Background

Even though radio astronomy was founded by Jansky at decametric wavelengths, the urgent quest for ever higher angular resolution and the fact that ionospheric structure limits interferometric imaging to short ($<$ 5 km) baselines at long wavelengths (LW, taken to be 2-20 m or 15-150 MHz), has left the LW region among the most poorly explored in the entire spectrum. This is despite the many important astrophysical questions which can be addressed only by LW observations, in addition to those problems which may be usefully addressed at a variety of wavelengths, but which may be done so with unparalleled efficiency in the LW regime. Thus the recent demonstration by the 74 MHz VLA system ([Kassim et al. 1993]), that self-calibration can accurately remove ionospheric distortions over arbitrarily long baselines, now offers the exciting opportunity to efficiently and economically open a new high-resolution, high-sensitivity window on the electromagnetic spectrum at the longest wavelengths ([Kassim & Erickson 1998]).

Figure 1: The supernova remnant Cas A at 74 MHz. This image was made with the prototype, 8-antenna 74 MHz VLA system. This image provided only the second example of unshocked ejecta interior to the reverse shock in a young supernova remnant ([Kassim et al. 1995b]).

Figure 2: The active radio galaxy Virgo A at 74 MHz. This figure demonstrates that the large scale radio halo is a response to the activity of the Virgo A black hole-jet system, and, contrary to conventional wisdom, is a relatively young feature ([Owen et al. 1999]).

Figure 3: The Coma cluster of galaxies at 74 MHz. (Top) The entire primary beam. This image illustrates the efficiency with which large sections of the sky can be mapped with a sensitive, low-frequency instrument. The rms noise level is 25 mJy beam$^{-1}$, and the field covers approximately 15$^\circ$ at a resolution of 1$^\prime$. (Bottom) A subimage of the wide-field image showing the central galaxies and the cluster halo.

The jointly developed NRL-NRAO 74 MHz observing system came fully on line in 1998 January with all 27 antennas equipped with 74 MHz receivers (prior to this a prototype system was installed on 8 antennas). The system has already attracted a wide variety of innovative observing proposals with encouraging initial results in solar system (planetary emission, solar bursts), Galactic (pulsars, supernova remnants), and extragalactic (cluster halos, radio galaxies) astrophysics. Figures 1-3 provide dramatic examples of the types of images currently being produced by this system. Figure 1 is an image of the Galactic supernova remnant (SNR) Cas A and provides only the second direct case (after SN 1006, [Hamilton & Fesen 1988]) of evidence for unshocked ejecta interior to the reverse shock in a young SNR ([Kassim et al. 1995b]), as predicted by various theoretical analyses (e.g., [Lozinskya 1992]). Figure 2 is an image of the radio galaxy Virgo A and provides important evidence that the large-scale radio halo is a response to the activity of the black hole-jet system and, contrary to conventional wisdom, is a relatively young feature ([Owen et al. 1999]). Figure 3 is a deep (rms noise level $\approx$ 25 mJy beam${}^{-1}$) image of the field containing the Coma cluster of galaxies (P. P. Kronberg 1999, private communication). The detection of the emission from the steep spectrum halo is important to theories of particle acceleration and magnetic fields in clusters of galaxies. Moreover, the amazing field of view ( $> 10\hbox{$^\circ$}$), angular resolution ( $\sim 1\hbox{$^\prime$}$), and sensitivity ($\approx$ 25 mJy beam${}^{-1}$) provided by this single VLA pointing testify to the dramatic imaging power offered by low frequency systems.

Now that the ionospheric barrier has been removed, such images can reveal unique information not only for important large-scale emission studies (e.g., SNRs, the Galactic center, galaxy and cluster halos), but also for studies which benefit from high sensitivity to small-diameter steep spectrum sources (e.g., pulsars and high redshift radio galaxies) which can now be obtained with incredible efficiency. Furthermore, a sensitive, high angular resolution, low frequency instrument would also be the ideal receiver for solar radar, an innovative concept which could image Earthward-bound coronal mass ejections (CMEs), thereby opening up a new field of space weather research and prediction.