A major goal of the Atacama Large Millimeter/submillimeter Array (ALMA) is to make accurate images with resolutions of tens of milliarcseconds, which at submillimeter (submm) wavelengths requires baselines up to ∼15 km. To develop and test this capability, a Long Baseline Campaign (LBC) was carried out from 2014 September to late November, culminating in end-to-end observations, calibrations, and imaging of selected Science Verification (SV) targets. This paper presents an overview of the campaign and its main results, including an investigation of the short-term coherence properties and systematic phase errors over the long baselines at the ALMA site, a summary of the SV targets and observations, and recommendations for science observing strategies at long baselines. Deep ALMA images of the quasar 3C 138 at 97 and 241 GHz are also compared to VLA 43 GHz results, demonstrating an agreement at a level of a few percent. As a result of the extensive program of LBC testing, the highly successful SV imaging at long baselines achieved angular resolutions as fine as 19 mas at ∼350 GHz. Observing with ALMA on baselines of up to 15 km is now possible, and opens up new parameter space for submm astronomy.

A new survey of the LMC has been completed in 2.6 mm carbon monoxide emission with NANTEN. This survey has revealed 107 giant molecular clouds, the first complete sample of giant molecular clouds in a single galaxy at a linear resolution of ∼ 40 pc. The cloud mass ranges from ∼ 6 × 104 to 2 × 106 M⊙, and the total molecular mass has been estimated to be 4–7 × 107 M⊙ for a molecular column density of ≳ 1.0 × 1021 cm-2, corresponding to 5–10% of the atomic mass. The molecular clouds exhibit a good spatial correlation with the youngest stellar clusters whose ages are ≲ 10 Myr, demonstrating that cluster formation is on-going in these clouds. On the other hand, they show little correlation with older clusters or with supernova remnants, suggesting that the molecular clouds are being rapidly dissipated in a several Myrs, probably due to the UV photons of massive stars in clusters.

The Atacama Large Millimeter-submillimeter Array (ALMA) radio telescope has commenced science observations of the Sun starting in late 2016. Since the Sun is much larger than the field of view of individual ALMA dishes, the ALMA interferometer is unable to measure the background level of solar emission when observing the solar disk. The absolute temperature scale is a critical measurement for much of ALMA solar science, including the understanding of energy transfer through the solar atmosphere, the properties of prominences, and the study of shock heating in the chromosphere. In order to provide an absolute temperature scale, ALMA solar observing will take advantage of the remarkable fast-scanning capabilities of the ALMA 12 m dishes to make single-dish maps of the full Sun. This article reports on the results of an extensive commissioning effort to optimize the mapping procedure, and it describes the nature of the resulting data. Amplitude calibration is discussed in detail: a path that utilizes the two loads in the ALMA calibration system as well as sky measurements is described and applied to commissioning data. Inspection of a large number of single-dish datasets shows significant variation in the resulting temperatures, and based on the temperature distributions we derive quiet-Sun values at disk center of 7300 K at λ = 3 mm and 5900 K at λ = 1.3 mm. These values have statistical uncertainties of order 100 K, but systematic uncertainties in the temperature scale that may be significantly larger. Example images are presented from two periods with very different levels of solar activity. At a resolution of order 25 ′′ , the 1.3 mm wavelength images show temperatures on the disk that vary over about a 2000 K range. Active regions and plage are amongst the hotter features while a large sunspot umbra shows up as a depression and filament channels are relatively cool. Prominences above the solar limb are a common feature of the single-dish images.

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We made observations of the 13CO and C18O J = 1−0 emission toward the Southern Coalsack. The 13CO observations covered the whole extent of the main cloud with a 2′.7 beam at an 8′ or 4′ grid spacing. The 13CO emitting gas is highly clumpy, consisting of small “cloudlets” of ≲ 0.4 pc diameter. The densest and most massive cloudlet, whose mass is ∼ 200 M⊙ , is coincident with the region of the heaviest visual extinction, located at the western edge of the cloud. The total mass of the 13CO emitting gas amounts to ∼ 600 M⊙ , corresponding to only ∼ 17% of the total cloud mass estimated from 12CO observations. In addition, toward the densest region in 13CO we made C18O J = 1−0 observations. We found five C18O dense cores, in agreement with well-known optically dark globules, and identified these dense cores to be potential sites of star formation, as indicated by their high column density, which is typical of star-forming cores. On the other hand, a comparison of the 13CO and C18O distributions with young stellar objects detected as IRAS point sources indicates no sign of recent star formation in the Southern Coalsack.

Observations of the Sun at millimeter and submillimeter wavelengths offer a unique probe into the structure, dynamics, and heating of the chromosphere; the structure of sunspots; the formation and eruption of prominences and filaments; and energetic phenomena such as jets and flares. High-resolution observations of the Sun at millimeter and submillimeter wavelengths are challenging due to the intense, extended, lowcontrast, and dynamic nature of emission from the quiet Sun, and the extremely intense and variable nature of emissions associated with energetic phenomena. The Atacama Large Millimeter/submillimeter Array (ALMA) was designed with solar observations in mind. The requirements for solar observations are significantly different from observations of sidereal sources and special measures are necessary to successfully carry out this type of observations. We describe the commissioning efforts that enable the use of two frequency bands, the 3 mm band (Band 3) and the 1.25 mm band (Band 6), for continuum interferometric-imaging observations of the Sun with ALMA. Examples of high-resolution synthesized images obtained using the newly commissioned modes during the solar commissioning campaign held in December 2015 are presented. Although only 30 of the eventual 66 ALMA antennas were used for the campaign, the solar images synthesized from the ALMA commissioning data reveal new features of the solar atmosphere that demonstrate the potential power of ALMA solar observations. The ongoing expansion of ALMA and solar-commissioning efforts will continue to enable new and unique solar observing capabilities.

We have developed a dual-polarization receiver for Band 4 of the Atacama Large Millimeter/submillimeter Array (ALMA). Band 4, which covers the 125 to 163 GHz spectral window, is one of the ten bands that form the ALMA Front End. The Band 4 receiver consists of three elements: a warm optics, a cold cartridge assembly, and a warm cartridge assembly. The cold cartridge includes a feed horn, an orthomode transducer, sideband-separating (2SB) superconductor–insulator–superconductor mixers, cold intermediate frequency (IF) amplifiers, IF isolators, bias-protection circuit boards, and component interconnections. The IF bandwidth is 4–8 GHz. The first eight receivers manufactured as preproduction models have demonstrated excellent performance within the stringent ALMA requirements. Stable astronomical fringes and closure phase have been successfully achieved during field performance tests of the Band 4 receivers installed in the ALMA antennas. Our well-established Band 4 receivers will contribute to various fields of astronomical research, such as the detection of high-redshift galaxies, characterization of cold molecular medium in normal field galaxies, and astrochemistry including observations of deuterated species.

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