Tobias Westmeier

Astrophysicist & Radio Astronomer

All-sky map of high-velocity clouds

High-velocity clouds (HVCs) are gas clouds in the vicinity of the Milky Way which are detectable in the 21-cm line of neutral hydrogen. They are characterised by high radial velocities which are inconsistent with Galactic rotation, suggesting that these clouds are located outside of the Galactic disc. HVCs are not homogeneously distributed across the sky. Instead, they form large complexes, some of which extend over tens of degrees. The most famous and extended complexes in the northern hemisphere are A, C, and M, the first two of which were among the first HVCs ever detected in H Ⅰ emission 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.

In addition to 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 (Braun & Burton 1999), and their distances and origin are not well constrained.

All-sky HVC map
Figure 1: All-sky false-colour map of high-velocity clouds. Intensity corresponds to H Ⅰ column density in the range of 0 (dark) to 10²⁰ cm⁻² (bright), while hue represents LSR radial velocity in the range of −480 (red) to +480 km s⁻¹ (purple).

The new all-sky HVC map

The all-sky HVC map (Fig. 1) was generated from the HI4PI Galactic H Ⅰ survey (HI4PI Collaboration 2016) on the basis of a simple model of the gas distribution in the Milky Way. 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 boundaries 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 70 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.

Although great care was taken to remove Galactic emission and radio frequency interference from the data, a few artefacts are still likely to be present in the final map, including residual Galactic emission (e.g. from the so-called Outer Arm and towards the Galactic Centre) and residual stray radiation in the northern sky. In addition, some HVC emission could be missing due to kinematic overlap with the H Ⅰ emission from the Galactic disc. Hence, the map should not be used as the basis for a quantitative analysis of column densities. Instead, column densities should be directly extracted from the corresponding data cubes of the HI4PI Survey which are available for download at the Centre de Données Astronomiques de Strasbourg (CDS).


Figure 2: Rendering of the HVC map from Fig. 1 onto a rotating globe.

Download the HVC map

H Ⅰ column density and radial velocity maps of the HVC sky in different projections and coordinate systems are available for download below in both FITS format (Table 1) and as PNG images. As I cannot guarantee the accuracy and correctness of the data products, any use of the data is at your own risk. Please include a reference to the original paper (Westmeier 2018, MNRAS, 474, 289) in any publication resulting from the use of these data products.

FITS files

Table 1: List of data products available for download. The projection codes are CAR for plate carée and AIT for Hammer–Aitoff projection. Note that images in Magellanic coordinates have Galactic coordinates defined in their header, as the FITS standard does not natively support the Magellanic coordinate system.
Data
product		 Coordinate
system		 Proj. Dimension
(pixels)		 Size
(MB)		 Download link
log(NH Ⅰ / cm⁻²) Galactic CAR 4323 × 2144 7.7 hi4pi-hvc-nhi-gal-car.fits.gz
log(NH Ⅰ / cm⁻²) Galactic AIT 3891 × 1947 4.6 hi4pi-hvc-nhi-gal-ait.fits.gz
log(NH Ⅰ / cm⁻²) Magellanic CAR 4323 × 2144 7.9 hi4pi-hvc-nhi-mag-car.fits.gz
vLSR / (km s⁻¹) Galactic CAR 4323 × 2144 4.5 hi4pi-hvc-vlsr-gal-car.fits.gz
vLSR / (km s⁻¹) Galactic AIT 3891 × 1947 2.9 hi4pi-hvc-vlsr-gal-ait.fits.gz
vGSR / (km s⁻¹) Galactic CAR 4323 × 2144 4.6 hi4pi-hvc-vgsr-gal-car.fits.gz
vGSR / (km s⁻¹) Galactic AIT 3891 × 1947 3.0 hi4pi-hvc-vgsr-gal-ait.fits.gz
vGSR / (km s⁻¹) Magellanic CAR 4323 × 2144 4.7 hi4pi-hvc-vgsr-mag-car.fits.gz

PNG images

HVC all-sky map
H Ⅰ column density colour-coded by LSR velocity
HVC all-sky map
H Ⅰ column density
HVC all-sky map
GSR velocity

Publication

If you would like to use the data provided on this page in your own research, please include a reference to the following paper in any publication resulting from your work:

  • A new all-sky map of Galactic high-velocity clouds from the HI4PI survey
    Westmeier, T., 2018, MNRAS, 474, 289
    (ADS | arXiv)

The paper was accompanied by a press release on 4 December 2017: