We present Atacama Large Millimeter/submillimeter Array (ALMA) observations from the 2014 Long Baseline Campaign in dust continuum and spectral line emission from the HL Tau region. The continuum images at wavelengths of 2.9, 1.3, and 0.87 mm have unprecedented angular resolutions of 0″. 075 (10 AU) to 0″. 025 (3.5 AU), revealing an astonishing level of detail in the circumstellar disk surrounding the young solar analog HL Tau, with a pattern of bright and dark rings observed at all wavelengths. By fitting ellipses to the most distinct rings, we measure precise values for the disk inclination (46 .72 0 .05 ± • •) and position angle (138 .02 0 .07).

Massive star formation occurs in Giant Molecular Clouds (GMCs); an understanding of the evolution of GMCs is a prerequisite to develop theories of star formation and galaxy evolution. We report the highest-fidelity observations of the grand-design spiral galaxy M51 in carbon monoxide (CO) emission, revealing the evolution of GMCs vis-a-vis the large-scale galactic structure and dynamics. The most massive GMCs (Giant Molecular Associations -GMAs) are first assembled and then broken up as the gas flow through the spiral arms. The GMAs and their H 2 molecules are not fully dissociated into atomic gas as predicted in stellar feedback scenarios, but are fragmented into smaller GMCs upon leaving the spiral arms. The remnants of GMAs are detected as the chains of GMCs that emerge from the spiral arms into interarm regions. The kinematic shear within the spiral arms is sufficient to unbind the GMAs against self-gravity. We conclude that the evolution of GMCs is driven by largescale galactic dynamics -their coagulation into GMAs is due to spiral arm streaming motions upon entering the arms, followed by fragmentation due to shear as they leave the arms on the downstream side. In M51, the majority of the gas remains molecular from arm entry through the inter-arm region and into the next spiral arm passage.

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.

We have mapped the northern area (30 × 20 ) of a Local Group spiral galaxy M33 in 12 CO(J = 1-0) line with the 45 m telescope at the Nobeyama Radio Observatory. Along with Hα and Spitzer 24 μm data, we have investigated the relationship between the surface density of molecular gas mass and that of star formation rate (SFR) in an external galaxy (Kennicutt-Schmidt law) with the highest spatial resolution (∼80 pc) to date, which is comparable to scales of giant molecular clouds (GMCs). At positions where CO is significantly detected, the SFR surface density exhibits a wide range of over four orders of magnitude, from Σ SFR 10 −10 to ∼10 −6 M yr −1 pc −2 , whereas the Σ H 2 values are mostly within 10-40 M pc −2 . The surface density of gas and that of SFR correlate well at an ∼1 kpc resolution, but the correlation becomes looser with higher resolution and breaks down at GMC scales. The scatter of the Σ SFR -Σ H 2 relationship in the ∼80 pc resolution results from the variety of star-forming activity among GMCs, which is attributed to the various evolutionary stages of GMCs and to the drift of young clusters from their parent GMCs. This result shows that the Kennicutt-Schmidt law is valid only in scales larger than that of GMCs, when we average the spatial offset between GMCs and star-forming regions, and their various evolutionary stages.

We present a method to derive positions of molecular clouds along the lines of sight from a quantitative comparison between 2.6‐mm CO emission lines and 18‐cm OH absorption lines, and apply it to the central kiloparsecs of the Milky Way. With some simple but justifiable assumptions, we derive a face‐on distribution of the CO brightness and corresponding radial velocity in the Galactic Centre without any help of kinematical models. The derived face‐on distribution of the gas is elongated and inclined so that the Galactic‐eastern (positive longitude) side is closer to us. The gas distribution is dominated by a bar‐like central condensation, whose apparent size is 500 × 200 pc. A ridge feature is seen to stretch from one end of the central condensation, though its elongated morphology might be artificial. The velocity field shows clear signs of non‐circular motion in the central condensation. The ‘expanding molecular ring’ feature corresponds to the peripheral region surrounding the central condensation, with the Galactic‐eastern end being closer to us. These characteristics agree with a picture in which the kinematics of the gas in the central kiloparsec of the Galaxy is under the strong influence of a barred potential. The face‐on distribution of the in situ pressure of the molecular gas is derived from the CO multiline analysis. The derived pressure is found to be highest in the central 100 pc. In this region, the gas is accumulating and is forming stars.

We report 350 and 230 GHz observations of molecular gas and dust in the starburst nucleus of NGC 253 at 20-40 pc (1 -2 ) resolution. The data contain CO(3-2), HCN(4-3), CO(2-1), 13 CO(2-1), C 18 O(2-1), and continuum at 0.87 mm and 1.3 mm toward the central kiloparsec. The CO(2-1) size of the galaxy's central molecular zone (CMZ) is measured to be about 300 pc × 100 pc at the half-maximum of intensity. Five clumps of dense and warm gas stand out in the CMZ at arcsecond resolution, and they are associated with compact radio sources due to recent massive star formation. They contribute one-third of the CO emission in the central 300 pc and have 12 CO peak brightness temperatures around 50 K, molecular gas column densities on the order of 10 4 M pc −2 , gas masses on the order of 10 7 M in the size scale of 20 pc, volume-averaged gas densities of n H 2 ∼ 4000 cm −3 , and high HCN-to-CO ratios suggestive of higher fractions of dense gas than in the surrounding environment. It is suggested that these are natal molecular cloud complexes of massive star formation. The CMZ of NGC 253 is also compared with that of our Galaxy in CO(2-1) at the same 20 pc resolution. Their overall gas distributions are strikingly similar. The five molecular cloud complexes appear to be akin to such molecular complexes as Sgr A, Sgr B2, Sgr C, and the l = 1.• 3 cloud in the Galactic center. On the other hand, the starburst CMZ in NGC 253 has higher temperatures and higher surface (and presumably volume) densities than its non-starburst cousin.

We have developed spectral line On-The-Fly (OTF) observing mode for the Nobeyama Radio Observatory 45-m and the Atacama Submillimeter Telescope Experiment 10-m telescopes. Sets of digital autocorrelation spectrometers are available for OTF with heterodyne receivers mounted on the telescopes, including the focal-plane 5 × 5 array receiver, BEARS, on the 45-m. During OTF observations, the antenna is continuously driven to cover the mapped region rapidly, resulting in high observing efficiency and accuracy. Pointing of the antenna and readouts from the spectrometer are recorded as fast as 0.1 second. In this paper we report improvements made on software and instruments, requirements and optimization of observing parameters, data reduction process, and verification of the system. It is confirmed that, using optimal parameters, the OTF is about twice as efficient as conventional position-switch observing method.

We report systematic variations in the emission line ratio of the CO J = 2 − 1 and J = 1 − 0 transitions (R 2−1/1−0 ) in the grand-design spiral galaxy M51. The R 2−1/1−0 ratio shows clear evidence for the evolution of molecular gas from the upstream interarm regions, passage into the spiral arms and back into the downstream interarm regions. In the interarm regions, R 2−1/1−0 is typically < 0.7 (and often 0.4-0.6); this is similar to the ratios observed in Galactic giant molecular clouds (GMCs) with low far infrared luminosities. However, the ratio rises to > 0.7 (often 0.8-1.0) in the spiral arms, particularly at the leading (downstream) edge of the molecular arms. These trends are similar to those seen in Galactic GMCs with OB star formation (presumably in the Galactic spiral arms). R 2−1/1−0 is also high, ∼ 0.8 − 1.0, in the central region of M51. Analysis of the molecular excitation using a Large Velocity Gradient radiative transfer calculation provides insight into the changes in the physical conditions of molecular gas between the arm and interarm regions: cold and low density gas ( 10 K, 300 cm −3 ) is required for the interarm GMCs but this gas must become warmer and/or denser in the more active star forming spiral arms. The ratio R 2−1/1−0 is higher in areas of high 24µm dust surface brightness (which is an approximate tracer of star formation rate surface density) and high CO(1-0) integrated intensity (i.e., a well-calibrated tracer of total molecular gas surface density). The systematic enhancement of the CO(2-1) line relative to CO(1-0) in luminous star forming regions suggests that some caution is needed when using CO(2-1) as a tracer of bulk molecular gas mass, especially when galactic structures are resolved.