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Extra-Galactic Sources

11.1 Options

There are a few extra-galactic sources that can be observed by amateurs. Two will be highlighted in this section - one is a strong source that is easily detected, the other is something of a challenge.

  • The strong source is Cygnus A (3C405) (RA 19h 59m 28.3566s DEC +40 44' 02.096")
  • The weaker source is Virgo A M87 NGC 4486 (RA 12h 30m 49.42338 DEC +12 23' 28.0439")

Cygnus A (3C 405) is one of the most famous radio galaxies, and among the strongest radio sources in the sky. It was discovered by Grote Reber in 1939. In 1951, Cygnus A, along with Cassiopeia A, and Puppis A were the first "radio stars" identified with an optical source; of these, Cygnus A became the first radio galaxy(19).

11.2 Cygnus A

The radio source can be located as shown in Figure 11.1. It is a peculiar-looking, 15th magnitude galaxy located in the constellation Cygnus which would probably never have come under scrutiny were it not for the fact that it is the host for one of the strongest radio sources in the sky (20).

Figure 11.1 Cygnus A

Located 600 million light years away, this galaxy is among the giants of the universe with a mass estimated at 100 trillion times the sun's mass. It consists, apparently, of two nuclei separated by 5500 light years, embedded in a galaxy extending some 450,000 light years across. The two nuclei of Cygnus-A are probably all that remain of two separate galaxies that passed too close to each other and merged together. See Figure 11.2

It is estimated that the total power radiated by the galaxy is 1038 Watts - millions of times more than from the entire Milky Way. The radio emission is produced from a vast area that dwarfs the size of the galaxy - See Figure 11.3

Figure 11.2 Cygnus A (optical)
Figure 11.3 Cygnus A (Radio emission)

 

A professional radio image of Cygnus A can be seen in Figure 11.4 (Image courtesy of NRAO/AUI).

Figure 11.4 Radio image of Cygnus A showing the galaxy & radio lobes

(image produced at cm wavelength)

A problem arises for the amateur observer because Cygnus A is located close to Cygnus X ( a powerful X ray source that also emits radio energy) and both lie within the galactic plane. It is therefore difficult to separate these components using small antennas with limited angular resolution. It is possible to observe at microwave frequencies where resolution is improved, but for frequencies below 2GHz the antenna beams are likely to encompass all the objects. See Figure 11.5 (Radio Eyes picture)

Figure 11.5 Cygnus sources embedded in the galactic plane

Probably the best way to pick out Cygnus A is to use an interferometer that will produce fringes for small diameter objects only (as discussed in section 8). A small sample of the fringe pattern recorded from Cygnus A with an amateur interferometer (21) is shown in Figure 11.6. Due to the strength of the source, the measurement goes 'off scale' but this serves to demonstrate that the detection and separation of Cygnus A from other objects is clearly possible for amateur observers to undertake.

Figure 11.6 Part of fringe pattern from Cygnus A

11.3 Virgo A (M87)

This is a more challenging object to observe. The power flux density at 1420MHz is low, approximately 560Jy - See Figure 2.1. Virgo A is a super-giant elliptical galaxy. It was discovered in 1781 by French astronomer Charles Messier and is the second brightest galaxy within the northern Virgo Cluster. See Figure 11.7. It is located about 53.5 million light years away from Earth.

Figure 11.7 Optical picture of the Virgo cluster of galaxies of which M87 is a member

M87 was identified with the radio source Virgo A by W. Baade and R. Minkowski in 1954. In 1956, a weaker radio halo was found by J.E. Baldwin and F.G. Smith of Cambridge. The galaxy has a spectacular jet which is better seen on short exposure photographs as shown in Figure 11.8 This is a directional beam of relativistic+ plasma issuing from the core of the galaxy and contributes to its radio emissions.

Figure 11.8 Virgo A(showing the relativistic jet)

The jet is thought to be produced by a violent active nucleus in the galaxy, probably a massive central object of several billion solar masses concentrated within the innermost sphere with a radius of 60 light years.

From Figure 11.9 it can be seen that it is fortunate that the Virgo A radio source lies well out of the galactic plane - toward the north galactic pole - as this enables it to be detected without clutter from the widely dispersed Galactic noise.

Figure 11.9 Location of Virgo A

+ Relativistic particles travel close to the speed of light

As the emission from Virgo A has a synchrotron-like spectrum - which can be seen in Figure 11.10 - to observe this source it is better to use as a low a wavelength as possible in order to receive the most signal.

Figure 11.10 Emission spectrum of Virgo A

This usually means having the disadvantage of a wide antenna beamwidth that smears out the point source and the background. Again, by employing an interferometer a narrow beam can be 'synthesised' making the object easier to detect. In Figure 11.9 we see the central lobe of the interferometer antenna pattern with a width of 1.4o in the E-W (Right Ascension) direction. The resulting fringe pattern is shown in Figure 11.11, confirming a good detection of this extra-galactic object.

Figure 11.11 Amateur detection of Virgo A (406.5MHz interferometer)

© Dr David Morgan 2011