Radio "View" Sky Survey Nears Completion with New Data Release

6/19/2006 - 31-06r
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Astronomers using the National Science Foundation's Very Large Array (VLA) have overcome longstanding technical hurdles to map the sky to unparalleled sensitivity at long wavelengths. The team of astronomers from the Naval Research Laboratory, the National Radio Astronomy Observatory (NRAO), and the University of Maryland released the unprecedented sky survey data over a nearly two-year period. The VLA Low-frequency Sky Survey (VLSS) Data Release 1, which contained the first half of the survey, occurred in late 2004.

The VLA Low-frequency Sky Survey Data Release 2 offers images of more than 90% of the northern sky visible to the VLA and a searchable database of approximately 67,000 sources. The survey was conducted at a wavelength of 4 meters (74 MHz frequency) making it one of the largest and most sensitive surveys conducted at these wavelengths, and a valuable complement to views of the sky at shorter wavelengths.

"The initial goal of this project was to exploit recent technical advances in imaging celestial sources at these wavelengths through the disrupting effects of the Earth's ionosphere," said Dr. Namir Kassim, of NRL in Washington, DC. A layer of the Earth's atmosphere that is ionized by the Sun's radiation and which extends from about 100 km (60 mi.) to more than 300 km (200 mi.) in altitude, the presence of the ionosphere is useful for long-distance communication, however, from an astronomical standpoint, it has hampered development at the longest wavelengths.

"Unveiling this new window on the Universe, we expect that the high-quality VLSS images will allow us to study familiar objects in new ways and potentially reveal some rare and entirely new objects," said NRL's Dr. Aaron Cohen of the Naval Research Laboratory. Among the objects the astronomers hope to find in these images are distant radio galaxies possibly containing the first supermassive black holes in the Universe, mysterious "halos" and "relics" produced by enormous collisions of galaxies in clusters, previously-undiscovered superdense, spinning neutron stars known as pulsars, and even "super Jupiters" circling stars beyond the Sun.

The survey has now used nearly 800 hours of VLA observing time. Images from the survey are being made available to other scientists as soon as they are completed. The newly released images and data are available via the NRAO Web site. "By doing this survey and making the results available, we are bringing low-frequency radio data, previously quite difficult to produce, to all astronomers in a simple and easy manner," Dr. Wendy Lane of NRL commented. "We probably will have to wait for years to know the full scientific benefit of this survey," she added.

Another key use of the survey will be to provide a crucial initial calibration grid for bringing an emerging new generation of powerful long wavelength instruments on-line, including the New Mexico-based Long Wavelength Array (LWA) Project led by the University of New Mexico. In fact it was the breakthrough in ionospheric calibration, demonstrated with the 74 MHz VLA system through projects like the VLSS, that led to the development of the US-based LWA and Dutch-based Low Frequency Array (LOFAR) projects. Other members in the Southwest Consortium developing the LWA include NRL, the Applied Research Laboratory at the University of Texas (Austin), and the Los Alamos National Laboratory.

In addition to Drs. Kassim, Cohen, and Lane, the VLSS team includes Drs. Rick Perley, James Condon, and William Cotton of NRAO; Dr. Joseph Lazio of NRL; and Dr. William Erickson of the University of Maryland. NRAO is a facility of the National Science Foundation, operated under cooperative agreement with Associated Universities, Inc.

More information on NRL's involvement with VLSS can be found on the web site:

The sky covered by the VLA Low-frequency Sky Survey. The green ellipses show the region of the sky covered in Data Release 2, the dark blue ellipses show the region covered in Data Release 1, and the light blue ellipses show the region covered in pilot observations. In this display, the North Pole is at the center of the figure and the celestial equator and the projection of the Earth's equator onto the sky is the dark circle labelled 0. The blow up shows a typical portion of the 74 MHz sky from Data Release 2.
A "rogues' gallery" of radio galaxy types seen in the VLSS. White regions indicate radio-bright emitting regions in the galaxies, while deep red/black indicate regions of little or no radio emission. In all cases, the radio galaxies are thought to shine because of jets of highly relativistic material being shot from the environment of a supermassive black hole in the center of the radio galaxy. The diversity of shapes probably reflects the environment of the radio galaxy itself as well as the history of the supermassive black hole and how much material has fallen into it.

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