The Owens Valley Long Wavelength Array

The Owens Valley Long Wavelength Array (OVRO-LWA) is a low-frequency radio interferometer composed of 288 crossed broadband dipole antennas. The OVRO-LWA band covers 58 MHz of instantaneous bandwidth below 88 MHz (FM radio stations drown out astrophysical signals above 88 MHz).

The OVRO-LWA currently hosts LEDA, the largest correlator (in terms of the number of input signals) ever built. This allows us to perform full cross-correlation of 512 signal paths. This unprecedented capability coupled with the all-sky sensitivity of a dipole antenna allows the array to image the entire visible sky as frequently as is computationally feasible (of order once per second). Once completed, the OVRO-LWA will be the most powerful radio telescope operational below 100 MHz.

Array Parameters

Antennas 288 crossed broadband dipoles Geometry 200 m diameter core array with 32 expansion antennas extending to 1.5 km baselines; antenna positions optimized to minimize snapshot imaging sidelobes (with a minimum spacing of 5 m to limit mutual coupling) Bandpass 27 MHz to 85 MHz (instantaneous) Back End 512 input correlator (LEDA) performing full cross-correlation of 256 antennas with a transient cluster imaging these visibilities on a 13 second timescale Field of View the full visible hemisphere Resolution ~9 arcmin at 80 MHz to ~23 arcmin at 30 MHz

Science Goals

The OVRO-LWA's full cross-correlation and all-sky sensitivity is designed primarily for the study of high redshift HI as a probe of the epoch of reionization and fast-cadence all-sky imaging for the detection of low frequency transients, such as coherent radio emission from exoplanets and compact object merger events. The OVRO-LWA's all-sky imaging will also enable the detection and monitoring of coronal mass ejections from nearby stars. A significant amount of observing time will also be devoted to solar dynamic imaging spectroscopy.

The Dark Ages and Cosmic Dawn

In the storybook picture of the universe, it all began with the big bang. A brief era of inflation was followed by 300,000 years where the universe was filled with a radiation dominated plasma. Soon after this radiation faded to the point where matter (dark matter, protons, electrons, and small amounts of heavier nuclei) dominated the universe, the electrons and protons were able to recombine and form neutral hydrogen. This is known from studying the distant cosmic microwave background (CMB) radiation that is just now reaching us from the period of recombination.

What follows is less well understood. There is a delay between recombination and the formation of the first stars and galaxies. This period of time, referred to as the dark ages, is where baryonic matter first begins to fall into dark matter halos. The first galaxies, clusters, and superclusters all began to form at this time. Eventually stars form and explode in supernovae that populate the universe with incrementally more heavy elements. These heavy elements make it easier for gas clouds to cool and collapse, forming even more stars. The universe is now awaking from the dark ages to a period of time known as the cosmic dawn.

In order to study the process of galaxy formation it makes sense to look for the most distant galaxies possible. The further away a galaxy is, the further back in time we are seeing it. This is problematic because distant galaxies are typically found and studied with optical and near-infrared telescopes. At large distances, the bound-free opacity of neutral hydrogen shrouds these galaxies from view.

The OVRO-LWA circumvents this problem by looking for highly redshifted 21 cm photons that are characteristic of neutral hydrogen. Instead of studying the proto-galaxies themselves, the OVRO-LWA makes it possible to study the gas around these galaxies. How do galaxies form and develop into the wonderfully complex and beautiful systems we see today? How did they interact with the surrounding gas during this process? When did galaxy formation start and how long did it take? All of these questions are wide-open. However, with the OVRO-LWA at Owen's Valley, we will begin to probe the answers to some of these questions.

The Transient Radio Sky

Time domain astronomy is a rich field with high potential for new discoveries, in all wavelength regimes. The success of transient searches in the optical (with the Palomar Transient Factory (PTF), the Catalina Real-time Transient Survey, the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS), etc.) and in the X-ray and gamma-ray skies (with the Swift Gamma-Ray Burst Mission and the Fermi Gamma-ray Space Telescope) highlights the vast scientific yield and exciting nature of time domain astronomy. The variable and transient radio sky, however, remains relatively poorly sampled, due to the limited fields of view, sensitivity, and survey speeds of traditional radio interferometers, despite the evidence that radio transient phase space is equally as rich as its counterparts in other wavelengths.

The OVRO-LWA will open up the field of radio transients -- with full cross-correlation of all of its 33,000 baselines and instantaneous imaging by a dedicated transient backend, the OVRO-LWA will produce all-sky images every second with approximately 10 arcminute resolution in all 4 polarizations (IQUV), reaching less than 10 mJy RMS noise in a 1 hour integration. This all-sky sensitivity means we can perform targeted transient searches as well as conduct blind surveys to better sample the transient phase space and reveal new and exciting populations of radio transients.

The OVRO-LWA transient search is particularly aimed at the detection of coherent radio emission from extrasolar planets, similar to the extremely bright electron cyclotron maser emission produced by magnetized planets in our own Solar System. The direct detection of extrasolar planets through their auroral radio emission would provide measurements of magnetic field strengths and rotation rate, as well as serving as an indirect probe of interior composition and dynamics.

Satellite Imagery

People

Gregg Hallinan (Caltech) OVRO-LWA PI Lincoln Greenhill (Harvard/SAO) LEDA PI Marin Anderson (Caltech) Grad Student Michael Eastwood (Caltech) Grad Student mweastwood at astro.caltech.edu Ryan Monroe (Caltech) Grad Student

Acknowledgements

The OVRO-LWA is located at the Owens Valley Radio Observatory, a Caltech-operated observatory near Bishop, California. Construction on the array began in 2012. The OVRO-LWA project was enabled by the kind donation of Deborah Castleman and Harold Rosen.