An overview of active galaxies
An overview of active galaxies

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An overview of active galaxies

8.5 Line spectra: Activity 7 Colours and broad lines

Activity 7: Colours and broad lines

Timing: 0 hours 20 minutes

Read Peterson Sections 1.3.3 and 1.3.4 [Tip: hold Ctrl and click a link to open it in a new tab. (Hide tip)] .


Peterson Figure 1.6 is a colour–colour diagram and the annotated line is the locus of the main sequence in this graph: that is to say, any main-sequence star should fall on (well, nearby!) this line. Any source of light which has intrinsic colours which deviate from this line cannot be a normal main-sequence star. Thus probable quasars can quickly be picked out.

In section 1.3.3 Peterson refers to the blackbody distribution, this is just shorthand for ‘blackbody spectral energy distribution’, which you know as the ‘blackbody spectrum’.

The Balmer continuum absorption edge is also known as the Balmer jump. This feature corresponds to the threshold (λ = 3646 Å) at which a photon can cause an ionization from the n = 2 level of hydrogen. Less energetic photons can only be absorbed by hydrogen in the n = 2 initial state if their wavelength corresponds to one of the Balmer series lines. Photons with energies just above the Balmer continuum absorption edge are very likely to be absorbed by any hydrogen atom in the n = 2 state that they encounter, causing a photoionization. Consequently, for stars hot enough for there to be significant numbers of hydrogen atoms in the n = 2 excited state, the emitted flux drops appreciably at wavelengths λ< 3646Å compared with λ > 3646 Å.

Keywords: Balmer jump (= Balmer continuum absorption edge), UV excess, flux-limited sample, equivalent width

SAQ 11

Question: Why are stars of spectral type O and B most likely to contaminate samples of quasars chosen for U-excess?


Because these are the hottest stars, and therefore will have the largest U-excess (see the vertical axis of Peterson Figure 1.6).

In the preceding reading from Peterson, the first of many possible selection effects arose. In section 1.3.4 Peterson discusses how quasars with z ≈ 2 appear bright in the U band, because the Lyman emission line falls within the U band for this redshift (see Figure 28). Selection effects can cause serious misunderstandings: one of the most important checks an astronomer must perform is to ascertain whether any observed trends are simply a by-product of the particular sample of objects chosen for study, rather than being an intrinsic property of the objects in general.

Figure 28
Figure 28 The wavelength at which Lyα is detected depends on the redshift. At redshift z1 (panels a and b) Lyα appears between 3000 Å and 4000 Å, while at redshift z2 (panels c and d) it does not

For example, if a sample of quasars was selected by looking for objects which are particularly bright in the U band (i.e. objects showing a U-excess relative to typical stars and galaxies), objects with z ≈2 would be appear to be even more plentiful than they really are.

SAQ 12

Question: What is the ‘Ly selection effect’?


This effect occurs for quasars with redshifts such that the Lyman emission line (which has rest wavelength 1216 Å) appears in the observed U band (i.e. between 3000 Å and 4000 Å). Because Lyman is such a strong emission line, it causes an appreciable enhancement in the U band apparent brightness. Consequently quasars whose continuum emission would have been too faint to detect without the contribution of the line emission can be detected. As a result it might appear that there are more quasars than otherwise expected at these redshifts.


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