© CAMBRIDGE UNIVERSITY PRESS 1997

1.1 Seyfert Galaxies

The first optical spectrum of an active galaxy was obtained at Lick Observatory by E.A. Fath in 1908 as part of his dissertation work. He noted the presence of strong emission lines in the nebula NGC 1068. V.M. Slipher at Lowell Observatory obtained a higher-quality, higher-resolution spectrum of NGC 1068, and commented that the emission lines are similar to those seen in planetary nebulae. He also made the important observation that these lines are resolved, and have widths of hundreds of kilometers per second.

Figure 1.1. The optical spectrum of the Seyfert 1 galaxy NGC 1275. The prominent broad and narrow emission lines are labeled, as are strong absorption features of the host galaxy spectrum. The vertical scale is expanded in the lower panel to show the weaker features. The full width at half maximum (FWHM) of the broad components is about 5900 km s-1, and the width of the narrow components is about 400 km s-1. The strong rise shortward of 4000 Å is the long-wavelength end of the ``small blue bump'' feature which is a blend of Balmer continuum and FeII line emission. This spectrum is the mean of several observations made during 1993 with the 3-m Shane Telescope and Kast spectrograph at the Lick Observatory. Data courtesy of A. V. Filippenko.

Figure 1.2. The ultraviolet spectrum of the Seyfert 1 galaxy NGC 5548. The prominent broad emission lines are labeled. The emission labeled with the Earth symbol (``'') arises in the extended upper atmosphere of the Earth and is known as ``geocoronal'' emission. Most of the labeled absorption features arise in our own Galaxy and thus apppear blueshifted from their rest wavelengths since the spectrum has been corrected for the redshift of NGC 5548 (z = 0.017). The labeled absorption features are O II 1302 (a), C II 1335 (b), Si IV 1394, 1403 (c), Si II 1527 (d), and C IV 1548, 1551 (e). Another weak C IV 1548, 1551 doublet (f) is only slightly displaced shortward of line center and presumably arises in NGC 5548 itself. This spectrum is the mean of several observations obtained with the Faint Object Spectrograph on the Hubble Space Telescope in 1993. Data courtesy of K. T. Korista.

Carl Seyfert (1943) was the first to realize that there are several similar galaxies which form a distinct class. Seyfert selected a group of galaxies on the basis of high central surface brightness, i.e., stellar-appearing cores. Seyfert obtained spectra of these galaxies and found that the optical spectra of several of these galaxies (NGC 1068, NGC 1275 ¹, NGC 3516, NGC 4051, NGC 4151, and NGC 7469) are dominated by high-excitation nuclear emission lines (Figs. 1.1 and 1.2). The important characteristics of these spectra were found to be:

1. The lines are broad (up to 8500 km s^-1, full width at zero intensity).

2. The hydrogen lines sometimes are broader than the other lines.

Seyfert galaxies received no further attention until 1955, when NGC 1068 and NGC 1275 were detected as radio sources.

Woltjer (1959) made the first attempt to understand the physics of Seyfert galaxies. He noted the following:

1. The nuclei are unresolved, so the size of the nucleus is less than 100 pc.

2. The nuclear emission must last more than 10⁸ years, because Seyfert galaxies constitute about 1 in 100 spiral galaxies. This is a simple argument. One extreme scenario is that galaxies which are Seyferts are always Seyferts, in which case their lifetime is the age of the Universe (10¹⁰ years). The opposite extreme is one where all spirals pass through a Seyfert phase (or phases) - since 1 spiral in 100 is currently in the Seyfert phase, it must last of order 10¹⁰ / 100 = 10⁸ years.

3. If the material in the nucleus is gravitationally bound, the mass of the nucleus must be very high. This is a simple virial argument, i.e.,

   (1.1)

   The velocity dispersion is obtained from the widths of the emission lines and is of order 10³ km s^-1. We have an upper limit to the size of the nucleus (r 100 pc) from the fact that it is spatially unresolved. The emission lines are characteristic of a low-density gas, which effectively provides a lower limit r 1 pc (eq. 6.15). Thus the mass of the nucleus can be inferred to be in the range M 10^9±1 M

Point (iii) tells us that something very extraordinary is occurring at the centers of Seyfert galaxies. If a large value of r is assumed, then it must be concluded that something like 10% of the mass of the galaxy is contained in a volume ~ 100 pc across. On the other hand, if r is much smaller than the upper limit set by ground-based spatial resolution, then the problem to be faced is how to generate an extraordinary amount of energy in a tiny volume.