and Cosmology

9.2 New Types of Galaxies
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Fig. 9.14. A section of the galaxy cluster Abell 2218
(z = 0.175), observed with the HST in four different filters.
This region was selected because the magnification by the
gravitational lens effect for sources at high redshift is expected
to be very large here. This fact has been established by a detailed
mass model of this cluster which could be constructed
from the geometrical constraints provided by the numerous
arcs and multiple images (Fig. 6.33). The red lines denote
the critical curves of this lens for source redshifts of z = 5,
6.5, and 7. A double image of an extended source is clearly
visible in the NIR image (on the right); this double image
was not detected at shorter wavelengths – the expected position
is marked by two ellipses in the two images on the
left. The direction of the local shear, i.e., of the expected image
distortion, is plotted in the second image from the right;
the observed elongation of the two images a and b is compatible
with the shear field from the lens model. Together
with the photometry of these two images, a redshift between
z = 6.8 andz = 7 is derived for the source of this double
image
tioned previously is another example. One important
result of such investigations of high-redshift sources
should be mentioned here. These galaxies and QSOs
have a high metal abundance, from which we conclude
that star formation must have already set in during a very
early phase of the Universe. We will later return to this
point.
The magnification effect is also utilized deliberately,
by searching for highly redshifted sources in fields
around clusters of galaxies. For a massive cluster, one
knows that distant sources located behind the cluster
center are substantially magnified. It is therefore not
surprising that some of the most distant galaxies known
have been detected in systematic searches for dropout
galaxies near the centers of massive clusters. One
example of this is shown in Fig. 9.14, where a galaxy
at z ∼ 7 is doubly imaged by the cluster Abell 2218
(see Fig. 6.33), and by means of this it is magnified by
a factor ∼ 25.
The Lyman-break galaxies discussed above are not the
only galaxies that are expected to exist at high redshifts.
We have argued that LBGs are galaxies with
active star formation. Moreover, the UV radiation from
their newly-born hot stars must be able to escape from
the galaxies. From observations in the local Universe we
know, however, that a large fraction of star formation
is hidden from our direct view, since the star-formation
region is enveloped by dust. The latter is heated by absorbing
the UV radiation, and re-emits this energy in the
form of thermal radiation in the FIR domain of the spectrum.
At high redshifts such galaxies would certainly not
be detected by the Lyman-break method.
Instrumental developments opened up new wavelength
regimes which yield access to other types of
galaxies. Two of these will be described in more detail
here: EROs (Extremely Red Objects) and submillimeter
(sub-mm) sources, the latter often being called SCUBA
galaxies because they were first observed in large numbers
by the SCUBA camera. But before we discuss these
objects we will first investigate starburst galaxies in the
relatively local Universe.
9.2 New Types of Galaxies
9.2.1 Starburst Galaxies
One class of galaxies, the so-called starburst galaxies,
is characterized by a strongly enhanced star-formation
rate, compared to normal galaxies. Whereas our Milky