An overview of active galaxies
An overview of active galaxies

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An overview of active galaxies

8.7 Luminosity functions

Samples of galaxies can be biased due to the flux limit of the sample that is observed. This is the so called Malmquist bias.

Activity 9: Radio-quiet quasars

Timing: 0 hours 20 minutes

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High Galactic latitude refers to directions which are away from the plane of the Milky Way Galaxy, see Figure 30. The plane of the Milky Way is at Galactic latitude = 0°, in the same way that the Earth's equator is at (terrestrial) latitude = 0°. Directions close to Galactic latitude = 0° go through the most densely populated parts of the disc of the Milky Way where dust obscures distant sources and there are many foreground stars belonging to the Milky Way. Extragalactic astronomers make observations primarily of objects at high Galactic latitude, where the line of sight is out of the disc, so that a higher proportion of the detected objects will be distant galaxies. The Hubble Deep Field (see Figure 31) is an example of a high Galactic latitude line of sight.

Keywords: luminosity function, radio-loud, radio-quiet, sample selections

Figure 30
Figure 30 Galactic latitude is measured from 0° in the plane of the Milky Way Galaxy. The figure shows lines of constant Galactic latitude and longitude overlaying (a) an optical image of the entire sky (b) an infrared image of the entire sky. In the optical image the dust in the plane of the Milky Way is clearly apparent. The dust is more transparent at infrared wavelengths, so the centre of the Milky Way is seen in image (b)
Figure 31
Figure 31 The Hubble Deep Field. Most of the objects in this picture are distant galaxies. As well as containing few foreground stars, this line of sight was chosen because it contains no bright nearby extragalactic objects

SAQ 14

Question: Why would astronomers wishing to measure the quasar luminosity function be unlikely to use the 3C catalogue as their sample?


Because there would be several serious selection effects:

  • The 3C catalogue is a radio catalogue, so all the entries are example of radio-loud quasars, which are only about 10% of the total population of quasars.

  • The 3C catalogue only contains objects which appear very bright, so suffers from a strong Malmquist bias.

  • The 3C catalogue was made with low angular resolution observations, and is limited by ‘confusion’ where sources overlap. This could cause additional selection effects, such as excluding objects in clusters of galaxies.

SAQ 15

Question: The 3C radio survey is biased, can optical surveys be regarded as unbiased?


Not necessarily. Dust in the vicinity of an AGN may absorb optical light, so infrared searches reveal more AGN.


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