New observations with the National Science Foundation’s Very Long Baseline Array (VLBA) indicate that the inner workings of active galaxies may be considerably more complex than astronomers have previously thought. Drs. Alan Roy and James Ulvestad of the National Radio Astronomy Observatory, together with Drs. Edward Colbert and Andrew Wilson of the Space Telescope Science Institute and the University of Maryland, used the VLBA to image a light-year-sized radio jet in NGC 4151, a relatively nearby spiral galaxy. The jet seen by the radio telescopes is not aligned as the scientists expected, and this misalignment may require changes to theoretical models of active galactic nuclei. The astronomers presented their findings today to the American Astronomical Society meeting in Winston- Salem, North Carolina.
The radio structure at the center of the Seyfert galaxy NGC 4151, located approximately 43 million light-years from Earth, was imaged with a resolution of better than 1 light-year. The radio images were made using the 25-meter (82-foot) telescopes of the VLBA, an array of 10 telescopes spread out over the full length and width of the United States, from the Virgin Islands to Hawaii. Seyfert galaxies are spiral galaxies that are nearby examples of galaxies containing active galactic nuclei (AGN), which are thought to be powered by black holes having masses millions of times greater than the Sun. They represent nearby cousins of the more distant and energetic quasars; their relative proximity to Earth permits images to be made with much finer spatial resolution than is possible for quasars.
The radio images of NGC 4151 reveal a chain of knots several light years in length, separated by a few light months, which then appear to make a fairly sharp turn — about 55 degrees — to merge with a previously known straight radio jet about 800 light-years in length. This large-scale radio jet is nearly coincident with a complex of gas clouds imaged at optical wavelengths with the Hubble Space Telescope. The large-scale optical and radio structures previously have been thought to indicate the symmetry axis of the active nucleus in the inner light-months of the galaxy, an idea now called into question by the new VLBA images.
Active galactic nuclei are classified in a variety of types, according to different phenomena seen by observers. Over the past decade, astronomers have suggested that the different types of AGN may all be the same type of object, with the different properties noted by observers resulting from different viewing angles as seen from Earth. These models, called “unification” schemes, usually rely on a single symmetry axis for an AGN, determined by the spin axis of a central black hole and a surrounding torus, or “doughnut” of material. However, the new radio images indicate that there are two distinctly different symmetry axes on scales less than and greater than a few light-years. According to Ulvestad, “This curvature is quite significant, because it implies that the symmetry axis of NGC 4151 changes substantially in its inner few light-years, and that the commonly accepted unified schemes for AGN are far too simplistic.” An answer to the puzzle of the two different symmetry axes in NGC 4151 could shed further light on other objects that do not easily fit into the simple unified scheme.
The Very Long Baseline Array is an instrument of the National Radio Astronomy Observatory, a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.