The Expanded Very Large Array

In almost 30 years of operation, the Very Large Array (VLA) has proved to be a remarkably flexible and productive radio telescope. However, the basic capabilities of the VLA have changed little since it was designed. A major expansion utilizing modern technology is currently underway to improve the capabilities of the VLA by at least an order of magnitude in both sensitivity and in frequency coverage. The primary elements of the Expanded Very Large Array (EVLA) project include new or upgraded receivers for continuous frequency coverage from 1 to 50 GHz, new local oscillator, intermediate frequency, and wide bandwidth data transmission systems to carry signals with 16 GHz total bandwidth from each antenna, and a new digital correlator with the capability to process this bandwidth with an unprecedented number of frequency channels for an imaging array. Also included are a new monitor and control system and new software that will provide telescope ease of use. Scheduled for completion in 2012, the EVLA will provide the world research community with a flexible, powerful, general-purpose telescope to address current and future astronomical issues.Comment: Added journal reference: published in Proceedings of the IEEE, Special Issue on Advances in Radio Astronomy, August 2009, vol. 97, No. 8, 1448-1462 Six figures, one tabl

A Kiloparsec-Scale Binary Active Galactic Nucleus Confirmed by the Expanded Very Large Array

We report the confirmation of a kpc-scale binary active galactic nucleus (AGN) with high-resolution radio images from the Expanded Very Large Array (EVLA). SDSS J150243.1+111557 is a double-peaked [O III] AGN at z = 0.39 selected from the Sloan Digital Sky Survey. Our previous near-infrared adaptive optics imaging reveals two nuclei separated by 1.4" (7.4 kpc), and our optical integral-field spectroscopy suggests that they are a type-1--type-2 AGN pair. However, these data alone cannot rule out the single AGN scenario where the narrow emission-line region associated with the secondary is photoionized by the broad-line AGN in the primary. Our new EVLA images at 1.4, 5.0, and 8.5 GHz show two steep-spectrum compact radio sources spatially coincident with the optical nuclei. The radio power of the type-2 AGN is an order-of-magnitude in excess of star-forming galaxies with similar extinction-corrected [O II] 3727 luminosities, indicating that the radio emission is powered by accretion. Therefore, SDSS J150243.1+111557 is one of the few confirmed kpc-scale binary AGN systems. Spectral-energy-distribution modeling shows that SDSS J150243.1+111557 is a merger of two ~10^{11} M_sun galaxies. With both black hole masses around 10^8 Msun, the AGNs are accreting at ~10 times below the Eddington limit.Comment: ApJL accepted. 6 pages, 3 figures, 1 tabl

The Expanded Very Large Array -- a New Telescope for New Science

Since its commissioning in 1980, the Very Large Array (VLA) has consistently demonstrated its scientific productivity. However, its fundamental capabilities have changed little since 1980, particularly in the key areas of sensitivity, frequency coverage, and velocity resolution. These limitations have been addressed by a major upgrade of the array, which began in 2001 and will be completed at the end of 2012. When completed, the Expanded VLA -- the EVLA -- will provide complete frequency coverage from 1 to 50 GHz, a continuum sensitivity of typically 1 microJy/beam (in 9 hours with full bandwidth), and a modern correlator with vastly greater capabilities and flexibility than the VLA's. In this paper we describe the goals of the EVLA project, its current status, and the anticipated expansion of capabilities over the next few years. User access to the array through the OSRO and RSRO programs is described. The following papers in this special issue, derived from observations in its early science period, demonstrate the astonishing breadth of this most flexible and powerful general-purpose telescope.Comment: 6 pages; 2 figures; emulateapj.cls; to appear in the ApJL EVLA special issu

Search for Electromagnetic Counterparts to LIGO-Virgo Candidates: Expanded Very Large Array

This paper summarizes a search for radio wavelength counterparts to candidate gravitational wave events. The identification of an electromagnetic counterpart could provide a more complete understanding of a gravitational wave event, including such characteristics as the location and the nature of the progenitor. We used the Expanded Very Large Array (EVLA) to search six galaxies which were identified as potential hosts for two candidate gravitational wave events. We summarize our procedures and discuss preliminary results.Comment: 4 pages; to appear in the New Horizons in Time Domain Astronomy, Proceedings of IAU Symposium 285, eds. R. E. M. Griffin, R. J. Hanisch & R. Seama

Exciting Maser Science with New Instruments - the Promise of the EVLA

In the near future, the Expanded Very Large Array (EVLA) will allow surveys for maser sources with unprecedented sensitivity, spectral coverage and spectroscopic capabilities. In particular, comprehensive surveys for many maser species with simultaneous sensitive continuum imaging and absorption studies will give a comprehensive radio picture of star formation in the Galactic plane and elsewhere. Very efficient EVLA surveys for H2O megamasers in Active Galacic Nuclei will be possible to practically arbitrary redshifts. EVLA and Atacama Large Millimeter Array (ALMA) studies of H2O and SiO masers will serve as high resolution probes of the innermost envelopes of oxygen-rich evolved stars and HCN masers of carbon-rich stars. Farther in the future, the Square Kilometer Array (SKA) promises the detection of OH gigamasers at all conceivable redshifts and maser astrometry with unprecedented accuracy.Comment: 10 pages, incl. 4 figures, iaus.cls, to appear the Procedings of IAU Symp. 242 (Astrophysical masers and their environments) eds. J. Chapman & W. Baan Replaced version with a few typos correcte

Protostellar Outflow Heating in a Growing Massive Protocluster

The dense molecular clump P1 in the infrared dark cloud (IRDC) complex G28.34+0.06 harbors a massive protostellar cluster at its extreme youth. Our previous Submillimeter Array (SMA) observations revealed several jet-like CO outflows emanating from the protostars, indicative of intense accretion and potential interaction with ambient natal materials. Here we present the Expanded Very Large Array (EVLA) spectral line observations toward P1 in the NH3 (J,K) = (1,1), (2,2), (3,3) lines, as well as H2O and class I CH3OH masers. Multiple NH3 transitions reveal the heated gas widely spread in the 1 pc clump. The temperature distribution is highly structured; the heated gas is offset from the protostars, and morphologically matches the outflows very well. Hot spots of spatially compact, spectrally broad NH3 (3,3) emission are also found coincident with the outflows. A weak NH3 (3,3) maser is discovered at the interface between an outflow jet and the ambient gas. These findings suggest that protostellar heating may not be effective in suppressing fragmentation during the formation of massive cores.Comment: 15 pages, 4 figures, 1 table, accepted to ApJ Letter

Expanded VLA Detection of 36.2 GHz Class I Methanol Masers in Sagittarius A

We report on the interferometric detection of 36.2 GHz Class I methanol emission with the new 27-40 GHz Ka band receivers available on the Expanded Very Large Array (EVLA). The brightness temperatures of the interferometric 36 GHz detections unambiguously indicate for the first time that the emission is maser emission. The 36 GHz methanol masers are not co-spatial with 1720 MHz OH masers, indicating that the two species trace different shocks. The 36 GHz and 44 GHz methanol masers, which both are collisionally pumped, do not necessarily co-exist and may trace different methanol gas. The methanol masers seem correlated with NH_3(3,3) density peaks. We favor an explanation in which the 36 GHz Class I methanol masers outline regions of cloud-cloud collisions, perhaps just before the onset of the formation of individual massive stars. The transition of the Very Large Array (VLA) to the EVLA is well under way, and these detections demonstrate the bright future of this completely renewed instrument.Comment: accepted to ApJ Letter

Expanded Very Large Array observations of the H66{\alpha} and He66{\alpha} recombination lines toward MWC 349A

We have used the greatly enhanced spectral capabilities of the Expanded Very Large Array to observe both the 22.3 GHz continuum emission and the H66{\alpha} recombination line toward the well-studied Galactic emission-line star MWC 349A. The continuum flux density is found to be 411 $\pm$ 41 mJy in good agreement with previous determinations. The H66{\alpha} line peak intensity is about 25 mJy, and the average line-to-continuum flux ratio is about 5%, as expected for local thermodynamic equilibrium conditions. This shows that the H66{\alpha} recombination line is not strongly masing as had previously been suggested, although a moderate maser contribution could be present. The He66{\alpha} recombination line is also detected in our observations; the relative strengths of the two recombination lines yield an ionized helium to ionized hydrogen abundance ratio y+ = 0.12 $\pm$ 0.02. The ionized helium appears to share the kinematics of the thermally excited ionized hydrogen gas, so the two species are likely to be well mixed. The electron temperature of the ionized gas in MWC 349A deduced from our observations is 6,300 $\pm$ 600 K.Comment: Accepted for publication in ApJ