Caption: Hands-on tutorial session at the April 18-19 ALMA Training in Toronto.
During the month of April, the NAASC
continued to provide training to interested members of the community via a series of ALMA Community Events in Baltimore, Toronto, Boston and Charlottesville, Virginia.
will be on the road for much of May and early June, providing training in Gainesville, Florida (May 2-3), Victoria, British Columbia (May 4-5), Iowa City, Iowa (May 9-10), Charlottesville (May 9-10), Quebec City(May 10), University of Arizona (May 12-13), Calgary (May 12-13), AAS/Boston (May23-24), Amherst, MA (May 25), Columbia University (May 27) and London, Ontario (June 3).
Information about these upcoming ALMA Community Training events can be found on the ALMA Community Day Events registration page. https://science.nrao.edu/facilities/alma/community1
NAASC Research Activities: Jim Braatz
Figure 1: Water maser spots imaged in the edge-on accretion disk in UGC 3789. The colors represent the direction of motion, with red spots moving toward the observer as they orbit the black hole, blue spots moving away, and green spots falling just in front of the black hole. Note the scale in milliarcseconds.
In active galaxies, as gas in the nucleus is pulled toward its fate by the central, supermassive black hole, it forms into a disk. The gas swirls around at millions of miles per hour, gets heated to temperatures hotter than the sun, and shines intensely in X-ray and UV radiation. Yet in this hostile environment, delicate water molecules can exist, and even thrive. They are buffered from the intense heat and the hard radiation field by dust in the disk. Energized by relatively gentle collisions with other particles in the disk, the water molecules shine brightly with beamed megamaser radiation. Masers are the radio frequency equivalent of lasers, and megamasers are the particularly powerful variety of masers associated with active galaxies. Observations of these megamasers, it turns out, make a powerful tool that can be used to explore a host of interesting problems in astrophysics, including the black hole itself, galaxy evolution, and the mysterious dark energy that causes the expansion of the universe to accelerate.
Jim Braatz leads the Megamaser Cosmology Project, a team that includes collaborators Mark Reid, Jim Condon, Christian Henkel, Fred Lo, Cheng-Yu Kuo, and Violetta Impellizzeri. We hunt for new masers with the Green Bank Telescope, and image interesting maser systems in high resolution using the Very Long Baseline Array, which we combine with the Green Bank Telescope and the Max-Planck Institute’s Effelsberg radio telescope to form a global VLBI array. Using these telescopes, we can pinpoint the relative locations of individual maser clouds to a precision of about 10 microarcseconds, or about 1000 times better than the resolution of the Hubble Space Telescope. By mapping the positions of maser clouds, we get a direct image of the gas in the AGN accretion disk, the only technique capable of imaging such disks directly. Combining these precise positions with line-of-sight Doppler velocities, we then have a valuable tool for probing the dynamics of the disk.
We can apply this tool to demonstrate that the object in the center of the active galaxy must be a black hole, and we can measure the mass of the black hole very precisely. Masers provide this information uniquely because they measure gas very close to the black hole, within about a light-year – so the masses we determine are accurate to about 10%, whereas other techniques can only get within a factor of two or so. With precise black hole masses, we can investigate important relationships between the black hole and its host galaxy, such as how the black hole mass compares to that of stars in the galactic bulge, or to the velocity dispersion of stars in the bulge. It is comparisons like these that lead to a better understanding of how galaxies evolve. So far, our initial findings have been surprising. We’ve found that not all black holes follow the tight correlations between the black hole mass and the bulge properties suspected from optical studies of big elliptical galaxies. A second problem that we can address with studies of water megamasers is a notoriously difficult one: measuring distances to galaxies. By analyzing the internal dynamics of a water maser system, we can calculate the radius of the accretion disk. Comparing this radius with the angular size of the disk measured from VLBI maps, we can (conceptually) triangulate to get a geometric distance to the galaxy. Because this method is geometric, it is a promising alternative to “standard candle” techniques that historically have been troubled with systematic uncertainties. While the maser technique is elegant and valuable for its simplicity, it can be difficult to apply in general because the most interesting masers are faint. The Megamaser Cosmology Project has measured a few galaxies this way, and more are on the way. Having measured distances to galaxies, we can then determine the expansion rate of the universe, the famous “Hubble Constant.” When combined with studies of the cosmic microwave background, a precise Hubble constant holds the key to understanding of the mysterious “dark energy” that seems to pervade the universe and causes its expansion to accelerate. ALMA will be a remarkable telescope for observing molecular gas in active galactic nuclei. There are numerous water maser transitions available in the millimeter and submillimeter wavelengths, but ALMA’s contributions will not be limited to observing those. It will be able to image emission from other molecules as well, masers or not. ALMA will open the door to getting a more complete picture of molecular gas in active galaxies and will reveal, directly, how the nuclear gas begins to interface with the host galaxy.
Meet the NAASC:
Jim Braatz is an astronomer at the National Radio Astronomy Observatory (NRAO) in Charlottesville, Virginia. A native Marylander, Jim received his B.A. in Physics from the Johns Hopkins University and Ph.D. in astronomy from the University of Maryland. Subsequently he held a postdoc at the Harvard-Smithsonian Center for Astrophysics and was a Jansky Fellow in Green Bank. Jim then joined the scientific staff at NRAO. He currently works with the North American ALMA Science Center, helping to prepare the astronomical community to use ALMA. Jim's research is centered on observations of radio emission from active galaxies. Jim is the PI of the Megamaser Cosmology Project, a multi-year effort that aims to use observations of water megamasers to measure distances to galaxies and black hole masses in AGNs.