and Cosmology

3. The World of Galaxies
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Fig. 3.38. Left: milliarcsecond structure of the two images
of the quasar QSO 0957+561, a VLBI map at 13 cm
wavelength by Gorenstein et al. Both quasar images show
a core-jet structure, and it is clearly seen that they are mirrorsymmetric,
as predicted by lens models. right: spectra of
the two quasar images QSO 0957+561A,B, observed by the
Faint Object Camera (FOC) on-board HST. The similarity
of the spectra, in particular the identical redshift, is a clear
indicator of a common source of the two quasar images.
The broad Lyα line, in the wings of which an NV line is
visible, is virtually always the strongest emission line in
quasars
The mass within θ E of a lens follows from the fact
that the mean surface mass density within θ E equals
the critical surface mass density Σ cr . A more accurate
determination of lens masses is possible by
means of detailed lens models. For quadruple image
systems, the masses can be derived with a precision
of a few percent – these are the most precise mass
determinations in (extragalactic) astronomy.
QSO 2237+0305: The Einstein Cross. A spectroscopic
survey of galaxies found several unusual emission lines
in the nucleus of a nearby spiral galaxy which cannot
originate from this galaxy itself. Instead, they are
emitted by a background quasar at redshift z s = 1.7 situated
exactly behind this spiral. High-resolution images
show four point sources situated around the nucleus
of this galaxy, with an image separation of Δθ ≈ 1 . ′′ 8
(Fig. 3.40). The spectroscopic analysis of these point
sources revealed that all four are images of the same
quasar (Fig. 3.41).
The images in this system are positioned nearly symmetrically
around the lens center; this is also a typical
lens configuration which may be caused by an elliptical
lens (see Fig. 3.36). The Einstein radius of this lens
is θ E ≈ 0 . ′′ 9, and we can determine the mass within this
radius with a precision of ∼ 3%.
Einstein Rings. More examples of Einstein rings are
displayed in Figs. 3.42 and 3.43. The first of these
is a radio galaxy, with its two radio components be-