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The Karl G. Jansky Very Large Array Sky Survey (VLASS). Science case and survey design

M. Lacy, S. A. Baum, C. J. Chandler, S. Chatterjee, T. E. Clarke, S. Deustua, J. English, J. Farnes, B. M. Gaensler, N. Gugliucci, G. Hallinan, B. R. Kent, A. Kimball, C. J. Law, T. J. W. Lazio, J. Marvil, S. A. Mao, D. Medlin, K. Mooley, E. J. Murphy, S. Myers, R. Osten, G. T. Richards, E. Rosolowsky, L. Rudnick, F. Schinzel, G. R. Sivakoff, L. O. Sjouwerman, R. Taylor, R. L. White, J. Wrobel, H. Andernach, A. J. Beasley, E. Berger, S. Bhatnagar, M. Birkinshaw, G. C. Bower, W. N. Brandt, S. Brown, S. Burke-Spolaor, B. J. Butler, J. Comerford, P. B. Demorest, H. Fu, S. Giacintucci, K. Golap, T. Guth, C. A. Hales, R. Hiriart, J. Hodge, A. Horesh, Z. Ivezic, M. J. Jarvis, A. Kamble, N. Kassim, X. Liu, L. Loinard, D. K. Lyons, J. Masters, M. Mezcua, G. A. Moellenbrock, T. Mroczkowski, K. Nyland, C. P. O'Dea, S. P. O'Sullivan, W. M. Peters, K. Radford, U. Rao, J. Robnett, J. Salcido, Y. Shen, A. Sobotka, S. Witz, M. Vaccari, R. J. van Weeren, A. Vargas, P. K. G. Williams, I. Yoon

arXiv:1907.01981v3astro-ph.IMastro-ph.GAastro-ph.HE

TL;DR

VLASS addresses the need for a high-resolution, wide-band radio survey providing spectral, polarimetric, and time-domain coverage. Its three-epoch VLA design demonstrates feasible survey operations, while early Quick Look products have already supported scientific applications despite accuracy caveats.

  • Problem

    Existing radio surveys motivate science-ready, high-resolution data products with broad frequency, polarization, and time-domain information.

  • Method

    VLASS uses approximately 5500 hours of VLA observations in three passes, imaging 2–4 GHz emission with calibrated Stokes I, Q, and U data.

  • Results

    Early observations demonstrated survey feasibility, and Quick Look images have already proven useful for some scientific applications.

  • Takeaways & Limitations

    Science-ready VLASS products can support broad radio studies, transient identification, cross-identification, and statistical polarization analyses.

  • Takeaways & Limitations

    Sources with diffuse emission larger than approximately 30 arcseconds may be poorly imaged and have underestimated total flux densities.

Abstract

from arXiv · show

The Very Large Array Sky Survey (VLASS) is a synoptic, all-sky radio sky survey with a unique combination of high angular resolution ($\approx$2.5"), sensitivity (a 1$σ$ goal of 70 $μ$Jy/beam in the coadded data), full linear Stokes polarimetry, time domain coverage, and wide bandwidth (2-4 GHz). The first observations began in September 2017, and observing for the survey will finish in 2024. VLASS will use approximately 5500 hours of time on the Karl G. Jansky Very Large Array (VLA) to cover the whole sky visible to the VLA (Declination $>-40^{\circ}$), a total of 33,885 deg$^2$. The data will be taken in three epochs to allow the discovery of variable and transient radio sources. The survey is designed to engage radio astronomy experts, multi-wavelength astronomers, and citizen scientists alike. By utilizing an "on the fly" interferometry mode, the observing overheads are much reduced compared to a conventional pointed survey. In this paper, we present the science case and observational strategy for the survey, and also results from early survey observations.

1. INTRODUCTION

VLASS is motivated by upgraded VLA capabilities, expanding optical/infrared survey coverage, and the demonstrated scientific value of accessible radio-survey data. Its design combines wideband, high-resolution, multi-epoch observations with products intended for broad scientific use, while accepting reduced sensitivity to diffuse emission.

  • 1. INTRODUCTION: Wideband backends increase VLA continuum sensitivity and provide frequency-dependent flux density and polarization across roughly a factor of two in frequency.Combined with on-the-fly mosaicking, these capabilities enable rapid, relatively deep imaging at high angular resolution.
  • 1. INTRODUCTION: More than 7500 FIRST images per day were served on average during a recent 18-month interval, demonstrating strong demand for science-ready radio-interferometric data products.Over 95% of queries were scripted, indicating substantial machine-accessed use.
  • 1. INTRODUCTION: Every 10 days, FIRST image-server usage represented an effective exposure time equal to the 4000 hours originally used to conduct the survey.The estimate combines 7500 images per day, three minutes per image, and ten days.
  • 1. INTRODUCTION: VLASS allocates approximately 5500 hours to cover the VLA-visible sky at ≈2.5 arcsec resolution in three passes separated by approximately 32 months.The survey spans 2–4 GHz in 2 MHz channels and provides calibrated Stokes I, Q, and U polarimetry.
  • 1. INTRODUCTION: The high-resolution design limits standard-image rms surface-brightness sensitivity to ≈1.5 K and poorly images diffuse emission on scales >∼30 arcseconds.Lack of short-baseline coverage can cause total flux densities of such sources to be underestimated.
  • 1. INTRODUCTION: VLASS is intended to provide information for every accessible radio-sky position to astronomers and citizen scientists, supporting machine-learning studies of millions of radio sources.An archive database and data-access tools are part of this intended use.

2. VLASS SCIENCE THEMES

VLASS design and science goals are organized around four themes that exploit the unique capabilities of the Karl G. Jansky VLA.

  • 2. VLASS SCIENCE THEMES: Four themes run throughout the VLASS design and science goals.
  • 2. VLASS SCIENCE THEMES: The themes are intended to exploit the unique capabilities of the Karl G. Jansky VLA.
  • 2. VLASS SCIENCE THEMES: The science themes provide the organizing framework for VLASS design and goals.

4. The New Milky Way Galaxy

VLASS combines multi-epoch radio imaging, broadband polarization, and high angular resolution to investigate transients, explosive events, magnetic fields, and magneto-ionic environments across the sky.

  • Transient searches: VLASS searches for slow transients in Quick Look images and fast transients through the commensal realfast and VLITE projects.Slow transients last weeks to years, whereas fast transients last milliseconds to seconds.
  • Transient classification: Follow-up VLA, optical, and near-infrared observations classify transients using radio light curves, 1–50 GHz SEDs, host-galaxy properties, distances, and energetics.The shallow depth of individual epochs is expected to associate known classes of events with optical or infrared host galaxies.
  • Polarization science: Faraday components separated by >100 rad m−2 can be resolved despite NVSS depolarization, with VLASS providing 10 rad m−2 resolution at SNR 10.The VLASS Faraday-depth response has width ≈200 rad m−2, while its native 2 MHz channels probe higher maximum rotation measures.
  • Polarization science: Six polarized sources per square degree are expected at SNR=10 in the combined survey, yielding at least six times the polarized-source density of NVSS.The estimate assumes 70 µJy rms and accounts for reduced wavelength-dependent depolarization in the 2–4 GHz band.
  • Magnetic-field studies: VLASS polarization data support statistical studies of depolarization, galactic magnetic fields, circumgalactic gas, and magnetized environments across galaxies and redshifts.The survey is expected to detect over 10^5 polarized sources above 0.75 mJy and increase samples for studies involving Mg II absorbers and distant galaxies.

2.3. Key Science Theme 3: Imaging Galaxies through Time and Space

VLASS is designed to study galaxy evolution across accretion activity, AGN feedback, quasar radio emission, mergers, star formation, and distant luminous infrared galaxies. Its large area, resolution, sensitivity, and dust-unaffected radio selection enable statistically substantial samples spanning redshift and source type.

  • The Evolution of Accretion Activity in Active Galactic Nuclei: VLASS can trace AGN populations from low-redshift radio-quiet sources to radio-loud quasars at z ∼5 using sensitivity, angular resolution, morphology, and spectral-index information.At low redshift it reaches radio luminosities of about 10^22−23 W Hz−1, while stacking extends studies to higher redshifts.
  • AGN Feedback: VLASS morphology enables large samples of extended FRI sources for radio-mode feedback and compact sources for quasar-mode feedback out to z > 2.These selections are intended to study feedback in groups, clusters, compact radio galaxies, and interactions between jets and ionized gas.
  • Quasar Radio Emission: VLASS will improve radio-quiet quasar demographics through direct detection to z ≈0.1 and stacking at greater distances.The radio emission of radio-quiet quasars may arise from failed jets, star formation, shocks, or combinations of these mechanisms.
  • Dual AGN: VLASS combines arcsecond resolution, enormous area, sub-mJy sensitivity, and dust-unaffected radio selection to identify thousands of merging dual AGN.Its sensitivity and resolution support applying the Stripe 82 mining technique over a 370× larger area, enabling uniformly selected samples and matched controls.
  • Star Formation in Galaxies: VLASS can detect central, spatially resolved radio emission in nearby star-forming galaxies, including compact regions or weak AGN, while diffuse outer-disk emission remains below its surface-brightness sensitivity.The eastern galaxy in the IC 5373 merger has a compact structure consistent with either a nuclear starburst or weak AGN.
  • Star Formation in Galaxies: VLASS detects star-formation emission in ULIRGs out to z ∼0.5 and is expected to increase radio detections by a factor ≈5 relative to FIRST-based studies.VLASS spectra combined with other data can assess ionized-gas column densities and estimate high-frequency free-free emission.

2.4. Key Science Theme 4: The New Milky Way Galaxy

VLASS expands the radio view of the Milky Way across much of the Galactic Plane, complementing other surveys while exploiting radio wavelengths' resistance to dust obscuration. Its frequency, sensitivity, resolution, and multi-epoch coverage support studies of compact objects, active stars, thermal sources, and Galactic structure.

  • The New Milky Way Galaxy: VLASS covers approximately 75% of the Galactic Plane and complements GLOSTAR's smaller, deeper 4–8 GHz survey region.Its broad Galactic longitude coverage creates discovery space for expected and unexpected radio phenomena.
  • The New Milky Way Galaxy: Radio wavelengths allow VLASS to identify Galactic objects that visible and near-infrared surveys can miss because of dust obscuration.This complements infrared, X-ray, and gamma-ray surveys probing the obscured Galactic Plane.
  • Compact Objects: At 2–4 GHz, VLASS is especially sensitive to highly scattered rare pulsar systems near the Galactic Plane.Spectral-index, polarization, and compactness filters can reduce detections to a feasible set for periodicity searches.
  • Coronal Magnetic Activity on Cool Stars: VLASS can detect ultracool dwarfs at 10–20 pc, active dwarfs at a few tens of parsecs, and active binaries at slightly less than 2 kpc.At 150–300 pc, its sensitivity probes stellar radio luminosities above 1–4 × 10^16 erg s−1 Hz−1 in young stellar objects.
  • Coronal Magnetic Activity on Cool Stars: Combining VLASS with LSST and eROSITA will support identification and study of nearby active stars across evolution from young stellar objects to the main sequence.Cross-correlating variable or transient sources across these surveys can constrain sources producing extreme magnetic activity.
  • Star Formation and Evolution, Distant Thermal Sources, and Galactic Structure: Arcsecond resolution and brightness-temperature sensitivity of order 10 K or better are sufficient to detect many distant H II regions and planetary nebulae.VLASS and complementary radio and infrared observations will expand samples of these thermal sources throughout 75% of the Galactic disk.

3. SURVEY STRATEGY

VLASS is designed as a high-resolution, three-epoch radio reference survey covering the VLA-visible sky, combining wideband polarimetry with sensitivity and cadence suited to source identification and transient science.

  • Calibration: 2–4 GHz observations provide calibrated Stokes I, Q, and U data, with flux-density accuracy required at 10% and polarization leakage below 0.75%.The survey has a 5% flux-density accuracy goal and a 0.25% polarization leakage goal.
  • Area and Depth: 33,885 deg^2 north of declination −40° is the target area, with at least 90% observed in each epoch.The survey area is 82% of the celestial sphere.
  • Angular Resolution: 2.5 arcsec resolution enables VLASS to decompose radio structures on roughly 30-kpc scales at redshifts ≳1 and distinguish crowded or ambiguous sources.The survey uses B and BnA configurations to provide this synthesized beam across the sky.
  • Cadence of Multiple Epochs: Three epochs separated by approximately 32 months provide independent high-resolution maps for transient discovery and future follow-up.The cadence exceeds the required two-year interval and matches the VLA configuration cycle.
  • Sensitivity: 70 µJy beam−1 is the combined three-epoch 1σ sensitivity goal, compared with 120 µJy beam−1 per epoch.The maximum survey speed is 23.83 square degrees per hour, constrained by the data rate.

4. SURVEY IMPLEMENTATION

VLASS implementation combines dynamic scheduling, on-the-fly mosaicking, staged data products, and rapid transient services to process and distribute a large multi-epoch survey.

  • Observing Plan: 5520 hours over seven years and six VLA B/BnA configuration cycles implement the planned three-epoch survey.A roughly 200-hour pilot survey tested the observing strategy before the main program.
  • Observing Plan: The first epoch was completed through two sky-half campaigns, with subsequent epochs preserving the approximately 32-month cadence.VLASS1.1 covered the first half from September 2017 to February 2018, and VLASS1.2 covered the remainder from February to July 2019.
  • Scheduling Considerations: 899 tiles are grouped into 4–8-hour scheduling blocks to support dynamic scheduling while reducing calibration overheads.Each tile is approximately 10°×4° and corresponds to about two hours of observing.
  • Data Products: Basic Data Products include calibrated data-generation products, Quick Look images, single-epoch images and catalogs, and cumulative images and catalogs.Products are designed to require little post-processing, while Enhanced and Commensal products require additional expertise or resources.
  • Transient Services: Quick Look images target production within two weeks to support rapid follow-up of short-timescale Galactic and relativistic transients.Automated quality assurance and source extraction are intended to identify artifacts, variables, and transients robustly.
  • Data Products: Fine 16-MHz cumulative cubes improve sensitivity to high rotation measures above 1000 rad m−2 compared with coarser products.The available 2–4 GHz frequencies provide a roughly 200 rad m−2 FWHM response in all cases.
  • Transient Services: The VLASS Transient Marshal and automated VOEvent alerts connect transient candidates with multi-wavelength information and the wider astronomical community.Alerts include links to the corresponding Transient Marshal entries.

5. COMMENSAL EXPERIMENTS

VLASS is supplemented by commensal systems that search for fast transients and observe at low radio frequencies without requiring separate primary observing programs.

  • The VLASS Commensal Sky Survey: realfast processes fast-sampled VLA visibilities in real time to detect candidate transients across the full field of view.A dedicated compute cluster addresses a data rate otherwise reaching roughly one terabyte per hour.
  • Realfast: 5–20 ms visibility sampling enables realfast searches, with a stated image sensitivity of 5 mJy in 5 ms for 1–5 GHz observations.The system dedisperses and images the fast-sampled data during VLASS observations.
  • The VLASS Commensal Sky Survey: VLITE simultaneously observes 320–384 MHz using 64 MHz bandwidth, full polarization, 100-kHz spectral resolution, and 2-second temporal resolution.It uses dedicated samplers, spare fibers, and a custom real-time DiFX correlator.
  • The VLASS Commensal Sky Survey: 15–16-antenna VLITE processing during the first epoch produced initial mosaics with roughly 3 mJy beam−1 noise and 12–25 arcsec resolution.Combined VCSS and VLASS data provide instantaneous point-source spectra, transient verification, and extended-source information.
  • The VLASS Commensal Sky Survey: VLITE data are archived for community scientific use, while expansion to the 27-antenna LOBO system remained a possibility during the VLASS program.Standard VLITE products include science-ready images, calibrated visibility data, and source catalogs.

6. CALIBRATION OF VLASS DATA

VLASS calibration combines standard interferometric processing with corrections for gain compression and full linear-polarization calibration. Early results show good flux-density consistency and improved Faraday-rotation measurements, while absolute polarization position angles remain limited.

  • Calibration pipeline: VLASS calibration begins with flagging known problems, calibrator modeling, and standard corrections for antenna gains, opacity, and positions.The pipeline is a modified CASA calibration system for ALMA and VLA data.
  • Gain compression: 30% gain compression can be corrected using switched-power data, while scheduling limited compression above 30% to <0.1% of the data.Gain compression is especially problematic in regions affected by satellite emission near the Clarke Belt.
  • Polarization calibration: Absolute polarization position angles are reliable only to approximately 5°, although relative angles and internal VLASS rotation measures are not expected to be significantly degraded.The limitation affects comparisons with other datasets.
  • Polarization: 200 rad m−2 is the Faraday-transfer-function width, with rotation-measure uncertainty of approximately 200/(2·SNR) rad m−2.The uncertainty estimate was confirmed using bright point sources with simple Faraday screens.
  • Flux-density scale: 1.9% and 0.7% RMS scatter about the mean was measured for J0318+1628 and J1326+3154, respectively, indicating good flux-calibration consistency.The measurements used 26 observations of J0318+1628 and 42 of J1326+3154; Quick Look images do not reach this accuracy.
  • Polarization: VLASS polarimetry reveals Faraday-rotation variations in 3C402 that were unresolved by NVSS and increases measured fractional polarization.For 3C402N, fractional polarization rises from 6.7% in NVSS to 13% in VLASS; for 3C402S, it rises from 1.7% to 11%.

7. EDUCATION AND OUTREACH

VLASS embeds education, outreach, visualization, and participation programs into survey operations. Workshops and NINE activities use VLASS data to develop visualization, computing, data-mining, and broader radio-astronomy engagement.

  • Outreach strategy: VLASS distributes information and data access through web, social, print, presentation, and community platforms for scientists and the public.The outreach plan includes catalogues and data products for astronomers, including non-specialists.
  • Visualization workshops: Two visualization workshops taught participants to balance aesthetic choices with scientific information when producing radio images.The programs aimed to create images that engage viewers and encourage further astronomy learning.
  • Visualization workshops: 20 participants completed a remote-instruction visualization workshop despite low-to-medium imaging skill levels.A moderator supported the online format by monitoring chat discussions.
  • Visualization workshops: Assigning colors to four frequency subbands allowed workshop participants to reveal spectral-index changes without contour-like plots.Teams used Quick Look images of 4C 48.49 and GIMP, followed by critique of color harmony and interpretation.
  • STEM programs: NINE workshops use VLASS imaging products to expose students to Python and data mining, with participants carrying these skills to their home institutions.The program targets broader participation by under-represented groups in radio astronomy.

8. SUMMARY

VLASS combines high-resolution, wideband, polarimetric, and time-domain radio coverage across 82% of the sky. Early observations demonstrate survey feasibility and useful applications, while later processing is intended to improve accuracy and add polarimetry.

  • Survey scope: 2.5 arcsec resolution, a 70 µJy rms sensitivity goal, full linear polarimetry, time-domain coverage, and 2–4 GHz bandwidth define VLASS over 82% of the sky.The survey is presented as a large-scale synoptic radio complement to optical and infrared surveys.
  • Survey scope: VLASS is described as the first large-scale synoptic radio survey, with expected long-term value for astronomical research.The paper states that it will open new areas of scientific discovery and benefit the astronomical community.
  • Early results: 2016–2019 pilot and first-epoch observations demonstrated survey feasibility and indicated that the stated goals could ultimately be reached.Quick Look images became available for these early observations and were already useful for some scientific applications.
  • Early results: Early Quick Look products require caution for science because of limitations in imaging and flux-density accuracy.A full Single Epoch processing suite was planned to provide more accurate products and include polarimetry.
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