By Erik Rosolowsky (University of Alberta), Joan Wrobel (NRAO)
In December a position paper on the Next Generation Very Large Array (ngVLA) was submitted
for the Mid-Term Review of LRP2020. A slightly modified version of the paper appears below
which we hope summarizes ngVLA developments over the past few years. If you want to get
more involved in ngVLA efforts in Canada, email rosolowsky [at] ualberta.ca
The ngVLA in a Nutshell
The ngVLA is a proposed new centimetre-wave interferometer that is being developed by the
US National Radio Astronomy Observatory (NRAO) to replace two existing aging facilities, the
Jansky Very Large Array (VLA) and the Very Long Baseline Array (VLBA). The ngVLA would
provide factors of 10 improvements in sensitivity and resolution relative to the VLA over the
critical frequency range of 1.2-116 GHz, positioning the new facility to become the premier world
observatory at frequencies between those covered by the Square Kilometre Array (SKA) and
the Atacama Large Millimeter/submillimeter Array (ALMA).
Conceptual design for the ngVLA. The ngVLA would comprise 244 antennas of 18-m diameter clustered in the US southwest but spread out between Hawaii and the US Virgin Islands, an 8860-km maximum baseline. Antenna placements in Canada and Mexico are under consideration. The ngVLA would also involve 19 antennas of 6-m diameter in a close-packed array at the current VLA site in New Mexico. Image credit: ophia Dagnello (NRAO/AUI/NSF)
The ngVLA would enable science not achievable with either ALMA or the SKA. For example, improved sensitivity at 30 GHz would allow the measurement of the optically thin Rayleigh- Jeans tail of dust emission from protoplanetary disks, resolving planet formation across 1000s of systems. Interferometer sensitivity is driven in part by collecting area, and ALMA lacks the sensitivity to make these measurements, even with the ALMA 2030 upgrade. Furthermore, the SKA would not reach sufficiently high frequencies to detect dust emission.
Strategically, the ngVLA would provide unmatched capabilities to:
- Observe planets in formation in the terrestrial zone of protoplanetary disks;
- Observe prebiotic molecules and chiral molecules in hot cores;
- Capture key organic and inorganic molecules like carbon-chain species, NH 3 , and radio
recombination lines that trace the dust and gas chemistry and kinematics of star
formation;
- Probe the thermal radio continuum and long wavelength dust emission;
- Access key spectral transitions like the fundamental transition of the CO gas tracer from
z=0 to z=12;
- Observe elusive pulsars toward the Galactic Centre to make precise tests of gravity;
- Probe the formation and evolution of black holes in the era of multi-messenger astronomy; and
- Observe targets in the northern hemisphere.
Key ngVLA Developments Since LRP2020
The ngVLA was evaluated in LRP2020 and presented as one of the Recommended Ground-Based future facilities for Canadian participation, with the caveat of its status in the US Decadal review. Astro2020 subsequently rated the ngVLA as one of the key new facilities for US ground-based astronomy, alongside an optical ELT and CMB-S4. Driven by the strong US Decadal support, NRAO has rapidly developed the ngVLA design, successfully passing a Design Review in September 2024 and preparing it for acceptance by the US National Science Foundation (NSF). The project is now in the US NSF’s Major Research Equipment and Facilities Construction queue, which is essential for the project to proceed to the construction phase. A Preliminary Design Review is expected to occur in approximately 18 months.
The US NSF has provided sufficient funding support to design, build and test a prototype 18-m ngVLA antenna. This prototype is now being assembled at the current VLA site in New Mexico and is expected to be turned over to NRAO for testing in early 2025. With continued support and development, the ngVLA could begin early science in the start of the next decade with full capabilities available in the late 2030s. While the US expects to be the primary funding source for the ngVLA, the project is seeking international partners. Thus far, early ngVLA developments have engaged the technical communities in Canada, Mexico, Germany, and Japan, and science communities worldwide. The ngVLA construction phase has a total project cost of US$3.2B (FY2020) according to Astro2020 TRACE analysis. External partners (both international and US domestic partners) are expected to provide ~25%. The science interests in the Canadian community should support a similar level of engagement as for ALMA or the SKA at approximately 5%.
One primary strength of the ngVLA is its mature facility design, underpinned by NRAO’s many decades of experience in the construction and operation of the VLA, VLBA, and ALMA. Overall, the ngVLA is low-risk in terms of technical design and has flexibility regarding exactly where its antennas will be located.
In the broader landscape of long-wavelength Canadian observatories, Canada has joined the SKA, securing access to cutting-edge low-frequency radio astronomy. Canada also remains actively engaged in submillimetre observations through ALMA, which has a robust upgrade and development plan to improve capacity, especially at >100 GHz. However, the key global facility in the 10-50 GHz range remains the 45-year old VLA.
Without being an ngVLA partner, Canada would need to access the facility through the Open Skies program, for which the US NSF has an expectation of reciprocity. It is anticipated that all SKA partners not involved in ngVLA construction or operations may collectively be limited to <5% of ngVLA time. In contrast, an SKA partner who contributes to ngVLA construction and operations can expect to be able to participate in the larger pool of Open Skies time.
Why the ngVLA Must Remain a Top Canadian Priority
- Participating in the ngVLA would provide an essential waveband to support Canadian science priorities as articulated in the LRP (galaxy evolution, star and planet formation, transients, pulsar astronomy, black hole growth). These opportunities would be unique to the ngVLA, with neither the SKA nor ALMA able to execute its key science goals.
- Our existing national expertise in low-cost antenna production, receiver development and correlators for interferometers, primarily through NRC, means that we would have clear opportunities for contributions to the ngVLA. This includes both design work, as is currently happening, and in the construction of the facility. Continuing our legacy of excellent hardware contributions would further develop our national STEM capacity and would continue to have spin-off applications for Canadian industry.
- Being a partner in the ngVLA would ensure a Canadian voice in its science steering and to lead major discoveries with the ngVLA. Key science projects are being designed now. Canadians are welcome at this point because of our expertise and potential engagement, but we risk being frozen out of defining the major science cases for this facility if we do not establish formal partnership. These science cases align well with LRP2020 goals and will remain active science themes throughout the next decade.
- Investment in the ngVLA would happen primarily in the next decade, but Canada should be planning for this transformational instrument now. Given the robust design and low- risk for construction, we could present a clear case and timeline for funding that happens after our SKA construction contributions. Furthermore, the US has historically been a reliable partner in radio astronomy (e.g., in the upgrades to the VLA and in our partnership in ALMA). Additionally, the ngVLA would be administered through partnership agreements that do not require extensive governance development. Hence, the path to Canadian participation in ngVLA would be easier than for the SKA.
