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Extra-Galactic Hydrogen Line Observations

A System for Observing Neutral Hydrogen Line Emissions in Nearby Galaxies


External Galaxies

Galaxies external to our home galaxy (the "Milky Way") are interesting and challenging targets for neutral hydrogen line emission observations.  In order to be detectable by neutral hydrogen line emissions at the large distances, the galaxies need to be hydrogen rich - usually meaning they are still "stellar nurseries" where dense nebulae are forming stars from molecular H2 gas and dust enriched with heavier elements. Associated with the H2 gas (and HII ionised gas) are quantities of atomic neutral hydrogen (HI) gas. It is this form of hydrogen gas that emits the hydrogen line signal at 21 cm.

Some targets of interest are described briefly here. A jump link table is provided below...

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...

See the 'Results' tab to see details of the results of observations at NRARAO and the verification against the LAB survey results for the Magellanic Clouds.

The Magellanic Stream

While not a galaxy, the Magellanic Stream is thought to be related to the Magellanic Clouds, forming a long ribbon of high-velocity clouds of hydrogen gas extending from the LMC and the SMC over a 100° arc...

...with velocities increasingly negative from the right (near the MCs) of the green curve, to the left, as this graph of velocity versus angle along the curve shows...

The Magellanic Stream - although not a galaxy as such - makes an interesting HI line extra-galactic target.

The Andromeda Galaxy (M31/NGC224 )Andromeda Galaxy (with h-alpha).jpg

The Andromeda Galaxy , also known as Messier 31, M31, or NGC 224, is a spiral galaxy approximately 780 kiloparsecs (2.5 million light-years) from Earth. It is the nearest major galaxy to the Milky Way and was often referred to as the Great Andromeda Nebula in older texts. It received its name from the area of the sky in which it appears, the constellation of Andromeda, which was named after the mythological princess Andromeda. Being approximately 220,000 light years across, it is the largest galaxy of the Local Group, which also contains the Milky Way, the Triangulum Galaxy, and about 44 other smaller galaxies [Wikipedia Reference].

This galaxy, while a good target (see F1EHN's observations - 'Links'), is too low in elevation (maximum 11°) for observation at NRARAO due to local obstructions (line of trees).

The Triangulum Galaxy (M33/NGC598 )M33 - Triangulum Galaxy.jpg

The Triangulum Galaxy is the third-largest member of the Local Group of galaxies, which includes the Milky Way, the Andromeda Galaxy and about 44 other smaller galaxies. It is one of the most distant permanent objects (2.8 million light-years) that can be viewed with the naked eye. The galaxy is the smallest spiral galaxy in the Local Group and it is believed to be a satellite of the Andromeda Galaxy due to their interactions, velocities and proximity to one another in the night sky [Wikipedia Reference].

This galaxy, another good target (see F1EHN's observations - 'Links'), is also too low in elevation (maximum 25°) for observation at NRARAO due to local obstructions (line of trees).

NGC300 - a Spiral Galaxy Far, Far Away...Composite Image of NGC 300.jpg

NGC 300 is a spiral galaxy in the constellation Sculptor. It is one of the closest galaxies to the Local Group, and probably lies between us and the Sculptor Group. It is the brightest of the five main spirals in the direction of the Sculptor Group. It is inclined at an angle of 42° when viewed from Earth and shares many characteristics of the Triangulum Galaxy (M33) [Wikipedia Reference].

At a distance variously estimated to be from 6 - 7 million light-years, this galaxy is a good substitute for M33 at NRARAO, located at a declination of −37° 41′ 04″.

It is largely face-on to Earth (like M33) and with angular dimensions of 21′.9 × 15′.5, covers an area of sky similar in extent to the Moon.

Unfortunately, compared to M33, NGC300 is a more difficult object to detect, lying at twice the distance of M33 - e.g. ~ 6 - 7 million light-years, or ~ 56,764,383,000,000,000,000 km.

Looking at the LAB Survey results for a 6° beamwidth, the comparison shows NGC300 is nearly 9 dB down on the brightness temperature of M33...

...and while the radial velocities are virtually mirror-images of each other, NGC300 is closer to the 0 km/s rest velocity - further complicating the extraction of the NGC300 signal from the foreground 'Milky Way' HI signals.

At the moment the detection of NGC300 remains a future activity.  Success or otherwise will be detailed on the results page in the future.