DEGAS — Dense Extragalactic GBT+Argus Survey

Image credit: Melissa Hoffman.
  • Receiver: Argus
  • Time granted: xxx h
  • PI: A. Kepley
  • Status: Observation completed
  • Data: <LINK>
  • Category: Extragalactic


The goal of DEGAS is to map four dense molecular gas tracers — HCN(1-0), HCO+(1-0), 13CO(1-0), and C18O(1-0) — over the central 4 arcmin2 of 36 nearby galaxies.

Resolved observations in the Milky Way and integrated measurements in nearby galaxies have shown that the presence of molecular gas, in particular dense molecular gas, is linked to the presence of star formation. This trend, however, has a large amount of scatter suggesting that additional physical parameters beyond the amount of dense gas play a role in setting this relationship. Disentangling these parameters requires resolved measurements of dense molecular gas in environments with a much wider range of conditions than found in the Milky Way.

A obvious population to do this experiment in is nearby (<20 Mpc) galaxies where one can obtain sensitive, relatively high resolution (<500 pc) observations. Such observations were extremely difficult with the previous generation of millimeter telescopes because the brightest dense molecular gas tracers (HCN and HCO+) are fainter than CO by a factor of 10-30.

DEGAS leverages the fast mapping speed, excellent surface brightness sensitivity, and good resolution made possible by the combination of the 100m Green Bank Telescope and Argus, a 16-pixel focal plane array, to map dense molecular gas in many nearby galaxies. When complete it will represent the largest extragalactic dense molecular gas survey to date.

Current Status

DEGAS officially started taking observations on February 1, 2017. The survey was awarded a set number of hours per semester (generally ~85hrs) and has been scheduled for approximately half as much (~45 hours). This trend is typical of other 4mm projects on the GBT. As of July 2019, we have obtained complete data sets for 9 galaxies and partial data sets for 5 more.

As of July 2019, the team is putting together data release 1 (DR1). This release will include all galaxies that have a full complement of maps (IC342, NGC2146, NGC2903, NGC3521, NGC4038, NGC4321, NGC4414, NGC4501, NGC4569, and NGC6946). We plan to release calibrated cubes and integrated intensity maps (with errors).


DEGAS is using a custom-built reduction pipeline. It was adapted from gbtpipe to use vane calibration instead of a noise diode to calculate the system temperature. The code is publicly available on Github.

Data Products

Publicly released datasets, included data cubes and property maps from the DR1 coming soon!


DR1 paper
Kepley et al., in prep.

IC342 detailed analysis
Li et al., in prep.



Amanda A. Kepley (NRAO)

Team Members

  • Alberto Bolatto
  • David Frayer
  • Andy Harris
  • Maria Jimenez-Donaire
  • Jialu Li
  • Jennifer Donovan Meyer
  • Erik Rosolowsky
  • Frank Bigiel
  • Sarah Church
  • Kieran Cleary
  • Molly Gallagher
  • Samantha Garza
  • Josh Gundersen
  • Annie Hughes
  • Sarah Kessler
  • Rebecca Levy
  • Matteo Luisi
  • Karin Sandstrom
  • Andreas Schruba
  • Matt Sieth
  • Eva Schinnerer
  • Yiqing Song
  • Jeremy Thorley
  • Antonio Usero

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