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The three-dimensional structure of all cloud complexes in the solar neighbourhood is revealed, showing a narrow and coherent 2.7-kpc arrangement of dense gas, in disagreement with the Gould Belt model.

For the past 150 years, the prevailing view of the local interstellar medium has been based on a peculiarity known as the Gould Belt(1-4), an expanding ring of young stars, gas and dust, tilted about 20 degrees to the Galactic plane. However, the physical relationship between local gas clouds has remained unknown because the accuracy in distance measurements to such clouds is of the same order as, or larger than, their sizes(5-7). With the advent of large photometric surveys(8) and the astrometric survey(9), this situation has changed(10). Here we reveal the three-dimensional structure of all local cloud complexes. We find a narrow and coherent 2.7-kiloparsec arrangement of dense gas in the solar neighbourhood that contains many of the clouds thought to be associated with the Gould Belt. This finding is inconsistent with the notion that these clouds are part of a ring, bringing the Gould Belt model into question. The structure comprises the majority of nearby star-forming regions, has an aspect ratio of about 1:20 and contains about three million solar masses of gas. Remarkably, this structure appears to be undulating, and its three-dimensional shape is well described by a damped sinusoidal wave on the plane of the Milky Way with an average period of about 2 kiloparsecs and a maximum amplitude of about 160 parsecs.

Since the original work on Bose-Einstein condensation(1,2), the use of quantum degenerate gases of atoms has enabled the quantum emulation of important systems in condensed matter and nuclear physics, as well as the study of many-body states that have no analogue in other fields of physics(3). Ultracold molecules in the micro- and nanokelvin regimes are expected to bring powerful capabilities to quantum emulation(4) and quantum computing(5), owing to their rich internal degrees of freedom compared to atoms, and to facilitate precision measurement and the study of quantum chemistry(6). Quantum gases of ultracold atoms can be created using collision-based cooling schemes such as evaporative cooling, but thermalization and collisional cooling have not yet been realized for ultracold molecules. Other techniques, such as the use of supersonic jets and cryogenic buffer gases, have reached temperatures limited to above 10 millikelvin(7,8). Here we show cooling of NaLi molecules to micro- and nanokelvin temperatures through collisions with ultracold Na atoms, with both molecules and atoms prepared in their stretched hyperfine spin states. We find a lower bound on the ratio of elastic to inelastic molecule-atom collisions that is greater than 50-large enough to support sustained collisional cooling. By employing two stages of evaporation, we increase the phase-space density of the molecules by a factor of 20, achieving temperatures as low as 220 nanokelvin. The favourable collisional properties of the Na-NaLi system could enable the creation of deeply quantum degenerate dipolar molecules and raises the possibility of using stretched spin states in the cooling of other molecules.

NaLi molecules are cooled to micro- and nanokelvin temperatures through collisions with ultracold Na atoms by using molecules and atoms in stretched hyperfine spin states and applying two evaporation stages.