and Cosmology

5.1 Introduction
5.1 Introduction
5.1.1 Brief History of AGNs
As long ago as 1908, strong and broad emission lines
were discovered in the galaxy NGC 1068. However,
only the systematic analysis by Carl Seyfert in 1943
drew the focus of astronomers to this new class of
galaxies. The cores of these Seyfert galaxies have an
extremely high surface brightness, as demonstrated in
Fig. 5.4, and the spectrum of their central region is dominated
by emission lines of very high excitation. Some
of these lines are extremely broad (see Fig. 5.1). The
line width, when interpreted as Doppler broadening,
Δλ/λ = Δv/c, yields values of up to Δv ∼ 8500 km/s
for the full line width. The high excitation energy of
some of the line-emitting atoms shows that they must
have been excited by photons that are more energetic
than photons from young stars that are responsible for
the ionization of HII regions. The hydrogen lines are often
broader than other spectral lines. Most of the Seyfert
galaxies are spirals, but one cD galaxy is also found in
his original catalog.
In 1959, Lodewijk Woltjer argued that the extent
of the cores of Seyfert galaxies cannot be larger than
r 100 pc because they appear point-like on optical
images, i.e., they are spatially not resolved. If the lineemitting
gas is gravitationally bound, the relation
GM
≃ v 2
r
between the central mass M(< r), the separation r of the
gas from the center, and the typical velocity v must be
Fig. 5.4a–c. Three images of the Seyfert galaxy NGC 4151,
with the exposure time increasing to the right. In short exposures,
the source appears point-like, with longer exposures
displaying the galaxy
satisfied. The latter is obtained from the line width: typically
v ∼ 1000 km/s. Therefore, with r 100 pc a mass
estimate is immediately ( obtained,
M 10 10 r
)
M ⊙ . (5.1)
100 pc
Thus, either r ∼ 100 pc, which implies an enormous
mass concentration in the center of these galaxies, or
r is much smaller than the estimated upper limit, which
then implies an enormous energy density inside AGNs.
An important milestone in the history of AGNs
was made with the 3C and 3CR radio catalogs
which were completed around 1960. These are surveys
of the northern (δ>−22 ◦ ) sky at 158 MHz and
178 MHz, with a flux limit of S min = 9 Jy (a Jansky
is the flux unit used by radio astronomers, where
1Jy= 10 −23 erg s −1 cm −2 Hz −1 ). Many of these 3C
sources could be identified with relatively nearby galaxies,
but the low angular resolution of radio telescopes
at these low frequencies and the resulting large positional
uncertainty of the respective sources rendered the
identification with optical counterparts very difficult. If
no striking nearby galaxy was found on optical photoplates
within the positional uncertainty, the source was
at first marked as unidentified. 1
In 1963, Thomas Matthews and Allan Sandage
showed that 3C48 is a point-like (“stellar-like”) source
of m = 16 mag. It has a complex optical spectrum consisting
of a blue continuum and strong, broad emission
lines which could not be assigned to any atomic transition,
and thus could not be identified. In the same
year, Maarten Schmidt succeeded in identifying the radio
source 3C273 with a point-like optical source which
also showed strong and broad emission lines at unusual
wavelengths. This was achieved by a lunar eclipse: the
Moon passed in front of the radio source and eclipsed it.
From the exact measurement of the time when the radio
emission was blocked and became visible again, the position
of the radio source was pinned down accurately.
Schmidt could identify the emission lines of the source
with those of the Balmer series of hydrogen, but at an,
for that time, extremely high redshift of z = 0.158. Presuming
the validity of the Hubble law and interpreting
1 The complete optical identification of the 3CR catalog, which was
made possible by the enormously increased angular resolution of interferometric
radio observations and thus by a considerably improved
positional accuracy, was finalized only in the 1990s – some of these
luminous radio sources are very faint optically.
177