Observing Conditions for Submillimeter Astronomy

Consistently superb observing conditions are crucial for achieving the scientific objectives of a telescope. Submillimeter astronomy is possible only at a few exceptionally dry sites, notably Mauna Kea, the Antarctic plateau, and the Chajnantor region in the high Andes east of San Pedro de Atacama in northern Chile. Long term measurements of 225 GHz and 350 \mu m atmospheric transparency demonstrate all three locations enjoy significant periods of excellent observing conditions. Conditions on the Chajnantor plateau and at the South Pole are better more often than on Mauna Kea. Conditions are better during winter and at night. Near the summit of Cerro Chajnantor, conditions are better than on the Chajnantor plateau

Submillimeter Atmospheric Transparency at Maunakea, at the South Pole, and at Chajnantor

For a systematic assessment of submillimeter observing conditions at different sites, we constructed tipping radiometers to measure the broad band atmospheric transparency in the window around 350 $\mu$m wavelength. The tippers were deployed on Maunakea, Hawaii, at the South Pole, and in the vicinity of Cerro Chajnantor in northern Chile. Identical instruments permit direct comparison of these sites. Observing conditions at the South Pole and in the Chajnantor area are better than on Maunakea. Simultaneous measurements with two tippers demonstrate conditions at the summit of Cerro Chajnantor are significantly better than on the Chajnantor plateau.Comment: Accepted by PAS

CO excitation in four IR luminous galaxies

The correlation between the CO and far infrared luminosities of spiral galaxies is well established. The luminosity ration, L sub FIR/L sub CO in IR luminous active galaxies is, however, systematically five to ten times higher than in ordinary spirals and molecular clouds in our Galaxy. Furthermore, the masses of molecular hydrogen in luminous galaxies are large, M (H2) approx. equals 10(exp 10) solar magnitude, which indicates the observed luminosity ratios are due to an excess of infrared output, rather than a deficiency of molecular gas. These large amounts of molecular gas may fuel luminous galaxies through either star formation or nuclear activity. This interpretation rests on applying the M (H2)/L sub CO ratio calibrated in our Galaxy to galaxies with strikingly different luminosity ratios. But are the physical conditions of the molecular gas different in galaxies with different luminosity ratios. And, if so, does the proportionality between CO and H2 also vary among galaxies. To investigate these questions researchers observed CO (2 to 1) and (1 to 0) emission from four luminous galaxies with the Institute for Radio Astronomy in the Millimeter range (IRAM) 30 m telescope. Researchers conclude that most of the CO emission from these Arp 193, Arp 220, and Mrk 231 arises in regions with moderate ambient densities similar to the clouds in the Milky Way molecular ring. The emission is neither from dense hot cloud cores nor from the cold low density gas characteristic of the envelopes of dark clouds

CCAT

Star formation, which drives the evolution of baryonic matter in the universe, occurs in the densest regions of the interstellar medium. As a result much of the emergent short wavelength radiation, UV to near IR, is absorbed by intervening dust and reradiated at longer wavelengths, far IR and subillimeter. Indeed the energy density of post primordial extragalactic light is divided equally between these short and long wavelengths, indicating equal amounts of radiation have passed through dusty and optically transparent environments over cosmic time. Comprehensive understanding of the processes of galaxy, star, and planetary formation requires, therefore, high sensitivity and high angular resolution observations, particularly surveys, in the far IR and submillimeter. A consortium led by Cornell and Caltech with JPL is now jointly planning the construction of a 25 m diameter telescope for submillimeter astronomy on a high mountain in northern Chile. This CCAT will combine high sensitivity, a wide field of view, and a broad wavelength range to provide an unprecedented capability for deep, large area, multi-color submillimeter surveys to complement narrow field, high resolution studies with ALMA. CCAT observations will address fundamental themes in contemporary astronomy, notably the formation and evolution of galaxies, the nature of the dark matter and dark energy that comprise most of the content of the universe, the formation of stars and planets, the conditions in circumstellar disks, and the conditions during the early history of the Solar system. The candidate CCAT site, at 5600m in northern Chile, enjoys superb observing conditions. To accommodate large format bolometer cameras, CCAT is designed with a 20 arcmin field of view. CCAT will incorporate closed loop active control of its segmented primary mirror to maintain a half wavefront error of 10 μm rms or less for the entire telescope. Instrumentation under consideration includes both short (650 μm–200 μm) and long (2 mm–750 μm) wavelength bolometer cameras, direct detection spectrometers, and heterodyne receiver arrays. In addition to Cornell and Caltech with JPL, the University of Colorado, the Universities of British Columbia and of Waterloo, the UK Astronomy Technology Centre on behalf of the UK community, and the Universities of Cologne and of Bonn have joined the CCAT consortium. When complete, CCAT will be the largest and most sensitive facility of its class as well as the highest altitude astronomical facility on Earth

Variation of Molecular Line Ratios and Cloud Properties in the Arp 299 Galaxy Merger

High resolution observations of 12CO (2.''3), 13CO (3.''9), and HCN (5.''4) J=1--0 in the galaxy merger Arp 299 (IC 694 and NGC 3690) show the line ratios vary dramatically across the system. The 12CO/13CO ratio is unusually large, 60 +- 15, at the IC 694 nucleus, where 12CO emission is very strong, and much smaller, 10 +- 3, in the southern extended disk of that galaxy. Elsewhere, the 12CO/13CO line ratio is 5-20, typical of spiral galaxies. The line ratio variation in the overlap between the two galaxies is smaller, ranging from 10 +- 3 in the east to 20 +- 4 in the west. The 12CO/HCN line ratio also varies across Arp 299, although to a lesser degree. HCN emission is bright towards each galaxy nucleus and in the extranuclear region of active star formation; it was not detected in the IC 694 disk, or the eastern part of the overlap region, leading to lower limits of 25 and 20 respectively. By contrast, at the nuclei of IC 694 and NGC 3690 the ratios are 9 +- 1 and 14 +- 3 respectively. In the western part of the overlap region it is 11 +- 3.Comment: 16 pages, 4 postscript figures, to appear in ApJ Letter

Submillimeter observing conditions on Cerro Chajnantor

Consistently superb observing conditions are crucial for achieving the scientific objectives of a ground based telescope. For observations at submillimeter wavelengths, choosing a site with very little atmospheric water vapor is paramount. In northern Chile, the high Andes near San Pedro de Atacama are among the highest and driest places on Earth. At the 5000 m Chajnantor plateau, long term measurements have demonstrated observing conditions are excellent for submillimeter astronomy. Even better conditions prevail on higher mountain peaks in the vicinity. For the CCAT, we have selected a candidate site at 5612 m near the summit of Cerro Chajnantor. Radiosonde measurements, meteorological data, and measurements of the 350 Consistently superb observing conditions are crucial for achieving the scientific objectives of a ground based telescope. For observations at submillimeter wavelengths, choosing a site with very little atmospheric water vapor is paramount. In northern Chile, the high Andes near San Pedro de Atacama are among the highest and driest places on Earth. At the 5000 m Chajnantor plateau, long term measurements have demonstrated observing conditions are excellent for submillimeter astronomy. Even better conditions prevail on higher mountain peaks in the vicinity. For the CCAT, we have selected a candidate site at 5612 m near the summit of Cerro Chajnantor. Radiosonde measurements, meteorological data, and measurements of the 350 μm transparency all indicate submillimeter observing conditions are consistently better at the CCAT site than at the plateau. transparency all indicate submillimeter observing conditions are consistently better at the CCAT site than at the plateau

CASIMIR, The Caltech airborne submillimeter interstellar medium investigations receiver

CASIMIR, the Caltech Airborne Submillimeter Interstellar Medium Investigations Receiver is a multiband, far infrared and submillimeter, high resolution, heterodyne spectrometer under development for SOFIA. It is a first generation, PI class instrument. CASIMIR is designed for detailed, high sensitivity observations of warm (100 K) interstellar gas both in external galaxies and Galactic sources, including molecular clouds, circumstellar envelopes, and protostellar cores. Combining the 2.5 m SOFIA mirror with state of the art superconducting mixers, will give CASIMIR unprecedented sensitivity. Initially, CASIMIR will have two bands, at 1000 and 1250 GHz, and a further three bands, 550, 750, 1400 GHz, will be added soon after. Any four bands will be available on each flight. The availability of multiple bands during each flight will allow for efficient use of flight time. For example, searches for weak lines from rare species in bright sources can be carried out on the same flight with observations of abundant species in faint or distant objects