7-Pixel 18-28 GHz Conventional Array

Kinetic temperature map of cold gas via NH3 (1,1) and (2,2) transition observations in the Galactic Center Sgr A* to Sgr B2 region. (Image courtesy J.Ott, NRAO.)

Overview

The K-band Focal Plane Array has seven beams total, each with dual circular polarization. Each beam covers the 18-27.5 GHz frequency range with fixed separations on the sky. The feeds have cooled polarizers producing circular polarization. The only internal switching modes is frequency switching. The seven feeds are laid out in a hexagon with one central feed. The hexagon is oriented such that the central feed is not at the same cross-elevation or the same elevation as any of the other beams. There is a noise diode for each beam ( 10% of the system temperature) for flux calibration. The maximum instantaneous bandwidth for each receiver is currently 1.8 GHz.  A complete and up-to-date description of the receiver can be found in the GBT proposal guide.

Key Science

While the overall impact of the KFPA will be large and will touch on areas ranging form mapping complex molecules in space  through determining the redshifts of galaxies at z= 3.5-4.5 through the CO line and studying highly  determining the properties of infrared dark clouds,  three key science areas have been identified for the instrument and are outlines below.  The complete science case for the KFPA can be found at this link.

I. How are stars assembled? The way in which stars are assembled from the interstellar medium and how their final masses are determined have been a long standing cause for debate. These questions remain among the major unsolved issues in modern astrophysics. Stars have a regular, near-universal mass distribution. This distribution, also known as the Initial Mass Function (IMF), has a peak around M~0.3 M_sun and at higher masses the distribution falls off as a power-law of dN/dM proportional to M_sun^-1.35 (Salpeter 1955). After more than 50 years of study, an observational clue to the origin of the IMF has finally come as large-format arrays like SCUBA have been able to map submillimeter emission at high resolution over large areas of nearby star-forming regions. These observations have uncovered populations of starless cores that appear to have mass distributions that are very similar to that of the IMF, suggesting a link between core formation and stellar mass. Only a few cores have been found in these clouds, so the similarity of the core mass distribution to the IMF remains formally uncertain. Furthermore, connections between potentially signicant variations in the core mass distributions and the parent cloud properties have not yet been explored.

Next-generation submillimeter instruments will be able to detect large numbers of starless cores in nearby star forming clouds, given their larger fields and greater intrinsic sensitivities. The K-band focal plane array for the GBT has unprecedented sky coverage, enabling the mapping of the regions pertinent to this problem. These maps can be at a higher resolution than that feasible with any other single-dish telescope (given the constraints of map size) and will probe the connection between core formation and stellar mass .

II. The Role of Turbulence in Star Formation: Dynamical theories of star-formation rest upon the relative importance of gravoturbulent motions and/or driven turbulence. The interplay between gravitational forces and supersonic turbulence potentially controls many stages of star formation, providing stability on large scales and initiating collapse on small scales (e.g., Larson 2003 and Mac-Low & Klessen 2004). The internal dynamics of molecular clouds may be traced by molecular transitions observable with the K-band focal plane array. The relative importance of the different physical processes in cloud dynamics and star-formation is still highly contentious. Observations with the K-band focal plane array will establish the true importance of turbulence in these regions and determine the nature and origin of its effects.

III. Tracing the formation of galaxy clusters: In order to study the first coherent large-scale structures in the Universe, many studies have focused on pointed sub-mm/mm-wavelength surveys toward `signposts' of overdense regions, mainly those high-redshift radio galaxies suspected of being near the centers of early proto-clusters (e.g. Stevens et al. 2003; Greve et al. 2007). The `negative K-correction' to the sub-mm/mm flux density of the sub-mm/mm galaxies (hereafter SMGs) identied in such surveys, results in a nearly constant flux density for a single object over redshifts z~1-10. Therefore, observations at sub-mm/mm wavelengths are ideally suited for identifying high redshift clusters. Indeed, early imaging studies have identifed an excess of SMGs within the central few square arcminutes of the radio galaxy elds, suggesting that some of these SMGs may be associated with the proto-cluster. Typically, redshifts for SMGs are obtained by optical spectroscopy of the proposed optical/infrared counterpart responsible for the sub-mm/mm emission, having been identied through radio-wavelength interferometry (e.g. Chapman et al. 2003, 2005). However, given the dificulty in using radio interferometry to identify optical/infrared counterparts, particularly for those objects at z»3.5, it is crucial that we explore alternative means of estimating redshifts for the submm/mm luminous galaxies in these fields, and ultimately confirm their cluster membership.

Project planning wiki pages can be found here