KNOW ABOUT GALACTIC NUCLEI

The nucleus of a galaxy is like a deep well: It is easy to fall in, but hard to get out. As a result, gas and stars accumulate there. In the 1960s, astronomers discovered that some galactic nuclei were truly remarkable: They could outshine an entire galaxy from a volume not much larger than

that of the solar system. These objects, termed quasars, are the most luminous type of active galactic nucleus. Theorists immediately conjectured that such prodigious power output could come only from the accretion of gas onto a supermassive black hole; later it was realized that energy could be extracted from the spin of the black hole as well. A consequence of these ideas is that many galaxies should harbor supermassive black holes in their nuclei.

Three decades later, this conjecture has been amply verified. Observations of both gas and stars have shown that even in our own “backyard,” the Milky Way Galaxy harbors a black hole 3 million times more massive than the Sun (Figure 2.14)—and that black hole masses in the nuclei of other galaxies can exceed a billion solar masses. Exquisitely precise measurements of the positions and three-dimensional velocities of water masers made with the Very Long Baseline Array (VLBA) toward the nucleus of the galaxy NGC 4258 provided incontrovertible evidence for the presence of a supermassive black hole (Figure 2.15). ARISE has the power to study the water emission in other galactic nuclei to search for black holes and determine their mass and the characteristics of the accreting gas.

15 Some of the data that provide strong evidence for the presence of a supermassive black hole in the center of the nearby spiral galaxy NCG 4258. The top panel is the actual image of the point-like maser clouds constructed from very long baseline interferometry (VLBI) data having a resolution of 200 microarcsec (with a wire grid depicting unseen parts of the disk). Also shown is the image of the continuum emission at 1.3-cm wavelength caused by synchrotron radiation from relativistic electrons emanating from the position of the dynamical center (black dot). The central mass required to gravitationally bind the system

is 39 million solar masses. Since all the mass must be within the inner boundary of the molecular disk of about 0.13 pc, this mass is probably in the form of a supermassive black hole. The bottom panel shows on a larger scale the synchrotron emission that arises from relativistic electrons ejected along the spin axis of the black hole. Observations with HST have confirmed that most nearby galaxies harbor supermassive black holes in their nuclei.

How do these supermassive black holes form and evolve? Do they grow from stellar “seeds” or do they originate at the very beginning of the formation of a galaxy? These key questions are ripe for a frontal attack now. Addressing them will require the observation of active galactic nuclei (AGNs) when they first turn on, over the entire electromagnetic spectrum. With its enormous sensitivity in the infrared, NGST will be able to detect AGNs out to redshifts beyond 10. Radiation emitted in the thermal infrared will be redshifted into the band detectable by SAFIR. The EVLA will detect much longer wavelength radio emission from AGNs to redshifts beyond 5. Constellation-X will be able to observe the first quasars even if they are heavily obscured by dust. EXIST will make a census of obscured, low-redshift AGNs over the whole sky; this sample can be compared with younger AGNs, seen at


high redshifts by Constellation-X, to study how the AGNs evolve. In this case the energies of the most penetrating hard x rays will be conveniently shifted by the expansion of the universe into the energy region of maximum sensitivity of the telescope. Furthermore, by observing the spectrum of hot gas as it disappears into supermassive black holes, Constellation-X will provide a laboratory for studying the physical processes occurring near the event horizons of black holes under conditions that differ substantially from those near stellar-mass black holes.

In a tremendously scaled-up version of the process of mass ejection from disks around protostars, massive black holes not only accrete material but also eject from their vicinity powerful jets at nearly the speed of light (Figure 2.16). This highly relativistic material is thought to generate extremely energetic photons, with frequencies more than 100 billion times that of visible light. VERITAS has the power to detect individual photons of this radiation interacting with Earth’s atmosphere, and can therefore probe the relativistic particle acceleration occurring near these massive black holes. Observing somewhat less energetic photons, GLAST will help determine how jets are powered and confined. ARISE has the spatial resolution to resolve the base of the jet and thereby provide a complementary probe of the acceleration region.




Galaxy mergers are inferred to be common, and it is quite possible that the massive black holes in their nuclei would merge as well. Such a cataclysmic event would produce powerful gravity waves that could be detected by LISA out to very large distances (redshifts up to at least 20). This gravitational radiation would be detectable for up to a year before the actual merger, enabling accurate prediction of the final event so that it could be observed by telescopes sensitive to the entire range of electromagnetic radiation. Observation of such a merger would provide a unique test of Einstein’s theory of general relativity in the case of strong gravitational fields. Further discussion of what scientists can learn about black holes can be found in the physics survey report Gravitational Physics: Exploring the Structure of Space and Time (NRC, 1999).

Galactic nuclei can become extremely luminous as a result of intense bursts of star formation or the presence of a supermassive black hole.