Observations of carbon monoxide in the starburst galaxy M82 with a 690 GHz-wide spectral bandwidth receiver

Abstract

A 690 GHz wide spectral bandwidth heterodyne receiver was developed to observe the J=6-5 rotational emission line of carbon monoxide (CO) in extragalactic sources. This receiver is based on a niobium superconductor-insulator-superconductor (SIS) mixer with a twin-slot antenna in a superconducting NbTiN ground plane. A 4-8 GHz low-noise amplifier was developed to amplify the intermediate frequency (IF) signal from the mixer with a spectral bandwidth of 1,700 km/s, enough to comfortably observe the entire emission line of the broadest extragalactic submillimeter sources with a single receiver tuning. This amplifier is a quasi-monolithic microwave integrated circuit (QMMIC); three 160 micron gate InP high-electron-mobility transistors (HEMTs) were bump-bonded to a thin-film GaAs substrate containing passive tuning and DC bias circuitry. The measured amplifier gain is 32 dB and the noise is approximately 8 Kelvin from 4 to 8 GHz at a physical temperature of 4 Kelvin. The complete receiver achieves a measured uncorrected double-sideband noise temperature of 180 Kelvin. Prior to this development effort, a versatile microwave simulation package was written to calculate and optimize the signal and noise performance of high-frequency circuits, especially those containing superconductors and superconducting tunnel junctions. Using this package, called SuperMix, C++ programs can be written to simulate and optimize circuits of arbitrary size, complexity, and topology. SuperMix was used to simulate the complete 690 GHz SIS receiver. The receiver was used at the Caltech Submillimeter Observatory (CSO) to map the 12CO J=6-5 emission line in the central kiloparsec of the nearby starburst galaxy M82 at a resolution of 14 arc seconds. Hot spots were found on either side of the dynamical center. A novel deconvolution technique was used to compute a 12CO J=6-5 / 12CO J=2-1 line ratio map based on high-resolution J=2-1 interferometer data. The 12CO J=6-5 map, along with observations of 12CO J=4-3, 12CO J=3-2, 13CO J=3-2, an upper limit for 13CO J=6-5, and five other measured CO lines from the literature, were analyzed in the context of a two-component large velocity gradient (LVG) excitation model. Likelihood curves were calculated for each of the model parameters as well as a variety of related physical quantities for the two hot spots based on the measured line intensities and their associated uncertainties. This approach reveals in an unbiased way how well various quantities can be constrained by the CO observations. The results of this analysis suggest that the warm gas is less dense than the cool gas, and that over half of the total molecular gas mass in these nuclear regions is warmer than 50 K