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“The past is but the beginning of a beginning, and all that is and has been is but the twilight of the dawn.”           

Herbert George Wells, The Discovery of the Future (1901)

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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 4arcmin^2 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 (<20Mpc) galaxies where one can obtain sensitive, relatively high resolution (<500pc) 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. We were awarded a set number of hours per semester (generally ~85hrs) and have 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 currently 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).

Software

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.  It is publicly available on github: https://github.com/GBTSpectroscopy/degas

DEGAS Publications

•  Kepley et al., in prep (DR1 paper)
•  Li et al., in prep (detailed IC342 analysis)
•  Frayer 2016, “The GBT Beam Shape at 109GHz”, GBT memo 296
•  Frayer et al., 2019, “Calibration of Argus and the 4mm Receiver on the GBT”, GBT memo 302

Publicly Released Datasets, Including Cubes and Property Maps

• DR1 coming soon!

Team Members

PI: Amanda A. Kepley (NRAO)

Alberto Bolatto, David Frayer, Andy Harris, Maria Jimenez-Donaire, Adam Leroy, 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 Liusi, Karin Sandstrom, Andreas Schruba, Matt Sieth, Eva Schinnerer, Yiqing Song, Jeremy Thorley, Antonio Usero

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Coming Soon…

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