The Magellanic Clouds - Our Near Galactic Neighbours

The two small irregular galaxies called the "Magellanic Clouds" are relatively near our home Galaxy and belong to the Local Group of galaxies. The Large Magellanic Cloud is at about 160,000 light years and the Small Magellanic Cloud is at about 200,000 light years from us. This compares with about 100,000 light years across our galaxy and about three million light years to the Andromeda galaxy, the nearest large galaxy - and also in the Local Group.

These smaller galaxies apparently looked somewhat like clouds to Ferdinand Magellan who with his men were the first Europeans to view them in the southern hemisphere.

Despite the naming, the Magellanic Clouds have been known since the first millennium. The first preserved mention of the Large Magellanic Cloud is by the Persian astronomer Al Sufi. In 964, in his Book of Fixed Stars, he called it al-Bakr ("the Sheep").

The radial velocities (receding) of the two galaxies are quite high - 266 km/s (LMC) and 150 km/s (SMC) - so their HI line signals can be readily distinguished from those of the intra-galactic hydrogen by filtering out lower velocities. The map below shows the distribution of HI signals with radial velocities near zero...

...which shows the ubiquitous nature of hydrogen gas throughout our Galaxy.

When a narrow band of positive radial velocities are selected (around +150 km/s) we see that the SMC stands out along with some other patches along the Galactic Equator...

...and if a yet higher narrow band of positive radial velocities are selected (around +270 km/s) we see the LMC appears almost alone...

This makes the task of unambiguously identifying HI line signals from both these external galaxies an easy one.

The relationship of the SMC and LMC velocities to 'stationary' hydrogen gas clouds is shown below in returned results from the LAB survey webpage...

...from which can be seen that in the direction of the SMC, the line of sight traverses intra-galactic hydrogen gas that has radial velocities clustered closely around 0 km/s. The radial velocity of the SMC directed away moves its band of HI signals well away in frequency from the foreground HI signals. Note also the spread of velocities within the cloud.

For the LMC essentially the same characteristic is displayed - except the higher radial velocity (away) moves the LMC band of HI line frequencies even further away from the foreground HI line signals.

Again note the similar spread of velocities within the cloud. Note also that the brightness temperature of the signals from the clouds are similar at around 6 K and therefore if one can be detected then so should the other. The brightness scale maximum has changed from 12 K for the SMC to 20 K for the LMC - because the foreground intra-galactic hydrogen signals are stronger in the vicinity of the LMC.

The characteristic shape of the velocity curve of each of the clouds allows positive verification of observed signal results.

'Flight of Fancy'

I am intrigued by the thought that the 1420.40575 MHz photons that are hitting my TVRO dish now, started on their 1.5 million million million kilometres journey from the Clouds around the time homo-sapiens were still using stone tools in the Nile Valley in Africa. Little did those photons know that in the time it took to complete their trip, we would evolve from using those simple tools to using USB dongles...