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We derived the three-dimensional distribution of H i gas in the Milky Way Galaxy using the latest H i survey data cubes and rotation curves. The distance of the H i gas was determined by the kinematic distance using a rotation curve. We solved the near-far problem in the inner Galaxy by a fitting method which involves introducing a model of vertical H i distribution. In our resultant maps we could trace three prominent arms: the Sagittarius-Carina arm, the Perseus arm, and the Outer arm. These three arms were found to be logarithmic spiral arms. The pitch angles of the Sagittarius-Carina, Perseus, and Outer arms were estimated to be about 11 • , 18 • , and 7 • , respectively. The Sun is located in a region rich in H i gas between the Sagittarius-Carina arm and the Perseus arm. The H i disk shows large and asymmetric warping in the outer disk: the H i disk goes up to about 1.5 kpc above the Galactic plane in the northern hemisphere, and down to about 1 kpc in the southern hemisphere, which means asymmetric warping. The inner H i disk is also found to be tilting. The radius of the H i disk is about 17 kpc and the Hi mass within this radius is estimated to be 2.5 × 10 9 M ⊙ , which corresponds to 1.5% of the dynamical mass predicted from the rotation curve. We also found that the H i outskirt is largely swelling in the fourth quadrant, and hence the Galaxy is significantly lopsided. The scale-height of the H i layer increases with the radius, and is correlated with the H i volume density at the centroid of the H i layer.

We created a three-dimensional distribution map of molecular gas throughout the Milky Way Galaxy using the latest 12 CO(J =1-0) survey data cube and rotation curve based on the kinematic distance. The radial distribution of the molecular gas shows a central peak and a second peak around 0.5 R 0 . The thickness of the molecular disk slightly increases from 48 pc to 160 pc with the galactocentric distance within a radius range of 0-11 kpc. We were able to trace the Outer, the Perseus, the Sagittarius-Carina, the Scutum-Crux, and the Norma arms as logarithmic spiral arms with pitch angles of 11• − 15• . Considering that the pitch angles of the spiral arms are within this range, the Norma and the Outer arms seem to be identified as the same spiral arm. We could also trace a midplane displacement, whose amplitude is nearly constant inside a 10 kpc radius and increases beyond this radius. The ridges of midplane displacement form leading spiral arms.

Context. Measurement of the Galactic neutral atomic hydrogen (H i) column density, N H i , and brightness temperatures, T B , is of high scientific value for a broad range of astrophysical disciplines. In the past two decades, one of the most-used legacy H i datasets has been the Leiden/Argentine/Bonn Survey (LAB). Aims. We release the H i 4π survey (HI4PI), an all-sky database of Galactic H i, which supersedes the LAB survey. Methods. The HI4PI survey is based on data from the recently completed first coverage of the Effelsberg-Bonn H i Survey (EBHIS) and from the third revision of the Galactic All-Sky Survey (GASS). EBHIS and GASS share similar angular resolution and match well in sensitivity. Combined, they are ideally suited to be a successor to LAB. Results. The new HI4PI survey outperforms the LAB in angular resolution (ϑ FWHM = 16 .2) and sensitivity (σ rms = 43 mK). Moreover, it has full spatial sampling and thus overcomes a major drawback of LAB, which severely undersamples the sky. We publish all-sky column density maps of the neutral atomic hydrogen in the Milky Way, along with full spectroscopic data, in several map projections including HEALPix.