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A 23 GHz Beamforming Array for the GBT

The GBT has been one of the world’s most prolific telescopes for the observation of interstellar and extragalactic ammonia for much of its lifetime. Since 2009/10 this rich seam of observational radio astronomy has been mined with the K-band Focal Plane Array (KFPA) instrument, a seven-pixel heterodyne receiver commissioned and located at the GBT.

With the sensitivity arising from the large collecting area of the GBT and low radio-frequency environment of the Green Bank site, the KFPA has been a popular instrument among GBT users, including those in the areas of galaxy evolution, planetary systems, as well as astrochemistry. The primary use of the KFPA, however, has been to observe star-forming material in Galactic clouds. In the last few years three separate large-scale surveys have used the GBT and KFPA to map out the moderately-dense gas tracer of ammonia in our galaxy.

  • The Green Bank Ammonia Survey (GAS; Friesen et al., 2017) is a project to observe all northern Gould Belt star-forming regions with Av>7
  • The K-band Examinations of Young Stellar Object Natal Environments (KEYSTONE; Keown et al., in press) survey proposed to map 11 giant molecular clouds out to a distance of 3 kpc
  • The Radio Ammonia Mid-Plane Survey (RAMPS; Hogge et al. 2018) which has recently completed its pilot phase with a low sensitivity, wide-field survey of the Galactic plane

Each of these surveys used 300-500 hours of highly-competitive time on the GBT and is revealing hidden details of the molecular gas which makes up a large portion of our galaxy. A demonstration of this power is shown in the figure below, which reveals the velocity, velocity dispersion and kinetic temperature of the Orion A giant molecular cloud.

The Green Bank Observatory recently submitted a project white paper to the National Academy of Science’s decadal survey, proposing that a phased-array feed receiver for the GBT be built. This instrument would use emerging beam-forming technologies to create 225 beams (pixels) on the sky, allowing the GBT to map the sky at a rate around ten times that of the KFPA.

NH3 properties of Orion A from GAS (Friesen et al., 2017). The left panel shows Herschel-derived column density data, inferred from dust emission, with NH3 integrated intensity overlaid and Spitzer protostars shown. The next panel shows the LSR velocity, exhibiting a clear north-south gradient. The third panel shows the velocity dispersion and the fourth shows the kinetic temperature derived from the (1,1) and (2,2) line ratios.