## An all-sky map of high-velocity clouds

### The distribution of HVCs

High-velocity clouds are not homogeneously distributed across the sky. Instead, they are forming large complexes, some of which extend over tens of degrees. The most famous and extended complexes in the northern hemisphere are complex A, C, and M, the first two of which were among the first HVCs ever detected by Muller, Oort, and Raimond (1963) with the 25-m radio telescope near Dwingeloo. The southern hemisphere is dominated by the H i emission of the Magellanic Clouds, the Magellanic Stream, and the Leading Arm.

Apart from these large and prominent HVC complexes there are numerous compact HVCs all across the sky. Some of them have been grouped into complexes, including complex L, the complexes in the direction of the Galactic centre (GCP and GCN), or the so-called Wannier clouds (WA–WE). Other compact HVCs are completely isolated from any of the known complexes (see, e.g., Braun & Burton 1999), and their origin is particularly controversial.

Fig. 1: All-sky map of high-velocity clouds as described below. Please click on the

### The all-sky map

The map was generated from the Leiden/Argentine/Bonn (LAB) Galactic H i Survey (Kalberla et al. 2005) on the basis of a model of the gas distribution in the Milky Way by Kalberla et al. (2007, in press). For each position in the sky, the allowed range of radial velocities for Galactic H i emission was derived by determining the lowest and highest radial velocity along the line of sight within the $2 \times 10^{-3}~\mathrm{cm}^{-3}$ density limits of the Milky Way model. The resulting radial velocity range of the Galactic hydrogen was then extended in both directions by a deviation velocity of $75~\mathrm{km \, s}^{-1}$. All emission outside this extended velocity range was considered high-velocity gas and was integrated to create the column density map above.

Part of the so-called Outer Arm of the Milky Way at Galactic longitudes between 30° and 120° was not accurately described by the model and had to be cut out by hand. A few more obvious artifacts from the LAB Survey were also removed by hand. These included signals from radio frequency interference as well as a few artifacts presumably resulting from the spectral baseline correction. Despite careful analysis, a few artifacts are still likely to be present in the final map. Therefore, the map should not be used for a quantitative analysis of column densities. For this purpose one should directly analyse the data cube of the LAB Survey which is available for download at the Centre de Données Astronomiques de Strasbourg (CDS).

The H i column density map of the HVC sky was finally converted into the Hammer-Aitoff projection which has the advantage of being an equal-area projection. Different versions of the map, including versions in rectangular projection, can be downloaded below.