Futures > Next Generation Very Large Array > ngVLA Community Studies Topics

ngVLA Community Studies Topics

by Davis Murphy last modified Aug 09, 2016 by Eric Murphy

Science Studies Topics

Terrestrial Planet Formation and Astrochemistry: Imaging of the dust continuum at optically thin frequencies (typically < 50GHZ) at 1AU resolution at 130pc (=10mas), plus astrochemistry, in particular the pathway to biochemistry. Areas of interest for further study include:

  • Imaging capabilities of different configurations using CASA simulator and disk models.
  • Number of systems that can be imaged in the dust continuum at 1AU resolution at 130pc (Taurus, Rho Oph). Possibilities for imaging at distance of Orion (500pc).
  • Sensitivity and spectral requirements for large molecule chemistry.

Chemistry of the Early Universe: Study of the molecular gas content, dynamics, and ISM physics of galaxies back to the epoch of reionization. Areas of interest for further study include:

  • Prospects for molecular deep fields and mapping out the dense gas history of the Universe using low order CO transitions.
  • Sensitivity required for imaging dense gas tracers, such as HCN and HCO+.
  • Imaging capabilities to study the dynamics, spatial distribution, and temperature distribution of molecular gas in main sequence galaxies at high redshift.

Exo-space Weather: study of stellar phenomena related to space weather and its potential impact on the development of life. Areas of interest for further study include:

  • Flares on dwarf stars: sensitivity vs. number of systems vs. time scales.
  • Stellar winds: sensitivity vs. mass loss rate.
  • Exo-aurorae: star - planet magnetospheric interactions: statistics and implications.
  • Imaging stellar photospheres: number of supergiants that can be resolved. Simulations of imaging capabilities. Temporal changes (rotation).
  • Radio HR diagram: number of stars that can be detected vs. type.
  • Radio novae: late time mass loss. Imaging capabilities and temporal variations.

The Dynamic Sky: time domain astronomy is a major growth area, with the advent of, e.g., LSST, Euclid, WFIRST, etc. Areas of interest for further study with the ngVLA include:

  • Capabilities and requirements for triggered fast response.
  • Possibility of finding EM counterparts to gravitational wave sources.
  • Searching for millisec bursts: Localizing FRBs.
  • Targeted search for millisecond pulsars at the galactic center.
  • Feasibility study of wide-area pulsar searches (including for example, effect of available frequency ranges, array configuration, field of view, correlator and computational requirements).
  • Pulsar timing; complementarity with current and future GHz-frequency instruments.
  • Active stars and stellar systems (flare stars, active binaries, T Tau objects, LMXRBs, …)
  • Novae, supernovae
  • Extreme scattering events

Near Earth Sensing: the ngVLA will offer significant opportunity into the field of Near Earth Sensing, which includes studies related to near Earth asteroids, space debris, and artificial satellites. At least 4 methods are possible and require further study:

  • Passive thermal observations. At the cm and millimeter wavelengths imaging ambient (200K) objects at 30 mas resolution will be possible. This resolution is equivalent to ~10cm at low earth orbit, 6m at geosynchronous orbit, and 50m at a lunar distance. Spin states, orientation, heat capacity, and material properties of satellites and asteroids could be probed at these scales. Such observations require no cooperation on behalf of the targets or other facilities.
  • Passive observing of satellite transmissions. Functioning artificial satellites invariably transmit in the radio band. Observing such transmissions could be used for radio "fingerprinting" and precise state vector determination.
  • Passive illumination of space debris. Radio transmissions of existing high-elevation satellites can illuminate known and unknown space debris. Training a small number of ngVLA antennas on the direct transmission of a satellite and the remaining away from the direct transmission one could perform a cross-correlation analysis and determine states of debris through time of arrival analysis.
  • Radar observations. Very precise astrometry of test mass satellites such as LAGEOS, which are intended for precise tests of gravity using laser ranging, could be used to improve frame ties between optical sensors, radio telescopes, and the quasar reference frame. Broad-band centimeter-wave radar imaging of near-earth asteroids could be used to complement passive thermal observations for improved material analysis

Baryon Cycling: Map the total and dense molecular gas content within nearby galaxies for a full census of mass cycles in galaxies. The ngVLA will enable local group-type ISM studies out to the Virgo cluster, using the suite of available diagnostic lines in the 3mm band. Areas of interest for further study include:

  • Array design required for low surface brightness sensitivity: antenna sizes, (re)configurations, total power.
  • Simulations of ngVLA capabilities to image the line and continuum emission on sub-arcsecond scales with sub-K sensitivity.

VLBI/Astrometry: microarcsec astronometry using VLBI has revolutionized the fields of Galactic structure, supermassive black studies, and determination of Ho. The ngVLA will play an anchoring role as the mega-element in a global VLBI array, pushing this field to new horizons. Areas of interest for further study include:

  • Simulation of astrometric performance of core ngVLA plus VLBI stations to transcontinental baselines.
  • Calculations of accuracy that can be obtained on spiral structure across the galaxy using masers.
  • Local group dynamics: explore possibility of determining 3D motions of local group galaxies.
  • Real time cosmology: explore possibility of detecting the expansion of the Universe in real-time using VLBI astrometry.

Plasma Physics: Use of the Sun, stars, supernovae, radio galaxies, ISM, and IPM as laboratories for understanding magnetic energy storage and release, particle acceleration and transport, shock formation and propagation, plasma turbulence, kinetic plasma processes, emission mechanisms. Areas of interest for further study include:

  • Required development for solar observing with ngVLA core
  • Required development for time and spectral domain observations of the above objects
  • Potential study targets include:
    • Pulsar radio emission mechanisms; especially new insights that may come from sensitive, wide-band >10 GHz observations.
    • solar wind tomography with the ngVLA
    • magnetic energy release on the Sun and starts using time-resolved imaging spectroscopy
    • magnetic reconnection in galactic and extragalactic sources using imaging spectroscopy
    • the SZ effect as a means of diagnosing shocks in clusters
    • radio filaments in radio galaxies and their relation to magnetic flux ropes
    • termination shocks in flares on the Sun and stars
    • spatiotemporal evolution of the electron distribution function in flares using imaging spectroscopy – the solar-stallar connection
    • coronal magnetography on the Sun and stars
    • coherent emission mechanisms and kinetic plasma processes on the Sun
    • a taxonomy of radio bursts on stars and stellar systems


Technical Studies Topics

Advanced Cryo-cooling Options: In addition to standard cryo techniques that are currently being pursued at NRAO and elsewhere, we are interested to see if there are substantial gains to be made through modifications to the Gifford-McMahon cycle, or the use of completely different thermodynamic cycles. Areas of interes for further study include:

  • Optimization of system power consumption to meet dynamic cooling requirements (cooldown, steady-state, standby modes) for individual receivers. Technology development to support this involves variable-speed drives for both cryocoolers and compressors, and intelligent monitor and control software.
  • Investigation of alternative (more theoretical/futuristic) thermodynamic cycles that will result in system that is more efficient, has increased intervals between maintenance, easily serviceable, and delivers significant power savings.

Ultra-wideband Feed Tradeoffs: There is strong interest by the community in ultra-wideband feed antennas for use in cryogenic receivers. Ongoing research and development in this area is currently going on at Caltech, NASA/JPL and CSIRO, among others. The advantages of such wide-band feeds include: Multi-octave signal bandwidths. Adds new science capabilities (spectral line searches, transient sources); Significant reduction in the receiver count, especially for low-frequency bands. This could dramatically reduce receiver construction and operations costs, with fewer cryocoolers per antenna. The challenges/areas that require further study include:

  • Achieving good aperture efficiency over multiple octaves (~6:1 bandwidth). Ongoing work with dielectrically-loaded quad-ridge feeds is promising, but needs additional development and testing.
  • Minimizing conductor loss. For quad-ridge feeds, the coax transitions could contribute significant additional noise. Noise calibrator injection is another challenge, particularly at high frequencies, because of coupler loss. Need to investigate alternatives.
  • Cooling the feed, particularly if it's large. May need more development on dewars with large windows and IR filters. Feeds have to be cooled, because of the coax terminations and their associated losses.

Phase Calibration Options: There are a number of choices for the phase calibration of ngVLA observations (e.g., fast switching, water vapor radiometer, and a stand-alone calibration array) each of which has a different suite of advantages and disadvantages. Areas that require additional study include:

  • A detailed investigation at the trade-offs (e.g., cost, observing efficience, etc.) amongst these various options.

Reconfigurability/Configuration/Total Power: The straw-man NGVLA contains ~300 18-m telescopes arranged in concentric fat rings. Its 300 km maximum baseline achieves ~10 mas resolution at 30 GHz to image the "dust" continuum of a nearby (130 pc) protoplanetary nebula and resolve gaps of AU width. Additionally, it is centrally concentrated, with ~60% of the collecting area <15 km from the center and 30% in a 1 km core, which improves the surface-brightness sensitivity enough for the NGVLA to image thermal spectral lines from cold molecular gas in high-redshift galaxies with ~100 mas to ~1 arcsec resolution, a few kpc at any 0.4 < z < 8. Areas that require further investigations include:

  • The impact on point source sensitivity due to various of weighting schemes that are able to yield a wide range of clean synthesized beamwidths.
  • The effectiveness of the compact core to be phased to search for pulsars near the Galactic Center or anchor a VLBA array measuring precise positions of simple compact sources such as masers.
  • The inclusion of longer north-south baseines that couple to make the ngVLA a better "SKA-High" to bridge the frequency gap between the SKA and ALMA for a larger number of southern hemisphere sources.
  • The use of a small array of tiny dishes or the GBT as a way to fill in the missing baselines smaller than 18m for studies of nearby galaxies.
  • Given the range or resolution and surface brightness sensitivity requirements of particular investigations, which could result in losing a factor of ~2 in effective collecting area, the option of reconfiguring the central array within a 15km radius warrants careful study.


Data Backhaul:   The data transmission system on the ngVLA includes the digitization of the RF signal from the antenna receivers and the transmission of the digitized signal via optical fiber to a central processing facility. Specific areas of data transmission that may require further investigation include:  

  • Establish the functional requirements for the ngVLA data transmission system, to include an analysis of the analog bandwidth, bit depth, sampling rate of digitizers, and how signals might be organized for the long distance transfer of wideband data. Investigate current and future technologies that might be implemented to address these requirements.  
  • The transmission medium for the ngVLA fiber optic system may be a combination of observatory-owned fiber, leased dark fiber, and commercial network bandwidth, due to expected the scale (~300km baselines). This arrangement will introduce issues with the time stamping of data. The system may need to cope with data packets arriving out of order and to provide padding for lost packets. Similarly, the technology adopted for the data transmission system at the center of the array may be different from that for the most distant antenna stations.

Time and Frequency Distribution:  The time and frequency distribution system of an interferometer provides clock signals for digital samplers and coherent frequency references for the up/down conversion of analog signals. Strict requirements are placed on the phase noise of these signals in order to preserve coherence. The signal phase should be stable on the timescale of an observation’s integration time in order to preserve the visibility phase.  Phase noise ultimately degrades the sensitivity, spatial resolution, and dynamic range of an observation. The total phase noise should be dominated by that of the atmosphere, and not that of the instrument. Specific areas in time and frequency distribution that may require further investigation include:

  • Propose the stability requirement for the ngVLA local oscillator (LO). Since the highest frequency on the ngVLA is higher than that for EVLA and SKA, the LO stability requirement for the ngVLA should be more stringent than that for those instruments.  However, there is no point in making the LO stability requirement better than the uncorrelated atmospheric stabilities on the longest baselines. Should the specified stability be a function of baseline length, thus allowing degraded stability on the longer baselines?  
  • Several technical solutions may exist for a given stability requirement.  Are the implementation and operations costs of a particular solution likely to be less than that of another?  Should fixed tones be distributed and then synthesized at each antenna? Or should the tones be synthesized at a central location?  
  • Polarization effects can become important for an LO stability specification of less than about 1 psec. If that is the case, then it is best not to transmit signals of different wavelengths because of differential delays. A conservative approach might be to limit the fiber-transmitted LO to a maximum frequency of 10 GHz, although this might be expensive since it requires a synthesizer at each antenna. Alternatively, one could transmit a tunable LO that is a sub-multiple of the desired first LO, and then perform a simple multiplication at the antenna.