OBSERVATIONS OF ACTIVE GALAXIES

The story of modern astrophysics is one of a dynamically evolving universe. On every scale, from planets to stars to galaxies, the objects that are present in this era differ from what they were during previous epochs. As we study the ancient light that arrives from distant corners of the universe, we are able to examine how galaxies looked and behaved in their youth. These observations reveal a level of activity in the centers of young, remote galaxies that is rarely found in nearer galactic nuclei.

Seyfert Galaxies

The first hint of the violent heritage of today's galaxies was found by EdwardA. Fath (1880– 1959), who in 1908 was observing the spectra of “spiral nebulae.”Although most showed an absorption-line spectrum produced by the combined light of the galaxy's stars, NGC 1068 displayed six bright emission lines. In 1926 Edwin Hubble recorded the emission lines of this and two other galaxies. Seventeen years later Carl K. Seyfert (1911–1960) reported that a small percentage of galaxies have very bright nuclei that are the source of broad emission lines produced by atoms in a wide range of ionization states. These nuclei are nearly stellar in appearance.

Today these objects are known as Seyfert galaxies, with spectra that are categorized into one of two classes. Seyfert 1 galaxies have very broad emission lines that include both allowed lines (H I, He I, He II) and narrower forbidden lines (such as [O III]). Seyfert 1 galaxies generally have “narrow” allowed lines as well, although even the narrow lines are broad compared to the spectral lines exhibited by normal galaxies. The width of the lines is attributed to Doppler broadening, indicating that the allowed lines originate from sources with speeds typically between 1000 and 5000 km s−1, while the forbidden lines correspond to speeds of around 500 km s−1. Seyfert 2 galaxies have only narrow lines (both permitted and forbidden), with characteristic speeds of about 500 km s−1.

THE HUBBLE SEQUENCE

As was mentioned at the beginning of Chapter 24, it was in the middle of the eighteenth century that Kant and Wright first suggested that the Milky Way represents a finite-sized disk-like system of stars. In the two centuries of scientific investigation since their proposal, we have indeed come to learn that a major component of our Galaxy is well represented by a disk of stars that also contains a significant amount of gas and dust. As an extension of their philosophical argument about the nature of the Galaxy, Kant went on to suggest that if the MilkyWay is limited in extent, perhaps the diffuse and very faint “elliptical nebulae” seen in the night sky might actually be extremely distant disk-like systems, similar to our own but well beyond its boundary. He called these objects island universes.

Cataloging the Island Universes

The true nature of the island universes became a matter of much investigation, and extensive catalogs of these objects were collected. One such catalog we owe to Charles Messier (1730– 1817), who, while hunting for comets, recorded 103 fuzzy objects that could otherwise be confused with the intended targets of his search. Although many of the members of the Messier catalog are truly gaseous nebulae contained within the Milky Way (such as the Crab supernova remnant and the Orion Nebula, M1 and M42, respectively), and others are stellar clusters (for instance, the Pleiades open cluster is M45 and the great globular cluster in Hercules is M13), the nature of other nebulae, such as M31 in Andromeda (Fig. 24.7), was unknown.

Another catalog of nebulae was produced by William Herschel and subsequently expanded by his son, Sir John Herschel (1792–1871), to include the southern hemisphere. Later, J. L. E. Dreyer (1852–1926) published the New General Catalog (NGC), which was based on the work of the Herschels and contained almost 8000 objects. Like Messier's catalog, the NGC includes many entries that are either gaseous nebulae or stellar clusters located within the Milky Way. However, the true nature of other objects in the catalog remained in question.