GBT W-band Focal Plane Array

The WFPA footprint superimposed on the HCN(1-0) (left) and HCO+(1-0) (right) images of comet Hale Bopp, as observed with the FCRAO 14m telescope whose resolution is given by the cross in the HCO+ image. (Images are courtesy of A. Lovell.)

Introduction

The combination of high sensitivity, unblocked aperture and wide field-of-view of the Robert C. Byrd Green Bank Telescope (GBT) is well-suited to focal-plane array receivers. Large spectroscopic focal-plane arrays at centimeter and millimeter wavelengths will enable a powerful new set of scientific capabilities with the GBT, both as a stand-alone instrument and in combination with the powerful EVLA and ALMA interferometers. The ability of the GBT to detect low surface brightness emission over wide areas is complementary to the capabilities of interferometers which are sensitive to small-scale structures. With the recent improvements to its surface rms, the GBT is the most sensitive single-dish telescope at W-band (3mm), and with a spatial resolution unmatched by any other single-dish telescope operating at 3mm. Combining this excellent sensitivity and resolution with a large spectroscopic focal plane array would provide unprecedented mapping ability and sensitivity. This will be a uniquely capable instrument for the investigation of the astrochemistry and physical properties and processes associated with molecular clouds and star formation within our Galaxy, nearby galaxies, and objects in the distant Universe.

Key Science

While the reach of the WFPA will be large, four key research areas have been identified for the GBT WFPA.   These are described below.  A complete description of the science case for the receiver can be found here.

• Fundamental Physics:

The GBT has helped to revolutionize the study of astrochemistry. Recently, 13 new organic molecules including CH6− (the first interstellar anion), have been found using the GBT. Contrary to prior expectations, many of these new species are not concentrated in compact hot cores, but are found in extended cool clouds. These discoveries have caused chemists to re-evaluate the chemical pathways and reaction rates in interstellar gas and have given rise to a new paradigm in the chemistry community - instead of using chemistry to understand astronomical phenomena, chemists are now using the unique conditions afforded by the interstellar clouds for the study of chemistry itself.  The GBT WFPA will be ideal both for studies of the ISM within the MilkyWay and other galaxies as well as for focussed studies of the Galactic Center, one of the prime astronomical targets for the study of astrochemistry.

• Origin of Life :


Finding the precursors to biological molecules in the Universe if vital to our understanding of the formation and evolution of life.  Transitions of complex organic molecules exist throughout the 3mm band, including those from simple sugar species such as glycolaldehyde and ethylene glycol.  Within the ISM, previous GBT results have shown that many organic species are not confined to hot molecular cores. Instead, many organic species, including interstellar aldehydes have spatial scales on the order of 1′ – 2′.  Existing telescope arrays are not sensitive enough to map the spatial distribution of these large organic bio-molecules in a reasonable amount of time. The GBT WFPA will be able to map the distribution of interstellar bio-molecules to locate the regions with the highest abundance, which is key to constraining the formation chemistry of these species.

Observations of comets are particularly important for our understanding of the formation of the solar system and the origin of life on Earth. Comets contain the primitive material left over from the formation of the solar system, and as such, their study provides key insight into the early formation life. Currently, there is a serious lack of data on the possible chemical variations among comets, which set boundary conditions for solar system chemistry, influencing theories of the origin of life on Earth. Given that the structure within comets can change within hours over varied spatial scales, studies of comets require rapid measurements over a wide field of view with high sensitivity to low surface-brightness lines and with <10″ spatial resolution. All of these needs are uniquely met by the specifications of the GBT WFPA, a unique tool that will transform cometary research by detecting comets at much great distance and by providing the ability to make quasi-realtime movies of spectral images of multiple molecular species.

• The Context of Star Formation:
It is becoming increasingly apparent that stars do not form in isolation, but rather form within stellar groups/clusters embedded in molecular clouds. While theoretically the formation of isolated low-mass stars has a well-developed “standard model”, there is no consensus model of how stars form within groups/clusters within the real complex environment of the galaxy. The WFPA instrument on the GBT will enable observations of molecular gas over a wide range of spatial scales which is key to understanding the processes associated with star-formation.   These studies will focus on: the cloud cores within Infrared Dark Clouds to determine the dynamics, structure, and content of the clouds; creating sensitive maps of the complex organic molecules in stellar outflows; and detailed analysis of the molecular gas complexes within Galactic molecular clouds to gain insight into the conditions and formation of the dense condensations of gas which become the stellar-birth sites.

• Galaxies Across Cosmic Time:
The GBT WFPA will be an extremely efficient instrument for mapping nearby and distant galaxies and galaxy clusters at much higher resolution and sensitivity than has previously been available.  In nearby galaxies the WFPA will allow for comparisons between the content, distribution, and dynamics of the low surface sensitivity gas in  the galaxies' inter-arm spurs for comparison against model predictions.   The WFPA will also be ideal for studying the HCN/CO ratio and distribution across nearby and infrared-bright galaxies for understanding star-formation and AGN activity within these systems, allowing for a much broader understanding of the physical scale, complexity, and chemical distribution of these systems.   The GBT WFPA will also be used to study the cold ISM/IGM and the associated star formation within cluster environments to provide insight into the evolution and formation of clusters.  Finally, the GBT WFPA will be used to study low surface brightness (LSB) galaxies and galaxy features. LSB galaxies sit at the extreme edge of star formation theories, making them ideal test cases for star formation theories.  The diffuse nature of low surface brightness galaxies, however, as made their study extremely difficult, and the GBT WFPA will be an ideal instrument to  understand the star formation potential and history of these systems.