We present a 12 CO J ¼ 6-5 map of the nuclear regions of the starburst galaxy M82 at a resolution of 14 00 taken at the Caltech Submillimeter Observatory (CSO). Hot spots were found on either side of the dynamical center. We compare our results with a high-resolution 12 CO J ¼ 2-1 interferometer map, and present a 12 CO J ¼ 6-5/ 12 CO J ¼ 2-1 line ratio map obtained using a novel deconvolution technique. This line ratio is highest at the two J ¼ 6-5 integrated intensity peaks, reaching 0.4 and 0.5 in the northeast and southwest peaks, respectively, and is typically 0.2 elsewhere in the nuclear region. We also present measurements of 12 CO J ¼ 4-3, 12 CO J ¼ 3-2, and 13 CO J ¼ 3-2, and an upper limit for 13 CO J ¼ 6-5. We analyze these observations in the context of a two-component large velocity gradient (LVG) excitation model. Likelihood density curves were calculated for each of the model parameters and a variety of related physical quantities for the northeast and southwest peaks 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. We find that the beam-averaged 12 CO and 13 CO column densities, the isotopomer abundance ratio, and the area filling factors are among the best constrained quantities, while the cool component H 2 density and pressure are less well constrained. 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.

Aims. This paper describes the Heterodyne Instrument for the Far-Infrared (HIFI) that was launched onboard ESA's Herschel Space Observatory in May 2009. Methods. The instrument is a set of 7 heterodyne receivers that are electronically tuneable, covering 480−1250 GHz with SIS mixers and the 1410−1910 GHz range with hot electron bolometer (HEB) mixers. The local oscillator (LO) subsystem comprises a Ka-band synthesizer followed by 14 chains of frequency multipliers and 2 chains for each frequency band. A pair of auto-correlators and a pair of acousto-optical spectrometers process the two IF signals from the dual-polarization, single-pixel front-ends to provide instantaneous frequency coverage of 2 × 4 GHz, with a set of resolutions (125 kHz to 1 MHz) that are better than 0.1 km s −1 . Results. After a successful qualification and a pre-launch TB/TV test program, the flight instrument is now in-orbit and completed successfully the commissioning and performance verification phase. The in-orbit performance of the receivers matches the pre-launch sensitivities. We also report on the in-orbit performance of the receivers and some first results of HIFI's operations.

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We report the first demonstration of a continuous wave coherent source covering 2.48-2.75 THz, with greater than 10% instantaneous tuning bandwidth and having 1-14 μW of output power at room temperature. This source is based on a 91.8-101.8 GHz synthesizer followed by a power amplifier and three cascaded frequency triplers. It demonstrates for the first time that purely electronic solid-state sources can generate a useful amount of power in a region of the electromagnetic spectrum where lasers (solid state or gas) were previously the only available coherent sources. The bandwidth, agility, and operability of this THz source have enabled wideband, high resolution spectroscopic measurements of water, methanol, and carbon monoxide with a resolution and signal-to-noise ratio unmatched by any other existing system, providing new insight in the physics of these molecules. Furthermore, the power and optical beam quality are high enough to observe the Lamb-dip effect in water. The source frequency has an absolute accuracy better than 1 part in 10(12) and the spectrometer achieves sub-Doppler frequency resolution better than 1 part in 10(8). The harmonic purity is better than 25 dB. This source can serve as a coherent signal for absorption spectroscopy, a local oscillator for a variety of heterodyne systems and can be used as a method for precision control of more powerful but much less frequency agile quantum mechanical terahertz sources.

Abstract-We report on the design, fabrication, and characterization of an 840-900-GHz frequency multiplier chain that delivers more than 1 mW across the band at room temperature with a record peak power of 1.4 mW at 875 GHz. When cooled to 120 K, the chain delivers up to 2 mW at 882 GHz. The chain consists of a power amplifier module that drives two cascaded frequency triplers. This unprecedented output power from an electronic source is achieved by utilizing in-phase power-combining techniques. The first stage tripler uses four power-combined chips while the last stage tripler utilizes two power-combined chips. The source output was analyzed with a Fourrer transform spectrometer to verify signal purity.

We discuss the detection of absorption by interstellar hydrogen fluoride (HF) along the sight line to the submillimeter continuum sources W49N and W51. We have used Herschel's HIFI instrument in dual beam switch mode to observe the 1232.4762 GHz J = 1−0 HF transition in the upper sideband of the band 5a receiver. We detected foreground absorption by HF toward both sources over a wide range of velocities. Optically thin absorption components were detected on both sight lines, allowing us to measure -as opposed to obtain a lower limit on -the column density of HF for the first time. As in previous observations of HF toward the source G10.6-0.4, the derived HF column density is typically comparable to that of water vapor, even though the elemental abundance of oxygen is greater than that of fluorine by four orders of magnitude. We used the rather uncertain N(CH) − N(H 2 ) relationship derived previously toward diffuse molecular clouds to infer the molecular hydrogen column density in the clouds exhibiting HF absorption. Within the uncertainties, we find that the abundance of HF with respect to H 2 is consistent with the theoretical prediction that HF is the main reservoir of gas-phase fluorine for these clouds. Thus, hydrogen fluoride has the potential to become an excellent tracer of molecular hydrogen, and provides a sensitive probe of clouds of small H 2 column density. Indeed, the observations of hydrogen fluoride reported here reveal the presence of a low column density diffuse molecular cloud along the W51 sight line, at an LSR velocity of ∼24 km s −1 , that had not been identified in molecular absorption line studies prior to the launch of Herschel.

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