The Green Bank Telescope Maps the Dense, Star-Forming Gas in the Nearby
  Starburst Galaxy M82
  Observations of the Milky Way and nearby galaxies show that dense molecular
gas correlates with recent star formation, suggesting that the formation of
this gas phase may help regulate star formation. A key test of this idea
requires wide-area, high-resolution maps of dense molecular gas in galaxies to
explore how local physical conditions drive dense gas formation, but these
observations have been limited because of the faintness of dense gas tracers
like HCN and HCO+. Here we demonstrate the power of the Robert C. Byrd Green
Bank Telescope -- the largest single-dish millimeter radio telescope -- for
mapping dense gas in galaxies by presenting the most sensitive maps yet of HCN
and HCO+ in the starburst galaxy M82. The HCN and HCO+ in the disk of this
galaxy correlates with both recent star formation and more diffuse molecular
gas and shows kinematics consistent with a rotating torus. The HCO+ emission
extending to the north and south of the disk is coincident with the outflow
previously identified in CO and traces the eastern edge of the hot outflowing
gas. The central starburst region has a higher ratio of star formation to dense
gas than the outer regions, pointing to the starburst as a key driver of this
relationship. These results establish that the GBT can efficiently map the
dense molecular gas at 90 GHz in nearby galaxies, a capability that will
increase further with the 16 element feed array under construction.
