Terawatt-level coherent radiation can be generated in the laboratory—but only for a few femtoseconds at a time. In space, continuous-wave coherent radiation of incredible power has been discovered. It has become known that there are water "megamaser" sources in remote galaxies that radiate in a single spectral line at more than a thousandfold higher power than the Sun at all wavelengths. Such an object has now also been found for the first time at the center of a so-called "radio-loud" radio galaxy by a German-Italian research team.1 Radio-loud galaxies had been believed to behave quietly with respect to maser emission.For their measurements, scientists from the Max-Planck Institute for Radio Astronomy (Bonn, Germany), the Institute of Radioastronomy of the Italian National Research Centre (CNR; Bologna, Italy), and the Osservatorio Astronomico di Cagliari (San Vito lo Capo, Italy), used the 100-m-diameter Effelsberg radio telescope near Bonn. The radioastronomers examined 3C403, an x-shaped radio-loud galaxy that is 750 million light years distant and is moving away from the Milky Way at a speed of more than 17000 km/s. Spectral lines emitted from 3C403 are correspondingly red-shifted in frequency. Radio-loud galaxies emit broadband radiation in the radio frequency band but were not known so far for megamaser emission.
The Very Large Array captured an image of the radio galaxy 3C403 at a wavelength of 3.6 cm, shown here in false color in units of Janskys (Jy). The red arrow points at the galaxy's nucleus, where a spectrum (inset) was taken with the Effelsberg 100-m telescope. The y-axis is flux density in Jy. The x-axis of the inset is in velocity units; the green arrow points to the recession velocity (in km/s)—that is, the speed at which 3C403 and the Milky Way are moving apart. The dual-line spectral structure arises from a single molecular water transition emitted from regions moving away from the Milky Way at slightly different velocities.
The Effelsberg instrument has a half-power acceptance beam width of 40 arcsec at a 1.3-cm wavelength and a pointing accuracy of better than 10 arcsec. To calibrate for background emission, the beam acceptance angle was altered by 2 arcmin with about 1 Hz frequency. The system temperature, including atmospheric contributions, was about 20 to 40 K on an antenna temperature scale.
A spectral structure split into two components was observed, which is attributed to the 1.3-cm water transition (see figure). The observed Doppler velocities bracket the systemic (recession) velocity of 3C403, as would be expected for a circumnuclear origin. A final verdict on the origin of the emission has to wait for very-long-baseline interferometric (VLBI) measurements, however. Interferometric measurements would presently be extremely difficult due to the weakness of the signal.
On the other hand, such maser emission is known to be variable and by luck the source might be observed during an outbreak, making VLBI observations feasible. Such observations would be extremely valuable, potentially providing the most-detailed information possible on the "monster within," the scientists say. High-resolution studies would permit discrimination between disk- and jet-maser emission.
Current thought on active galactic nuclei is that they are surrounded by dense accretion disks consisting largely of dust and molecules. To date, the only direct evidence for molecular disks around black holes is from maser emission.
REFERENCE
- A. Tarchi et al., to be published in Astronomy and Astrophysics Lett..