An Introduction to Modern Astrophysics

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.