Imaging the Galactic Center at 74 MHz: Viewing Our Galaxy through the Last Electromagnetic Window

M.E. Nord, T.J.W. Lazio, and N.E. Kassim
Remote Sensing Division

Introduction: Although radio astronomy originated at frequencies below 100 MHz, the quest for greater angular resolution, combined with the problems resulting from ionospheric structure, has driven radio astronomers to higher frequencies. NRL has built, installed, and now successfully operated 74 MHz receivers on The National Radio Astronomy Observatory's Very Large Array (VLA) radio interferometer. Coupled with techniques for mitigating ionospheric distortions, these 74 MHz receivers have opened the last unexplored electromagnetic frequency window to high-resolution astronomical observations.

Astrophysically, low-frequency imaging opens up a wealth of opportunities for studying relativistic particles and magnetic fields. One example is that synchrotron radiation (from relativistic electrons spiraling around magnetic fields) peaks at low radio frequencies. The DOD's interests in low-frequency imaging lie in ionospheric and space physics. These observations are sensitive to short time-scale and small length-scale fluctuations in the ionosphere. The techniques used to compensate for the ionosphere during astronomical observations may also be applicable to problems encountered in HF and VHF communications and geo-location. Furthermore, these techniques could have application in low-frequency space radar of events such as coronal mass ejections, which are a major cause of geomagnetic storms.

Methodology: NRL has equipped the 27-antenna VLA radio interferometer at the National Radio Astronomy Observatory (NRAO) near Socorro, New Mexico, with 74 MHz receivers designed and built in the Remote Sensing Division. The NRLNRAO 74 MHz system has made the VLA the highest angular resolution, highest dynamic range radio interferometer operating below 100 MHz.

Phase errors due to density fluctuations in the ionosphere have long been a problem in the HF, VHF, and UHF bands. NRL scientists have recently shown that pre-existing "self-calibration" techniques can be used to remove the ionospheric effects.¹ The ionosphere affects the line of sight to the individual N telescopes, but the interferometer measures the phase differences between N^*(N - 1)/2 baselines. For arrays with N > 4, the individual antenna phases are over-determined and ionospheric corrections can be calculated for every line of sight.

Data acquired with the NRL-NRAO 74 MHz system are calibrated, self-calibrated, and imaged threedimensionally at NRL by using high-speed PCs running NRAO's Astrophysical Image Processing Software (AIPS). Radio frequency interference is mitigated by breaking the frequency band into many small channels and then excising the affected channels and times.

Images: The antennas of the VLA interferometer are able to move into several different configurations. The more compact configurations have lower resolution, but higher surface brightness sensitivity. The two figures shown here are from the more compact configurations. North is up, and the plane of the Milky Way galaxy passes diagonally across the image in both figures. The D configuration (Fig. 4) has maximum antenna spacing of 1 km, and the C configuration (Fig. 5) has maximum antenna spacing of 3 km. Data in the more extended B and A configurations have been taken and are being analyzed.

FIGURE 4
The galactic center region at 74 MHz as imaged by the VLA in "D" configuration. Although this image is the lowest resolution produced by the VLA, it is still much higher resolution than any pre-existing image at these frequencies. This image is ~13 X 13 degrees, has a resolution of 17 arcminutes, and a pixel size of 4 arcminutes (1 arcminute = 0.3 milliradians). Note the two prominent absorption regions M8 (upper left) and NGC6357 (lower right).

Fig5 Image

FIGURE 5
The galactic center region at 74 MHz as imaged by the VLA in "C" configuration. This image is ~13 X 13 degrees, has a resolution of 6 arcminutes, and a pixel size of 1 arcminute (1 arcminute = 0.3 milliradians). The two absorption regions seen in Fig. 4 are beginning to be resolved out, and point sources are now more apparent.

Figures 4 and 5 are images of the region around the center of our Milky Way galaxy. (In both figures, north is up, and the plane of the Milky Way galaxy passes diagonally across the image.) Of interest in the galactic center region are numerous relics of exploded stars called supernova remnants (SNRs), which are strong emitters at low frequencies. The radio source Sagittarius A, thought to be the location of the super-massive black hole central to our galaxy, is the brightest source in the field at higher radio frequencies. At 74 MHz, this source is not present, probably because foreground absorption obscures the source entirely.

Figure 5 has roughly three times the resolution of Fig. 4, with a corresponding factor of 10 decrease in surface brightness sensitivity. Because of this decreased sensitivity, Fig. 5 is less sensitive to regions of extended emission or absorption. Increased resolution brings increased sensitivity to point sources, and many more point sources are seen. We are in the process of matching these sources to known galactic and extragalactic sources.

Analysis: The initial effort in exploiting these new images focuses on the distribution of galactic cosmic rays. Although discovered almost 90 years ago, the origin, distribution, and energy spectrum of these high-energy subatomic particles is still largely unknown. Interstellar free-free absorption causes ionized regions that appear in emission at higher frequencies, to appear in absorption against the galactic background synchrotron radiation produced by cosmic ray electrons. These foreground ionized regions are located at well-determined, independently known distances. By comparing the sky brightness in the direction of the absorption and nearby lines of sight, we can constrain both the foreground and background components of the galactic magnetic field and cosmic ray electron density with respect to a specific location in three-dimensional space.

Figure 4 shows two prominent absorption regions, M8 and NGC 6357. These regions have been found to coincide with ionized hydrogen emission regions detected at other frequencies. The hydrogen in these regions is being ionized from the emission of stars being formed in the cloud. The emissivity of the cosmic ray electrons between the observer and the ionized region are found to be 0.2 and 0.4 Kelvin per parsec, respectively (1 parsec ~ 3.1 X 10¹⁶ meters). With many such observations, a three-dimensional map of cosmic ray electrons in the galaxy could be constructed. Thus these observations, coupled with similar observations in other regions of the sky, are allowing us to attack the nearly century-old mystery of the origin and distribution of cosmic rays as well as revealing sources and structure visible only through this last electromagnetic window.

[Sponsored by ONR]

References
¹ N.E. Kassim, R.A. Perley, W.C. Erickson, and K.S. Dwarakanath, "Subarcminute Resolution Imaging of Radio Sources at 74 MHz with the Very Large Array,""Astron. J. 106, 2218-2228 (1993).