has.
Occasionally, two galaxies collide and merge, leading initially to a rapid increase in the rate of star formation and a rapid funneling of gas fuel to the supermassive black hole engine in the center. This active galactic nucleus (or AGN) sometimes shines so brightly that all we can see on earth is a quasar at its center. In fact, the radiation pressure and powerful jets generated by the AGN can drive out all the gas accreted in the merger, thereby shutting off the supply of gas that fueled star formation and nuclear activity in the first place.
On the very large scale, galaxies appear to have formed out of the expanding universe shortly after the Big Bang. So the study of galaxy evolution requires understanding not only star formation and supermassive black holes, but also Cosmology --- the birth and evolution of the universe itself.
Selected Current Projects
Spitzer Telescope
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| The high redshift (z = 1.070) galaxy cluster ISCS J1433.1+3334 at a distance of about 7.9 billion light years. The boxed and circled objects are galaxies in this cluster. |
In addition to studying the Origin of Stars and Planets, the Spitzerinfrared observatory also is used to study galaxies at distances so great that we are seeing them as they were billions of years ago. All light from these galaxies is redshifted by the expansion of the universe to about twice its normal wavelength. (For example, the emission line of doubly-ionized oxygen and the Hα line of neutral hydrogen [0.501 and 0.656 micron wavelengths, respectively] are redshifted to over 1.0 and 1.3 microns.) So, whereas local galaxies can be studied at optical wavelengths, it is better to study "high redshift" galaxies in the infrared.
Recently the Spitzer telescope's InfraRed Array Camera (IRAC) was used to search for clusters of galaxies nearly 8 billion light years away. In addition to discovering 335 new young galaxy candidates, the study also determined that the total mass of some of the clusters in which they lie weight more than 100 trillion suns each! Detailed studies of the stars in these galaxies indicate that they formed shortly after the Big Bang, over 11 billion years ago.
The Spitzer IRAC instrument also was used to look at over 3000 galaxies with very strong X-ray emission. It was found that in at least 90% of these galaxies the X-ray and infrared emission is coming from an active nucleus, i.e., a brightly emitting supermassive black hole engine at the center of the galaxy.
The Deep Space Network Ground Radio Telescopes
While the main job of NASA’s Deep Space Network is to track distant spacecraft like Voyager, Cassini, and Spitzer, these communication antennas also can be used as ground radio telescopes - either as single dishes or as some of the most powerful elements in world-wide Very Long Baseline Interferometry (VLBI) arrays, which also may include a space radio telescope as well (Space VLBI).
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| Artist’s conception of the central few light years of the active galaxy NGC 4258. The black hole lies at the center and produces a jet. A warped disk of molecular gas encircles the hole and produces masers in the direction of the earth (bright white spots on the disk). The radio spectrum at the bottom, indicating the velocities of the masers away from the earth, shows that the gas indeed orbits the black hole just like the planets orbit our sun (i.e., in a Keplerian-type orbit). |
Astronomical masers (microwave lasers) are regions of interstellar (or circumstellar) gas where radio emission of a particular molecular transition piles up in one direction, producing an extremely bright source toward that one direction. Occasionally some of these focused emissions point toward the earth. Masers of OH, SiO, methanol, and other molecules are common in giant molecular cloud regions where stars form. In addition, enormously bright water megamasers often occur in the centers of mildly active galaxies (AGN), in the warm dense molecular gas that circles within a couple of light years of the central supermassive black hole. By measuring the detailed motions of water megamaser spots we can measure very accurately not only the mass of the central black hole, but also the distance of the galaxy from earth.
Recently NASA’s Deep Space Network antennas were used to find eight new water megamasers in the centers of nearby and moderately distant galaxies. This has significantly increases the number, and distance, of known megamasers. Further studies of these systems with VLBI and Space VLBI will help determine an accurate distance scale to the universe - the so-called Hubble constant.
NASA-Funded Theoretical Investigations
The behavior of normal and active galaxies also can be studied with supercomputers and other forms of theoretical calculation. Indeed, it is with such studies, and detailed comparison with observations, that the greatest understanding of these objects is achieved. Similar to weather prediction or airplane aerodynamic studies on the earth, these astrophysical simulations build galaxy or black hole systems inside a supercomputer’s memory and use the laws of physics to determine how the system evolves. Sometimes the simulation follows the flow of cosmic fluid or the interaction of hundreds of thousands of star and dark matter "particles."